This article may be too technical for most readers to understand. Please help improve this article to make it understandable to non-experts, without removing the technical details. The talk page may contain suggestions. (September 2010)

HD Radio, which originally stood for "Hybrid Digital", is the trademark for iBiquity's in-band on-channel (IBOC) digital radio technology used by AM and FM radio stations to transmit audio and data via a digital signal in conjunction with their analog signals. It was selected by the United States Federal Communications Commission (FCC) in 2002 as a digital audio broadcasting method for the United States,^[1][2] and is the only digital system approved by the FCC for digital AM/FM broadcasts in the United States. It is officially known as NRSC-5, with the latest version being NRSC-5B.^[3] Other digital radio systems include FMeXtra, Digital Audio Broadcasting (DAB) (Eureka 147), Digital Radio Mondiale (DRM30 and DRM+ configurations), and Compatible AM-Digital (CAM-D).

As a standard practice, the kilohertz signal rate is written next to its corresponding data transfer rate kilobits/s or kbits/s in HD Radio documentation. While HD Radio does allow for an all-digital mode, this system currently is used by some AM and FM radio stations to simulcast both digital and analog audio within the same channel (a hybridized digital-analog signal) as well as to add new FM channels and text information. Although HD Radio broadcasting's content is currently subscription-free, listeners must purchase new receivers in order to receive the digital portion of the signal. As of May 2009, there were more stations in the world on the air with HD Radio technology than any other digital radio technology. More than 1,900 stations covering approximately 84% of the United States^[4] are broadcasting with this technology, and more than 1,000 HD2 and HD3 multicast channels are on the air.^[4] According to iBiquity's website, the "HD" is simply a brand name and has no meaning. There is no connection with high-definition television (HDTV), although like HDTV the HD Radio specification provides enhanced capabilities over the old analog format, such as 5.1 surround sound.^[5]

The FCC has not indicated any intent to force off analog radio broadcasts as it has with analog television broadcasts,^[1] as it would not result in the recovery of any radio spectrum rights which could be sold. Thus, there is no deadline by which consumers must buy an HD Radio receiver. In addition, there are many more analog AM/FM radio receivers than there were analog televisions, and many of these are car stereos or portable units that cannot be upgraded.

Overview[link]

Digital information is transmitted using COFDM with an audio compression algorithm called PAC (Perceptual audio coder). However, audio compressed with this method led to complaints about poor sound quality, so in 2003 iBiquity combined it with an enhancer called SBR (spectral band replication) that improves audio quality at very low bit rates and branded the codec to HDC (High-Definition Coding). (HDC is a proprietary codec based upon, but incompatible with, the MPEG-4 standard HE-AAC). HD Radio equipped stations pay a one-time licensing fee for converting their primary audio channel to iBiquity's HD Radio technology, and 3% of incremental net revenues for any additional digital subchannels.^[6] The cost of converting a radio station can run between $100,000 and $200,000.^[7] Receiver manufacturers pay a royalty.^[8]

If the primary digital signal is lost the HD Radio receiver will revert to the analog signal, thereby providing seamless operation between the newer and older transmission methods. The extra HD-2 and HD-3 streams are not simulcast on analog, causing the sound to drop-out or "skip" when digital reception degrades (similar to HDTV drop-outs). Alternatively the HD Radio signal can revert to a more-robust 20 kilobit per second stream, though the sound is reduced to AM-like quality. Datacasting is also possible, with metadata providing song titles or artist information.

iBiquity Digital claims that the system approaches CD quality sound and offers reduction of both interference and static;^[9] however, some listeners have complained of increased interference on the analog AM band (see AM, below).

AM[link]

An example of information displayed by an AM HD station locking.

Sending pure digital data through the approximately 20 kilohertz AM channel is roughly equivalent to sending data through two 33 kbit/s analog telephone lines, thus limiting the maximum throughput possible. By using spectral band replication the HDC+SBR codec is able to simulate the recreation of sounds up to 15,000 Hz, thus achieving FM quality on the bandwidth-tight AM band.^[10] The HD Radio AM hybrid mode offers two options which can carry approximately 40 or 60 kbit/s of data, but most AM digital stations default to the more-robust 40 kbit/s mode which features redundancy (same signal broadcast twice). HD Radio also provides a pure digital mode, which lacks an analog signal for fallback and instead reverts to a 20 kbit/s signal during times of poor reception. The pure digital mode transmissions will stay within the AM station's channel instead of spilling into the channels next to the station transmitting "HD radio" as the hybrid digital stations do.

The AM version of HD Radio technology uses the 20 kHz channel (+/- 10 kHz), and overlaps 5 kHz into the opposite sideband of the adjacent channel on both sides. When operating in pure digital mode, the AM HD Radio signal fits inside a standard 20 kHz channel (20-40 kbit/s) or an extended 30 kHz channel (40-60 kbit/s), at the discretion of the station manager. As AM radio stations are spaced at 9 kHz (Europe) or 10 kHz (Americas) intervals, much of the digital information overlaps adjacent channels when in hybrid mode. Some nighttime listeners have expressed concern this design harms reception of adjacent channels^[11][12] with one formal complaint filed regarding the matter: WYSL owner Bob Savage against WBZ in Boston. The digital radio signal received on a conventional AM receiver tuned to an adjacent channel sounds like a large waterfall or similar white noise-like hiss, as can be heard in the audio soundclip referenced in the previous sentence. Listeners (and some professional broadcasters) call it IBUZ because of the "buzz saw" like noise.

FM[link]

HD radio DX during a band opening

The FM hybrid digital/analog mode offers four options which can carry approximately 100, 112, 125, or 150 kbit/s of lossy data depending upon the station manager's power budget and/or desired range of signal. The HD Radio also provides several pure digital modes with up to 300 kbit/s bitrate, and enabling extra features like surround sound. Like AM, pure digital FM provides a "fallback" condition where it reverts to a more-robust 25 kbit/s signal.

FM stations have the option to subdivide their datastream into sub-channels (e.g., 88.1 HD1, HD2, HD3) of varying audio quality. The multiple services are similar to the digital subchannels found in ATSC-compliant digital television using multiplexed broadcasting. For example, some top 40 stations have added hot AC and classic rock to their digital subchannels to provide more variety to listeners.^[13] Stations may eventually go all-digital, thus allowing as many as three full-power channels and four low-power channels (seven total). As defined by iBiquity these channels could be sub-divided into CD-quality (100 kbit/s), FM-quality (25-50 kbit/s), AM-quality (12 kbit/s), or Talk-quality (5 kbit/s) channels. Alternatively, they could broadcast one single channel at 300 kbit/s.

Where the digital signal fails, the analog signal is used as a fallback for the main digital channel (normally HD1), requiring synchronization of the two. Current FCC rules require that one channel be a simulcast of the analog signal. In some cases, particularly during tropospheric ducting events, an HD Radio receiver will lock on to the digital sidebands of a distant station, even though there is a much stronger local analog-only station on the same frequency. Because there is no standard identification or other indication sent on the analog signal, there is no way for the receiver to recognize that there is no correlation between the two. The listener can possibly turn HD reception off (to listen to the local station, or avoid random flipping between the two stations), or enjoy listening to the distant stations and trying to get a station ID.

Although the signals may be synchronized at the transmitter and reach your equipment simultaneously, what you hear through an HD unit and an analog radio played together will be distinctly unsynchronized. This is because all analog receivers process analog signals faster than HD radios can process digital signals. The resulting unmistakable "reverb" or echo effect from playing HD and analog in the same room or house can be annoying. It's more noticeable with simple voice transmission than with complex musical signals. (Multiple HD receivers or multiple analog receivers in the same room or house will not cause noticeable echo.) The only way to truly "synchronize" HD with analog radio would be to delay the analog transmission by a few hundred milliseconds, but this is normally not done.

Stations can transmit HD Radio through their existing antennas using a diplexer as on AM, or are permitted by the FCC to use a separate antenna at the same location, or at a site licensed as an analog auxiliary, provided it is within a certain distance and height referenced to the analog main signal. This limitation assures that the two have about the same broadcast range, and that they maintain the proper ratio of signal strength to each other so as not to cause destructive interference with each other at any given location where they may be received.

Bandwidth and power[link]

Currently, FM stations in the United States and Canada are licensed to carry 100 kilohertz of bandwidth, requiring approximately 200 kilohertz of spectrum. Only 15 kHz of the modulation bandwidth is used by analog monaural audio (baseband), with the remainder used for stereo, RBDS, paging, radio reading service, rental to other customers, or as a transmitter/studio link for in-house telemetry.

In regular hybrid mode a station has its full ±100 kHz of analog bandwidth and adds an extra ±30 kHz guard band and ±70 kHz for its digital signals, thus taking a full 400 kHz of width. FM stations also have the option to discontinue existing subcarrier services (usually at 92 kHz and 67 kHz) in order to carry extended HD Radio, though such services can be restored through use of the digital subchannels that are then made available. However, this requires the replacement of all related equipment both for the broadcasters and all of the receivers that use the discontinued services.

The ratio of power of the analog signal to the digital signal is standardized at 100:1 (−20 dBc), making the digital signal 1% of the analog carrier power. Unlike with subcarriers, where the total baseband modulation is reduced, there is no reduction to the analog carrier power. The National Association of Broadcasters (NAB) requested a 10 dB (10×) increase^[14] in the digital signal from the FCC. This equates to an increase to 10% of the analog carrier power, but no decrease in the analog signal. This was shown to reduce analog coverage because of interference, but results in a dramatic improvement in digital coverage. Other levels have also been tested, including a 6dB or fourfold increase to 4% (−14dBc or 25:1). National Public Radio is opposed to any increase because it is likely to increase interference to their member stations, particularly to their broadcast translators which are secondary and therefore left unprotected from such interference. Other broadcasters are also opposed (or indifferent), as increasing power would require expensive changes in equipment for many, and the already-expensive system has so far given them no benefit.

There are still some concerns that HD Radio on FM will increase interference between different stations even though HD Radio at the 10% power level fits within the FCC spectral mask.^[15] North American FM channels are spaced 200 kHz apart. An HD Radio station will not generally cause interference to any analog station within its 1 mV/m service contour, the limit above which the FCC protects most stations. However, the IBOC signal resides within the analog signal of the first-adjacent station. With the proposed power increase of 10dB, the potential exists to cause the degradation of the second-adjacent analog signals within its 1 mV/m service contour.^{[16][17][18][19]} Ironically, the National Association of Broadcasters claims this is not a problem, at the same time using it as justification to keep LPFM stations off the air on second-adjacent channels, excluding NAB members' own translators that are allowed higher power than local community radio LPFMs.

On January 29, 2010, the U.S. FCC approved a report and order to voluntarily increase the maximum digital effective radiated power (ERP) to 4% of analog ERP (−14dBc), up from the previous maximum of 1% (−20dBc).^[20] Individual stations may apply for up to 10% (−10dBc) if they can prove it will not cause destructive interference to any other station. If at least six verified complaints of ongoing RF interference to another station come from locations within the other station's service contour, the interfering station will be required to reduce to the next level down of 4%, 2% (−17dB), or 1%, until the FCC makes a final determination.^[21] The station to which the interference is caused bears the burden of proof and its associated expenses, rather than the station that causes the problem. For grandfathered FM stations which are allowed to remain over the limit for their class, these numbers are relative to that lower limit rather than their actual power.

Comparison to other digital radio standards[link]

HD Radio versus DAB[link]

Some countries have implemented Eureka-147 Digital Audio Broadcasting (DAB). DAB broadcasts a single station that is approximately 1500 kilohertz wide (~1000 kilobits per second). That station is then subdivided into multiple digital streams of between 9 and 12 programs. In contrast FM HD Radio is assigned to the traditional 200 kilohertz-wide channels, with capability of 300 kbit/s in pure digital mode. Thus HD Radio is approximately twice as data-efficient as DAB.

The first generation DAB uses the MPEG-1 Audio Layer II (MP2) audio codec which has less efficient compression than newer codecs. The typical bitrate for DAB programs is only 128 kbit/s and as a result most radio stations on DAB have a lower sound quality than FM, prompting a number of complaints.^[22] The newer FM HD Radio uses a codec based upon the MPEG-4 HE-AAC standard. Several psychoacoustic studies/tests have shown HE-AAC codecs achieve the same perceived quality as MP2, but at one-quarter the bit rate (CD quality at 64 kbit/s rather than 256 kbit/s).

A directly related issue with DAB's original inefficient compression is "downgrading" stations from stereophonic to monaural, in order to include more channels into the limited 1000 kbit/s bandwidth,^[23] smaller coverage of markets as compared to analog FM, radios that are overly expensive, poor reception inside vehicles or buildings, and a general lack of interest in DAB in many countries. HD Radio shares most of these same flaws (see criticisms below).

HD Radio versus DRM[link]

Digital Radio Mondiale (DRM) is a system designed primarily for world band radio with compatible radios already available for sale. DRM is very similar to AM HD Radio in that each station is broadcast via a channel 20 kilohertz wide (+/-10 kHz), and the radio is hand-tuned to each individual station's location on the dial. The two standards also share the same modulation scheme (COFDM) and codecs based upon MPEG-4 AAC.

Just like HD Radio which allows either hybrid digital-analog broadcasts or pure digital broadcasts, DRM allows broadcasters to use multiple options:

• Hybrid mode (digital/analog) — 10 kHz analog plus 5 kHz digital bandwidth allows 5-16 kbit/s data rate;
• 10 kHz digital-only bandwidth (equal to one-half of one channel) allows 12-35 kbit/s;
• 20 kHz digital-only bandwidth (equal to one channel with both sidebands) allows 24-72 kbit/s.

DRM Plus additionally permits operation in the FM band with 100 kHz digital-only bandwidth which allows 700 kbit/s data rate.

Actual DRM bit rates vary depending on day versus night transmission (groundwave versus skywave) and the amount of bits dedicated for error correction. For AM stations DRM offers a growth path for broadcasters. Unfortunately DRM shares many of the same flaws as DAB and HD Radio technology: shorter broadcast distance as compared to analog AM signal when in hybrid mode; poor reception inside vehicles and buildings; and interference with adjacent channels (though in all-digital mode the signal fits inside the designated channel mask).

Criticisms[link]

Awareness, coverage[link]

There is low awareness among consumers, and even lower uptake. According to a survey dated August 8, 2007 by Bridge Ratings, when asked the question, "Would you buy an HD radio in the next two months?" only 1.0% responded "yes".^[24] Some broadcast engineers have also expressed distrust or dislike of the new system.^[25] Also, a survey conducted in September 2008 saw a small percentage still confused HD radio with satellite radio.^[26]

Most of the first-generation HD Radio tuners have been noted as being very insensitive, making reception problematic. The HD Radio signal is 10 to 20 dB below a station's analog signal. In addition it has been noted that the analog section of some tuners displays poor reception capabilities compared to older non-digital models.^[27]

Proprietary and incompatible[link]

Even though DAB and DRM standards are open-standards and pre-date HD Radio, HD Radio receivers cannot be used to receive these stations when sold or moved overseas (with certain exceptions; there are HD Radio stations in Sri Lanka,^[28] Brazil, Thailand, Taiwan, Japan, Romania and a few other countries). DAB and DRM receivers cannot receive HD Radio signals in the US. The HD Radio system, which enables AM and FM stations to upgrade to digital without changing frequencies, is a different digital broadcasting standard. The lack of a common standard means that HD Radios cannot receive DAB format broadcasts of other countries and vice-versa, and that manufacturers must develop separate products for different countries, which typically are not dual-format. Whereas the AAC (Advanced Audio Coding) family of codecs are publicly documented standards, the HDC codec exists only within the HD Radio system, and is an iBiquity trade secret. Similarly DRM and DAB are open specifications, while iBiquity's HD Radio specification is partly open but mostly private.^[29]

HD Radio is also incompatible with ATSC, the standard for digital television in the United States. In the days of analog television, the low end of the FM broadcast band (87.7 to 87.9 MHz) overlapped with the audio subcarrier of analog television's channel 6, which allowed the audio of television stations that broadcast on that channel to be heard on most FM radios. Several television stations exploited this overlap and operated as radio stations (a process that still continues with some low-powered stations, which are still allowed to broadcast in analog for the time being). Full-powered television stations were forced to cease analog broadcasting in June 2009. Because the digital television and digital radio standards are incompatible with each other, HD radios are not able to receive digital TV signals on the 87.7 frequency, eliminating the dual-medium compatibility of channel 6 television stations.

Reduced-quality concerns[link]

Promotion for HD Radio does not always make clear that some of its capabilities are mutually incompatible with other of its capabilities. For example, the FM system has been described as "CD quality;" however, the FM system also allows multiplexing the data stream between two or more separate programs. A program utilizing one half or less of the data stream does not attain the higher audio quality of a single program allowed the full data stream. The FCC has declared "one free over-the-air digital stream [must be] of equal or greater quality than the station’s existing analog signal".^[30] (If the FCC discontinues analog simulcasting, each station will have over 300 kbit/s bandwidth available, allowing for CD-quality or even surround sound audio together with multiple sub-channels.)

The broadcasting industry is seeking FCC approval for conditional access, that is, enabling the extra programs to be available only by paid subscription (on future models of HD Radio).^{[citation needed]} Conditional access will also enable special services for the blind (such as radio reading services) and hearing impaired.^[31] NDS, a maker of digital media encryption technology, has a deal with iBiquity to provide HD Radio with an encrypted content-delivery system called RadioGuard.^[32] NDS claims that RadioGuard will "provide additional revenue-generating possibilities".

A few existing FM tuners tuned to a channel broadcasting an HD Radio signal are prone to increased noise on the analog signal, called "HD Radio self-noise" ([2]), due to analog demodulation of the digital signal(s). Some rare high fidelity FM tuners in quality playback systems, cause this noise to be audible and irritating. A few existing FM tuners might require major internal modifications to the internal filters ([3]) or the addition of a post-detection filter ([4]) may be required to prevent degradation of the analog signal quality on stations broadcasting with HD Radio.

Reduced analog signal[link]

Radio stations are licensed in the United States to broadcast at a specific effective radiated power level. NPR Labs recently did a study of predicted HD radio operation if power levels were increased to 10% of maximum power as is now allowed by the FCC under certain circumstances, and found the digital signal would increase RF interference on FM. However the boosted digital HD signal coverage would then exceed analog coverage, with 17% more population covered in vehicles but 17% less indoors.^[33]

High costs[link]

The costs of installing the system, including fees, vary from station to station, according to the station's size and existing infrastructure. Typical costs are at least several tens of thousands of dollars at the outset (including transmitter, diplexer or antenna/feedline, and labor), plus per-channel annual fees to be paid to iBiquity. Large companies in larger media markets (such as Clear Channel or Citadel Broadcasting) can afford to implement the technology for their stations. However, community radio stations, both commercial and noncommercial, cannot. During mid 2010 a new generation of HD Radio broadcasting equipment was introduced greatly lowering the technical costs of implementing the system.

Current HD Radio receivers cost anywhere from around $50 to several hundred dollars (US), compared to regular FM radios which can even be found at dollar stores. By contrast, all of the R&D work for DAB (and much of DAB+) had already been done using an existing codec, and requires no licensing fees. FMeXtra is even less expensive, and requires no installation labor or cost for the broadcaster, other than plugging it into the transmitter. It also requires no FCC approval, coming under a station's subsidiary communications authority. FMeXtra requires no license fees, and does not interfere with adjacent channels. Other digital broadcast technologies have not been approved for use in the United States on the FM and AM bands. The United States uses DRM for HF or "Shortwave" broadcasts.^[34] The number and cost of HD Radio receivers greatly dropped during 2009 and 2010.

Programming[link]

Currently the HD Digital Radio Alliance, a consortium of major owners such as ABC, CBS, and ClearChannel, is acting as a liaison for stations to choose unduplicated formats for the extra channels (HD2, HD3, etc.). This is designed to provide additional choices for listeners instead of several stations all independently deciding to create the same format. HD1 stations broadcast the same format as the regular FM (and some AM) stations, and most of these stations offer an HD2 subchannel to complement their current programming.

Clear Channel is selling programming of several different music genres to other competing stations, in addition to airing them on its own stations. Some stations are simulcasting their local AM or lower-power FM broadcasts on sister stations' HD2 or HD3 channels, such as KMBZ-FM in Kansas City simulcasting 610 KCSP's programming on 98.1-HD2. It is common practice to broadcast an older, discontinued format on HD2 channels; for example, with the recent disappearance of the smooth jazz format from the analog radio dial in many markets, stations such as WDZH in Detroit, Michigan (formerly WVMV), WQCD in New York City, WPGB in Pittsburgh, Pennsylvania and WNWV in Cleveland, Ohio program smooth jazz on their HD2 or HD3 bands. Other recent additions include introduction of air staff on HD2 stations, like KDWB's Party Zone channel in Minneapolis-Saint Paul. Some HD2 or HD3 stations are even simulcasting sister AM stations. In St. Louis, Missouri for example, clear-channel KMOX AM (1120 kHz analog and HD) is simulcast on KEZK FM 102.5 HD3. KBCO in Boulder, Colorado uses its HD2 channel to broadcast exclusive live recordings from their private recording studio. CBS Radio recently announced plans to introduce its more popular superstations into distant markets (KROQ-FM into New York City, WFAN into Florida, and KFRG and KSCF into Los Angeles) via HD2 and HD3 channels.

On March 8, 2009, CBS Radio inaugurated the first station with an HD4 subchannel, WJFK-FM in Washington, DC, a sports radio station which also carries sister sports operations WJZ-FM from Baltimore, Philadelphia's WIP and WFAN from New York.^[35]

Public broadcasters are also embracing HD Radio. Minnesota Public Radio offers a few services: KNOW, the MPR News station in the Twin Cities, offers music service Radio Heartland on 91.1 HD2 and additional news programming called BBC News and More on 91.1 HD3; KSJN, the Classical MPR station in the Twin Cities, provides Classical 24 service on 99.5 HD2; and The Current, on 89.3 in the Twin Cities, offers Wonderground Radio, music for kids and their parents, on 89.3 HD2.^[36]

Southern California Public Radio, heard on 89.3 FM in Los Angeles, offers Ahora, the Spanish-language service of Radio Netherlands on 89.3 HD2 and MPR's music service The Current on 89.3 HD3 in Los Angeles.^[37]

WNYC in New York City broadcasts a locally programmed, all-classical music service called Q2, on 93.9 HD2. The service launched in March 2006.^[38] On October 8, 2009, the format was moved to WQXR-HD2 on 105.9 when WQXR was acquired by WNYC as part of a frequency swap with Univision Radio for their former frequency. The programming on the WNYC HD2 channel now is a rebroadcast of WQXR in order to give full coverage of WQXR programming in some form, as the 105.9 signal is weaker and does not cover the whole area.

WMIL-FM in Milwaukee has offered an audio simulcast of Fox affiliate WITI on their HD3 subchannel since August 2009 as part of a news and weather content agreement between Clear Channel and WITI. This restored WITI's audio to the Milwaukee radio dial after a two month break after the digital transition; as a Channel 6 analog television station WITI exploited the 87.7 FM audio quirk as an advantage in order to allow viewers to hear the station's newscasts and Fox programming on their car radios.

College radio has also been impacted by HD radio, stations such as WBJB which is a public station on a college campus offer a student run station as one of the multicast channels.

Some commercial broadcasters also use their HD2's to broadcast the programming of noncommercial broadcasters. Bonneville International uses its HD2 and HD3 channels to broadcast Mormon Channel which is entirely noncommercial and operates solely as a public service from Bonneville's owner, The Church of Jesus Christ of Latter-day Saints. That network of eight HD2 and HD3 stations was launched on May 18, 2009 and was fully functional within two weeks. Also, in Detroit, WMXD, an urban adult contemporary station, airs the contemporary Christian K-Love format on its HD2 band (the HD2 also feeds several analog translators around the metropolitan area—see below), due to an agreement between Clear Channel and K-Love owner Educational Media Foundation allowing EMF to program WMXD's HD2.

Translators[link]

Although broadcast translators are prohibited from originating their own programming, the FCC has controversially allowed translator stations to rebroadcast in standard analog FM the audio of a HD Radio channel of the primary station the translator is assigned to (see, for instance, W237CS, which is licensed as a translator to WXMT but originates its own schedule). This also allows station owners, who already usually own multiple stations locally and nationally, to avoid the rulemaking process of changing the table of allotments as would be needed to get a new separately-licensed station, and to avoid exceeding controlling-interest caps intended to prevent the excessive concentration of media ownership. Such new translator stations can block new LPFM stations from going on the air in the same footprint. Translator stations are allowed greater broadcast range (via less restrictive height and power limitations) than locally-originated LPFMs, so they may occupy a footprint in which several LPFMs might have been licensed otherwise.

In addition to the controversial practice of converting the HD Radio-only secondary channels of a primary station into analog FM in areas where the primary station's signal can already readily be received, translators can also be used in a more traditional manner to extend the range of the full content of the primary station, including the unmodified main signal and any HD radio sub-channels, in areas where the station has poor coverage or reception, as is done at K202BD in Manti, Utah, which rebroadcasts both the analog and digital signals of KUER from Salt Lake City. In order to do this, HD Radio may be passed along from the main station via a "bent pipe" setup, where the translator simply makes a frequency shift of the entire channel, often by heterodyning it through the use of an intermediate frequency. This may require an increase in bandwidth in both the amplifier and radio antenna if they are too narrowband to pass the wider signal, meaning one or both would have to be replaced. Baseband translators which use a separate receiver and transmitter require an HD Radio transmitter, just as does the main station. Translators are not required to pass through HD Radio, and the vast majority of existing translators which repeat FM stations running hybrid HD Radio signals do not repeat the HD Radio part of the broadcast due to technical limitations in equipment designed before the prominence of HD Radio.

Receivers[link]

This section may contain promotional material and other spam. Please remove any content which is not encyclopedic, and any promotional external links in accordance with the external links guideline. (January 2011)

Automotive and home/professional[link]

There several HD Radio receivers to choose from. Receivers are expensive, starting at around US$50.

The list of HD Radio automotive receiver manufacturers includes Alpine Electronics, Clarion, Delphi Corporation, Directed Electronics, Dual, Insignia, Jensen, JVC, Kenwood, Pioneer, Sony and Visteon.

Home and office listening equipment is currently available from roughly a dozen companies, in both component tuner and tabletop models, including Audio Design Associates, Boston Acoustics, DaySequerra, Denon, DICE Electronics, Directed Electronics, Insignia, Jensen Electronics, LG, Marantz, Onkyo, Polk Audio, Radiosophy, Radio Shack, Rotel, Sangean, Sony, TEAC, Visteon and Yamaha.

Portable[link]

Previously, portable HD Radio receivers were unavailable due to the early chipsets either being too large for a portable enclosure or needing too much power to be practical for a battery-operated device. However, in January 2008 at the Consumer Electronics Show (CES) in Las Vegas iBiquity unveiled a prototype of a new portable receiver, roughly the size of a cigarette pack. Two companies are currently making low-power chipsets for HD Radio receivers: Samsung^[39] and SiPort.^[40]

The portable Coby HDR-700 unit receives AM HD and FM HD stations.^[41] Griffin Technology produces an HD Radio tuner designed to be plugged into the dock connector of an Apple iPod or iPhone, with tuning functionality provided via software through the device's multi-touch display. This product is now discontinued.

On July 12, 2009, Best Buy started selling a house brand portable unit, the Insignia NS-HD01, which was the second portable HD Radio to come to the general market and features FM-only playback and a non-removable rechargeable battery which charges via mini USB. Coby produced the first portable HD radio (HDR-700). The Insignia unit sells for around $50 and is the least expensive receiver currently available.^[42] This product is also discontinued.

On September 15, 2009, Microsoft released the Zune HD, which includes an HD Radio receiver within the media device.^[43] As with the other portable HD radio units, the Zune HD is now discontinued.

iBiquity is trying to get HDR chipsets into mobile phones by 2012.

References[link]

External links[link]

• HD Radio official site
• U.S. Federal Communications Commission IBOC Page
• Site with Standards Documents for the NRSC formats of HD Radio

How radio communication works. Information such as sound is transformed into an electronic signal which is applied to a transmitter. The transmitter sends the information through space on a radio wave (electromagnetic wave). A receiver intercepts some of the radio wave and extracts the information-bearing electronic signal, which is converted back to its original form by a transducer such as a speaker.

Radio is the transmission of signals through free space by electromagnetic waves with frequencies significantly below visible light, in the radio frequency range, from about 3 kHz to 300 GHz.^[1] These waves are called radio waves. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space.

Information, such as sound, is carried by systematically changing (modulating) some property of the radiated waves, such as their amplitude, frequency, phase, or pulse width. When radio waves strike an electrical conductor, the oscillating fields induce an alternating current in the conductor. The information in the waves can be extracted and transformed back into its original form.

Classic radio receiver dial

Etymology[link]

The etymology of "radio" or "radiotelegraphy" reveals that it was called "wireless telegraphy," which was shortened to "wireless" in Britain. The prefix radio- in the sense of wireless transmission, was first recorded in the word radioconductor, a description provided by the French physicist Édouard Branly in 1897. It is based on the verb to radiate (in Latin "radius" means "spoke of a wheel, beam of light, ray").

The word "radio" also appears in a 1907 article by Lee De Forest. It was adopted by the United States Navy in 1912, to distinguish radio from several other "wireless" communication technologies in use at the time, such as the photophone. The term became common by the time of the first commercial broadcasts in the United States in the 1920s. (The noun "broadcasting" itself came from an agricultural term, meaning "scattering seeds widely.") The term was adopted by other languages in Europe and Asia. British Commonwealth countries continued to commonly use the term "wireless" until the mid-20th century, though the magazine of the BBC in the UK has been called Radio Times ever since it was first published in the early 1920s.

In recent years the term "wireless" has gained renewed popularity through the rapid growth of short-range computer networking, e.g., Wireless Local Area Network (WLAN), Wi-Fi, and Bluetooth, as well as mobile telephony, e.g., GSM and UMTS. Today, the term "radio" often refers to the actual transceiver device or chip, whereas "wireless" refers to the system and/or method used for radio communication; hence one talks about radio transceivers and Radio Frequency Identification (RFID), but about wireless devices and wireless sensor networks.

Processes[link]

Radio systems used for communications will have the following elements. With more than 100 years of development, each process is implemented by a wide range of methods, specialized for different communications purposes.

Transmitter and modulation[link]

Each system contains a transmitter. This consists of a source of electrical energy, producing alternating current of a desired frequency of oscillation. The transmitter contains a system to modulate (change) some property of the energy produced to impress a signal on it. This modulation might be as simple as turning the energy on and off, or altering more subtle properties such as amplitude, frequency, phase, or combinations of these properties. The transmitter sends the modulated electrical energy to a tuned resonant antenna; this structure converts the rapidly changing alternating current into an electromagnetic wave that can move through free space (sometimes with a particular polarization).

An audio signal (top) may be carried by an AM or FM radio wave.

Amplitude modulation of a carrier wave works by varying the strength of the transmitted signal in proportion to the information being sent. For example, changes in the signal strength can be used to reflect the sounds to be reproduced by a speaker, or to specify the light intensity of television pixels. It was the method used for the first audio radio transmissions, and remains in use today. "AM" is often used to refer to the mediumwave broadcast band (see AM radio).

Frequency modulation varies the frequency of the carrier. The instantaneous frequency of the carrier is directly proportional to the instantaneous value of the input signal. Digital data can be sent by shifting the carrier's frequency among a set of discrete values, a technique known as frequency-shift keying.

FM is commonly used at VHF radio frequencies for high-fidelity broadcasts of music and speech (see FM broadcasting). Normal (analog) TV sound is also broadcast using FM.

Angle modulation alters the instantaneous phase of a carrier wave to transmit a signal. It is another term for Phase modulation.

Antennas and propagation[link]

Electromagnetic waves travel through space either directly, or have their path altered by reflection, refraction or diffraction. The intensity of the waves diminishes due to geometric dispersion (the inverse-square law); some energy may also be absorbed by the intervening medium in some cases. Noise will generally alter the desired signal; this electromagnetic interference comes from natural sources, as well as from artificial sources such as other transmitters and accidental radiators. Noise is also produced at every step due to the inherent properties of the devices used. If the magnitude of the noise is large enough, the desired signal will no longer be discernible; this is the fundamental limit to the range of radio communications.

Resonance[link]

Electrical resonance of tuned circuits in radios allow individual stations to be selected. A resonant circuit will respond strongly to a particular frequency, and much less so to differing frequencies. This allows the radio receiver to discriminate between multiple signals differing in frequency.

Receiver and demodulation[link]

A Crystal Receiver, consisting of an antenna, rheostat, coil, crystal rectifier, capacitor, headphones and ground connection.

The electromagnetic wave is intercepted by a tuned receiving antenna; this structure captures some of the energy of the wave and returns it to the form of oscillating electrical currents. At the receiver, these currents are demodulated, which is conversion to a usable signal form by a detector sub-system. The receiver is "tuned" to respond preferentially to the desired signals, and reject undesired signals.

Early radio systems relied entirely on the energy collected by an antenna to produce signals for the operator. Radio became more useful after the invention of electronic devices such as the vacuum tube and later the transistor, which made it possible to amplify weak signals. Today radio systems are used for applications from walkie-talkie children's toys to the control of space vehicles, as well as for broadcasting, and many other applications.

A radio receiver receives its input from an antenna, uses electronic filters to separate a wanted radio signal from all other signals picked up by this antenna, amplifies it to a level suitable for further processing, and finally converts through demodulation and decoding the signal into a form usable for the consumer, such as sound, pictures, digital data, measurement values, navigational positions, etc.^[2]

Radio band[link]

Radio frequencies occupy the range from a few hertz to 300 GHz, although commercially important uses of radio use only a small part of this spectrum.^[3] Other types of electromagnetic radiation, with frequencies above the RF range, are microwave, infrared, visible light, ultraviolet, X-rays and gamma rays. Since the energy of an individual photon of radio frequency is too low to remove an electron from an atom, radio waves are classified as non-ionizing radiation.

Communication systems[link]

A radio communication system sends signals by radio.^[4] Types of radio communication systems deployed depend on technology, standards, regulations, radio spectrum allocation, user requirements, service positioning, and investment.^[5]

The radio equipment involved in communication systems includes a transmitter and a receiver, each having an antenna and appropriate terminal equipment such as a microphone at the transmitter and a loudspeaker at the receiver in the case of a voice-communication system.^[6]

The power consumed in a transmitting station varies depending on the distance of communication and the transmission conditions. The power received at the receiving station is usually only a tiny fraction of the transmitter's output, since communication depends on receiving the information, not the energy, that was transmitted.

Classical radio communications systems use frequency-division multiplexing (FDM) as a strategy to split up and share the available radio-frequency bandwidth for use by different parties communications concurrently. Modern radio communication systems include those that divide up a radio-frequency band by time-division multiplexing (TDM) and code-division multiplexing (CDM) as alternatives to the classical FDM strategy. These systems offer different tradeoffs in supporting multiple users, beyond the FDM strategy that was ideal for broadcast radio but less so for applications such as mobile telephony.

A radio communication system may send information only one way. For example, in broadcasting a single transmitter sends signals to many receivers. Two stations may take turns sending and receiving, using a single radio frequency; this is called "simplex." By using two radio frequencies, two stations may continuously and concurrently send and receive signals - this is called "duplex" operation.

History[link]

19th century[link]

The meaning and usage of the word "radio" has developed in parallel with developments within the field of communications and can be seen to have three distinct phases: electromagnetic waves and experimentation; wireless communication and technical development; and radio broadcasting and commercialization. Many individuals—inventors, engineers, developers, businessmen - contributed to produce the modern idea of radio and thus the origins and 'invention' are multiple and controversial. Early radio designs could not transmit sound or speech and were called the "wireless telegraph."

Development from a laboratory demonstration to a commercial entity spanned several decades and required the efforts of many practitioners. In 1878, David E. Hughes noticed that sparks could be heard in a telephone receiver when experimenting with his carbon microphone. He developed this carbon-based detector further and eventually could detect signals over a few hundred yards. He demonstrated his discovery to the Royal Society in 1880, but was told it was merely induction, and therefore abandoned further research.

Experiments were undertaken by Thomas Edison and his employees of Menlo Park. Edison applied in 1885 to the U.S. Patent Office for a patent on an electrostatic coupling system between elevated terminals. The patent was granted as U.S. Patent 465,971 on December 29, 1891. The Marconi Company would later purchase rights to the Edison patent to protect them legally from lawsuits.^[7]

Tesla demonstrating wireless transmissions during his high frequency and potential lecture of 1891. After continued research, Tesla presented the fundamentals of radio in 1893.

In 1893, in St. Louis, Missouri, Nikola Tesla made devices for his experiments with electricity. Addressing the Franklin Institute in Philadelphia and the National Electric Light Association, he described and demonstrated the principles of his wireless work.^[8] The descriptions contained all the elements that were later incorporated into radio systems before the development of the vacuum tube. He initially experimented with magnetic receivers, unlike the coherers (detecting devices consisting of tubes filled with iron filings which had been invented by Temistocle Calzecchi-Onesti at Fermo in Italy in 1884) used by Guglielmo Marconi and other early experimenters.^[9]

A demonstration of wireless telegraphy took place in the lecture theater of the Oxford University Museum of Natural History on August 14, 1894, carried out by Professor Oliver Lodge and Alexander Muirhead. During the demonstration a radio signal was sent from the neighboring Clarendon laboratory building, and received by apparatus in the lecture theater.

In 1895 Alexander Stepanovich Popov built his first radio receiver, which contained a coherer. Further refined as a lightning detector, it was presented to the Russian Physical and Chemical Society on May 7, 1895. A depiction of Popov's lightning detector was printed in the Journal of the Russian Physical and Chemical Society the same year. Until recently, mistakenly believed that it was the first description (publication of the minutes 15/201 of this session — December issue of the journal RPCS^[10]), but in fact the first description of the device gave Dmitry Aleksandrovich Lachinov in July 1895 in the 2nd edition of his course "Fundamentals of Meteorology and climatology" — the first in Russia.^[11][12] Popov's receiver was created on the improved basis of Lodge's receiver, and originally intended for reproduction of its experiments.

In 1895, Marconi built a wireless system capable of transmitting signals at long distances (1.5 mi./ 2.4 km).^[13][14] In radio transmission technology, early public experimenters had made short distance broadcasts.^[15] Marconi achieved long range signalling due to a wireless transmitting apparatus and a radio receiver claimed by him.^[16][17] From Marconi's experiments, the phenomenon that transmission range is proportional to the square of antenna height is known as "Marconi's law."^[18]

Marconi's experimental apparatus proved to be a complete, commercially successful radio transmission system.^[17][19][20] According to the Proceedings of the United States Naval Institute in 1899, the Marconi instruments had a "[...] coherer, principle of which was discovered some twenty years ago, [and was] the only electrical instrument or device contained in the apparatus that is at all new."^[21]

Telephone Herald in Budapest, Hungary (1901).

In 1896, Marconi was awarded British patent 12039, Improvements in transmitting electrical impulses and signals and in apparatus there-for, for radio. In 1897, he established a radio station on the Isle of Wight, England. Marconi opened his "wireless" factory in Hall Street, Chelmsford, England in 1898, employing around 60 people. Shortly after the 1900s, Marconi held the patent rights for radio.

20th century[link]

In 1900, Brazilian priest Roberto Landell de Moura transmitted the human voice by wireless. According the newspaper Jornal do Comercio (June 10, 1900), he conducted his first public experiment on June 3, 1900, in front of journalists and the General Consul of Great Britain, C.P. Lupton, in São Paulo, Brazil, for a distance of approximately 8 km. The points of transmission and reception were Alto de Santana and Paulista Avenue.^[22]

One year after that experiment, he received his first patent from the Brazilian government. It was described as "equipment for the purpose of phonetic transmissions through space, land and water elements at a distance with or without the use of wires." Four months later, knowing that his invention had real value, he left Brazil for the United States of America with the intent of patenting the machine at the US Patent Office in Washington, DC.

Having few resources, he had to rely on friends to push his project. In spite of great difficulty, three patents were awarded: "The Wave Transmitter" (October 11, 1904) which is the precursor of today's radio transceiver; "The Wireless Telephone" and the "Wireless Telegraph," both dated November 22, 1904.

"The Wireless Telephone" U S Patent Office in Washington, DC

The next advancement was the vacuum tube detector, invented by Westinghouse engineers. On Christmas Eve 1906, Reginald Fessenden used a synchronous rotary-spark transmitter for the first radio program broadcast, from Ocean Bluff-Brant Rock, Massachusetts. Ships at sea heard a broadcast that included Fessenden playing O Holy Night on the violin and reading a passage from the Bible.^[23]

This was, for all intents and purposes, the first transmission of what is now known as amplitude modulation or AM radio. The first radio news program was broadcast August 31, 1920 by station 8MK in Detroit, Michigan, which survives today as all-news format station WWJ under ownership of the CBS network. The first college radio station began broadcasting on October 14, 1920 from Union College, Schenectady, New York under the personal call letters of Wendell King, an African-American student at the school.^[23]

That month 2ADD (renamed WRUC in 1947), aired what is believed to be the first public entertainment broadcast in the United States, a series of Thursday night concerts initially heard within a 100-mile (160 km) radius and later for a 1,000-mile (1,600 km) radius. In November 1920, it aired the first broadcast of a sporting event.^[23][24] At 9 pm on August 27, 1920, Sociedad Radio Argentina aired a live performance of Richard Wagner's opera Parsifal from the Coliseo Theater in downtown Buenos Aires. Only about twenty homes in the city had receivers to tune in this radio program. Meanwhile, regular entertainment broadcasts commenced in 1922 from the Marconi Research Centre at Writtle, England.

Sports broadcasting began at this time as well, including the college football on radio broadcast of a 1921 West Virginia vs. Pittsburgh football game.^[25]

In 1943, the United States Supreme Court invalidated one of the Marconi patents, number 763,772 (1904), on the basis it had been anticipated by Tesla, Lodge, and others. After years of patent battles by Marconi's company, the United States Supreme Court, in the 1943 case of Marconi Wireless Telegraph co. of America v. United States, 320 U.S. 1 (1943), said that "it is now held that in the important advance upon his basic patent Marconi did nothing that had not already been seen and disclosed."^[26][27][28]

Although Marconi claimed that he had no knowledge of prior art from Tesla's patents, the Supreme Court considered his claim false.^[29] In addition to that ruling from the Supreme Court, the United States Court of Claims invalidated the fundamental 1935 Marconi patent.^[30] This case defined radio by the statement: "A radio communication system requires two tuned circuits each at the transmitter and receiver, all four tuned to the same frequency."^[31] The court determined that Tesla's patent clearly was the first to disclose a system which could be used for wireless communication of intelligible messages (such as human voice and music) and used the four-circuit tuned combination.^[32]

An American girl listens to a radio during the Great Depression.

In contrast, related developments in the United Kingdom saw the High Court of Justice uphold Marconi's British Patent, issued on April 26, 1900. This patent disclosed a four-circuit system, which was strikingly similar to a four-circuit system disclosed in U.S. patent #645,576 that was issued earlier to Tesla on March 20, 1900. On the matter of invention, it is held that Marconi knowingly and unknowingly used the scientific and experimental work of others who were devising their own radio tuning apparatus' around the same time, such as the work of American electrical engineer John Stone Stone who was issued several U.S. patents between 1904 and 1908. However, what made Marconi more successful than any other was his ability to commercialize radio and its associated equipment into a global business.^{[33][dubious – discuss]}

One of the first developments in the early 20th century was that aircraft used commercial AM radio stations for navigation. This continued until the early 1960s when VOR systems became widespread.^[34] In the early 1930s, single sideband and frequency modulation were invented by amateur radio operators. By the end of the decade, they were established commercial modes. Radio was used to transmit pictures visible as television as early as the 1920s. Commercial television transmissions started in North America and Europe in the 1940s.

The Regency TR-1 which used Texas Instruments' NPN transistors was the world's first commercially produced transistor radio.

In 1954, the Regency company introduced a pocket transistor radio, the TR-1, powered by a "standard 22.5 V Battery." In 1955, the newly formed Sony company introduced its first transistorized radio.^[35] It was small enough to fit in a vest pocket, powered by a small battery. It was durable, because it had no vacuum tubes to burn out. Over the next 20 years, transistors replaced tubes almost completely except for high-power transmitters.

By 1963, color television was being broadcast commercially (though not all broadcasts or programs were in color), and the first (radio) communication satellite, Telstar, was launched. In the late 1960s, the U.S. long-distance telephone network began to convert to a digital network, employing digital radios for many of its links. In the 1970s, LORAN became the premier radio navigation system.

Soon, the U.S. Navy experimented with satellite navigation, culminating in the launch of the Global Positioning System (GPS) constellation in 1987. In the early 1990s, amateur radio experimenters began to use personal computers with audio cards to process radio signals. In 1994, the U.S. Army and DARPA launched an aggressive, successful project to construct a software-defined radio that can be programmed to be virtually any radio by changing its software program. Digital transmissions began to be applied to broadcasting in the late 1990s. 2012 saw the first transmission of the Passover classic "Matzah" on Harvard's WHRB station.

Uses of radio[link]

Early uses were maritime, for sending telegraphic messages using Morse code between ships and land. The earliest users included the Japanese Navy scouting the Russian fleet during the Battle of Tsushima in 1905. One of the most memorable uses of marine telegraphy was during the sinking of the RMS Titanic in 1912, including communications between operators on the sinking ship and nearby vessels, and communications to shore stations listing the survivors.

Radio was used to pass on orders and communications between armies and navies on both sides in World War I; Germany used radio communications for diplomatic messages once it discovered that its submarine cables had been tapped by the British. The United States passed on President Woodrow Wilson's Fourteen Points to Germany via radio during the war. Broadcasting began from San Jose, California in 1909,^[36] and became feasible in the 1920s, with the widespread introduction of radio receivers, particularly in Europe and the United States. Besides broadcasting, point-to-point broadcasting, including telephone messages and relays of radio programs, became widespread in the 1920s and 1930s. Another use of radio in the pre-war years was the development of detection and locating of aircraft and ships by the use of radar (RAdio Detection And Ranging).

Today, radio takes many forms, including wireless networks and mobile communications of all types, as well as radio broadcasting. Before the advent of television, commercial radio broadcasts included not only news and music, but dramas, comedies, variety shows, and many other forms of entertainment (the era from the late 1920s to the mid-1950s is commonly called radio's "Golden Age"). Radio was unique among methods of dramatic presentation in that it used only sound. For more, see radio programming.

Audio[link]

A Fisher 500 AM/FM hi-fi receiver from 1959.

AM radio uses amplitude modulation, in which the amplitude of the transmitted signal is made proportional to the sound amplitude captured (transduced) by the microphone, while the transmitted frequency remains unchanged. Transmissions are affected by static and interference because lightning and other sources of radio emissions on the same frequency add their amplitudes to the original transmitted amplitude.

In the early part of the 20th century, American AM radio stations broadcast with powers as high as 500 kW, and some could be heard worldwide; these stations' transmitters were commandeered for military use by the US Government during World War II. Currently, the maximum broadcast power for a civilian AM radio station in the United States and Canada is 50 kW, and the majority of stations that emit signals this powerful were grandfathered in (see List of 50 kW AM radio stations in the United States). In 1986 KTNN received the last granted 50,000 watt license. These 50 kW stations are generally called "clear channel" stations (not to be confused with Clear Channel Communications), because within North America each of these stations has exclusive use of its broadcast frequency throughout part or all of the broadcast day.

Bush House, home of the BBC World Service.

FM broadcast radio sends music and voice with higher fidelity than AM radio. In frequency modulation, amplitude variation at the microphone causes the transmitter frequency to fluctuate. Because the audio signal modulates the frequency and not the amplitude, an FM signal is not subject to static and interference in the same way as AM signals. Due to its need for a wider bandwidth, FM is transmitted in the Very High Frequency (VHF, 30 MHz to 300 MHz) radio spectrum.

VHF radio waves act more like light, traveling in straight lines; hence the reception range is generally limited to about 50–200 miles. During unusual upper atmospheric conditions, FM signals are occasionally reflected back towards the Earth by the ionosphere, resulting in long distance FM reception. FM receivers are subject to the capture effect, which causes the radio to only receive the strongest signal when multiple signals appear on the same frequency. FM receivers are relatively immune to lightning and spark interference.

High power is useful in penetrating buildings, diffracting around hills, and refracting in the dense atmosphere near the horizon for some distance beyond the horizon. Consequently, 100,000 watt FM stations can regularly be heard up to 100 miles (160 km) away, and farther (e.g., 150 miles, 240 km) if there are no competing signals.

A few old, "grandfathered" stations do not conform to these power rules. WBCT-FM (93.7) in Grand Rapids, Michigan, US, runs 320,000 watts ERP, and can increase to 500,000 watts ERP by the terms of its original license. Such a huge power level does not usually help to increase range as much as one might expect, because VHF frequencies travel in nearly straight lines over the horizon and off into space. Nevertheless, when there were fewer FM stations competing, this station could be heard near Bloomington, Illinois, US, almost 300 miles (500 km) away.^{[citation needed]}

FM subcarrier services are secondary signals transmitted in a "piggyback" fashion along with the main program. Special receivers are required to utilize these services. Analog channels may contain alternative programming, such as reading services for the blind, background music or stereo sound signals. In some extremely crowded metropolitan areas, the sub-channel program might be an alternate foreign-language radio program for various ethnic groups. Sub-carriers can also transmit digital data, such as station identification, the current song's name, web addresses, or stock quotes. In some countries, FM radios automatically re-tune themselves to the same channel in a different district by using sub-bands.

Aviation voice radios use VHF AM. AM is used so that multiple stations on the same channel can be received. (Use of FM would result in stronger stations blocking out reception of weaker stations due to FM's capture effect). Aircraft fly high enough that their transmitters can be received hundreds of miles (or kilometres) away, even though they are using VHF.

Degen DE1103, an advanced world mini-receiver with single sideband modulation and dual conversion

Marine voice radios can use single sideband voice (SSB) in the shortwave High Frequency (HF—3 MHz to 30 MHz) radio spectrum for very long ranges or narrowband FM in the VHF spectrum for much shorter ranges. Narrowband FM sacrifices fidelity to make more channels available within the radio spectrum, by using a smaller range of radio frequencies, usually with five kHz of deviation, versus the 75 kHz used by commercial FM broadcasts, and 25 kHz used for TV sound.

Government, police, fire and commercial voice services also use narrowband FM on special frequencies. Early police radios used AM receivers to receive one-way dispatches.

Civil and military HF (high frequency) voice services use shortwave radio to contact ships at sea, aircraft and isolated settlements. Most use single sideband voice (SSB), which uses less bandwidth than AM. On an AM radio SSB sounds like ducks quacking, or the adults in a Charlie Brown cartoon. Viewed as a graph of frequency versus power, an AM signal shows power where the frequencies of the voice add and subtract with the main radio frequency. SSB cuts the bandwidth in half by suppressing the carrier and one of the sidebands. This also makes the transmitter about three times more powerful, because it doesn't need to transmit the unused carrier and sideband.

TETRA, Terrestrial Trunked Radio is a digital cell phone system for military, police and ambulances. Commercial services such as XM, WorldSpace and Sirius offer encrypted digital Satellite radio.

Telephony[link]

Mobile phones transmit to a local cell site (transmitter/receiver) that ultimately connects to the public switched telephone network (PSTN) through an optic fiber or microwave radio and other network elements. When the mobile phone nears the edge of the cell site's radio coverage area, the central computer switches the phone to a new cell. Cell phones originally used FM, but now most use various digital modulation schemes. Recent developments in Sweden (such as DROPme) allow for the instant downloading of digital material from a radio broadcast (such as a song) to a mobile phone.

Satellite phones use satellites rather than cell towers to communicate.

Video[link]

Television sends the picture as AM and the sound as AM or FM, with the sound carrier a fixed frequency (4.5 MHz in the NTSC system) away from the video carrier. Analog television also uses a vestigial sideband on the video carrier to reduce the bandwidth required.

Digital television uses 8VSB modulation in North America (under the ATSC digital television standard), and COFDM modulation elsewhere in the world (using the DVB-T standard). A Reed–Solomon error correction code adds redundant correction codes and allows reliable reception during moderate data loss. Although many current and future codecs can be sent in the MPEG transport stream container format, as of 2006 most systems use a standard-definition format almost identical to DVD: MPEG-2 video in Anamorphic widescreen and MPEG layer 2 (MP2) audio. High-definition television is possible simply by using a higher-resolution picture, but H.264/AVC is being considered as a replacement video codec in some regions for its improved compression. With the compression and improved modulation involved, a single "channel" can contain a high-definition program and several standard-definition programs.

Navigation[link]

All satellite navigation systems use satellites with precision clocks. The satellite transmits its position, and the time of the transmission. The receiver listens to four satellites, and can figure its position as being on a line that is tangent to a spherical shell around each satellite, determined by the time-of-flight of the radio signals from the satellite. A computer in the receiver does the math.

Radio direction-finding is the oldest form of radio navigation. Before 1960 navigators used movable loop antennas to locate commercial AM stations near cities. In some cases they used marine radiolocation beacons, which share a range of frequencies just above AM radio with amateur radio operators. LORAN systems also used time-of-flight radio signals, but from radio stations on the ground.

VOR (Very High Frequency Omnidirectional Range), systems (used by aircraft), have an antenna array that transmits two signals simultaneously. A directional signal rotates like a lighthouse at a fixed rate. When the directional signal is facing north, an omnidirectional signal pulses. By measuring the difference in phase of these two signals, an aircraft can determine its bearing or radial from the station, thus establishing a line of position. An aircraft can get readings from two VORs and locate its position at the intersection of the two radials, known as a "fix."

When the VOR station is collocated with DME (Distance Measuring Equipment), the aircraft can determine its bearing and range from the station, thus providing a fix from only one ground station. Such stations are called VOR/DMEs. The military operates a similar system of navaids, called TACANs, which are often built into VOR stations. Such stations are called VORTACs. Because TACANs include distance measuring equipment, VOR/DME and VORTAC stations are identical in navigation potential to civil aircraft.

Radar[link]

Radar (Radio Detection And Ranging) detects objects at a distance by bouncing radio waves off them. The delay caused by the echo measures the distance. The direction of the beam determines the direction of the reflection. The polarization and frequency of the return can sense the type of surface. Navigational radars scan a wide area two to four times per minute. They use very short waves that reflect from earth and stone. They are common on commercial ships and long-distance commercial aircraft.

General purpose radars generally use navigational radar frequencies, but modulate and polarize the pulse so the receiver can determine the type of surface of the reflector. The best general-purpose radars distinguish the rain of heavy storms, as well as land and vehicles. Some can superimpose sonar data and map data from GPS position.

Search radars scan a wide area with pulses of short radio waves. They usually scan the area two to four times a minute. Sometimes search radars use the Doppler effect to separate moving vehicles from clutter. Targeting radars use the same principle as search radar but scan a much smaller area far more often, usually several times a second or more. Weather radars resemble search radars, but use radio waves with circular polarization and a wavelength to reflect from water droplets. Some weather radar use the Doppler effect to measure wind speeds.

Data (digital radio)[link]

2008 Pure One Classic digital radio

Most new radio systems are digital, see also: Digital TV, Satellite Radio, Digital Audio Broadcasting. The oldest form of digital broadcast was spark gap telegraphy, used by pioneers such as Marconi. By pressing the key, the operator could send messages in Morse code by energizing a rotating commutating spark gap. The rotating commutator produced a tone in the receiver, where a simple spark gap would produce a hiss, indistinguishable from static. Spark-gap transmitters are now illegal, because their transmissions span several hundred megahertz. This is very wasteful of both radio frequencies and power.

The next advance was continuous wave telegraphy, or CW (Continuous Wave), in which a pure radio frequency, produced by a vacuum tube electronic oscillator was switched on and off by a key. A receiver with a local oscillator would "heterodyne" with the pure radio frequency, creating a whistle-like audio tone. CW uses less than 100 Hz of bandwidth. CW is still used, these days primarily by amateur radio operators (hams). Strictly, on-off keying of a carrier should be known as "Interrupted Continuous Wave" or ICW or on-off keying (OOK).

Radioteletype equipment usually operates on short-wave (HF) and is much loved by the military because they create written information without a skilled operator. They send a bit as one of two tones using frequency-shift keying. Groups of five or seven bits become a character printed by a teleprinter. From about 1925 to 1975, radioteletype was how most commercial messages were sent to less developed countries. These are still used by the military and weather services.

Aircraft use a 1200 Baud radioteletype service over VHF to send their ID, altitude and position, and get gate and connecting-flight data. Microwave dishes on satellites, telephone exchanges and TV stations usually use quadrature amplitude modulation (QAM). QAM sends data by changing both the phase and the amplitude of the radio signal. Engineers like QAM because it packs the most bits into a radio signal when given an exclusive (non-shared) fixed narrowband frequency range. Usually the bits are sent in "frames" that repeat. A special bit pattern is used to locate the beginning of a frame.

Modern GPS receivers.

Communication systems that limit themselves to a fixed narrowband frequency range are vulnerable to jamming. A variety of jamming-resistant spread spectrum techniques were initially developed for military use, most famously for Global Positioning System satellite transmissions. Commercial use of spread spectrum began in the 1980s. Bluetooth, most cell phones, and the 802.11b version of Wi-Fi each use various forms of spread spectrum.

Systems that need reliability, or that share their frequency with other services, may use "coded orthogonal frequency-division multiplexing" or COFDM. COFDM breaks a digital signal into as many as several hundred slower subchannels. The digital signal is often sent as QAM on the subchannels. Modern COFDM systems use a small computer to make and decode the signal with digital signal processing, which is more flexible and far less expensive than older systems that implemented separate electronic channels.

COFDM resists fading and ghosting because the narrow-channel QAM signals can be sent slowly. An adaptive system, or one that sends error-correction codes can also resist interference, because most interference can affect only a few of the QAM channels. COFDM is used for Wi-Fi, some cell phones, Digital Radio Mondiale, Eureka 147, and many other local area network, digital TV and radio standards.

Heating[link]

Radio-frequency energy generated for heating of objects is generally not intended to radiate outside of the generating equipment, to prevent interference with other radio signals. Microwave ovens use intense radio waves to heat food. Diathermy equipment is used in surgery for sealing of blood vessels. Induction furnaces are used for melting metal for casting, and induction hobs for cooking.

Amateur radio service[link]

Amateur radio station with multiple receivers and transceivers

Amateur radio, also known as "ham radio," is a hobby in which enthusiasts are licensed to communicate on a number of bands in the radio frequency spectrum non-commercially and for their own enjoyment. They may also provide emergency and public service assistance. This has been very beneficial in emergencies, saving lives in many instances.^[37]

Radio amateurs use a variety of modes, including nostalgic ones like Morse code and experimental ones like Low-Frequency Experimental Radio. Several forms of radio were pioneered by radio amateurs and later became commercially important, including FM, single-sideband (SSB), AM, digital packet radio and satellite repeaters. Some amateur frequencies may be disrupted illegally by power-line internet service.

Unlicensed radio services[link]

Unlicensed, government-authorized personal radio services such as Citizens' band radio in Australia, the US, and Europe, and Family Radio Service and Multi-Use Radio Service in North America exist to provide simple, (usually) short range communication for individuals and small groups, without the overhead of licensing. Similar services exist in other parts of the world. These radio services involve the use of handheld units.

Free radio stations, sometimes called pirate radio or "clandestine" stations, are unauthorized, unlicensed, illegal broadcasting stations. These are often low power transmitters operated on sporadic schedules by hobbyists, community activists, or political and cultural dissidents. Some pirate stations operating offshore in parts of Europe and the United Kingdom more closely resembled legal stations, maintaining regular schedules, using high power, and selling commercial advertising time.^[38][39]

Radio control (RC)[link]

Radio remote controls use radio waves to transmit control data to a remote object as in some early forms of guided missile, some early TV remotes and a range of model boats, cars and airplanes. Large industrial remote-controlled equipment such as cranes and switching locomotives now usually use digital radio techniques to ensure safety and reliability.

In Madison Square Garden, at the Electrical Exhibition of 1898, Nikola Tesla successfully demonstrated a radio-controlled boat.^[40] He was awarded U.S. patent No. 613,809 for a "Method of and Apparatus for Controlling Mechanism of Moving Vessels or Vehicles."^[41]

See also[link]

Radio portal

References[link]

General information

• A História da Rádio em Datas (1819-1997) (in Portuguese) - notes on etymology
• L. de Forest, article in Electrical World 22 June 1270/1 (1907), early use of word "radio."
• http://web.mit.edu/varun_ag/www/bose.html - It contains a proof that Sir Jagadish Chandra Bose invented the Mercury Coherer which was later used by Guglielmo Marconi and along with other patents.
• Cheney, Margaret (1981). Tesla - Man Out of Time. New York: Simon & Schuster. ISBN 978-0-7432-1536-7. 

Footnotes

Further reading[link]

• Sewall, C. H. (1904). Wireless telegraphy: its origins, development, inventions, and apparatus. New York: D. Van Nostrand.
• Mills, J. (1917). Radio communication, theory and methods, with an appendix on transmission over wires. New York: McGraw-Hill book company [etc., etc.].
• Lauer, H., & Brown, H. L. (1920). Radio engineering principles. New York: McGraw-Hill book company; [etc., etc.].
• Cockaday, L. M. (1922). Radio-telephony for everyone; the wireless: how to construct and maintain modern transmitting and receiving apparatus. New York: Frederick A. Stokes.
• Hausmann, E., Goldsmith, A. N., Hazeltine, L. A., Hogan, J. V. L., Morecroft, J. H., Canavaciol, F. E., et al. (1922). Radio phone receiving; a practical book for everybody. New York: D. Van Nostrand.
• Buga, N.; Falko A., Chistyakov N.I. (1990) Chistyakov N.I. ed. Radio Receiver Theory Translated from the Russian by Boris V. Kuznetsov Moscow: Mir Publishers ISBN 5-03-001321-0  First published in Russian as «Радиоприёмные устройства» 
• Da Silva, E. (2001). High frequency and microwave engineering. Oxford: Butterworth-Heinemann.
• Clint Smith, Curt Gervelis (2003). Wireless Network Performance Handbook. McGraw-Hill Professional. ISBN 0-07-140655-7. http://books.google.com/books?id=nZAVGBoPevUC&pg=PA25&lr=&as_brr=3&ei=F4-sSKuzO6XmtgO4393BBA&sig=ACfU3U0g0vtYCOzP0LCCzdRfr7bgKwvTNg#PPA24,M1. 
• Hugh G. J. Aitkin: The Continuous Wave: Technology and the American Radio, 1900-1932 (Princeton University Press, 1985).
• Asa Briggs: The History of Broadcasting in the United Kingdom (Oxford University Press, 1961).
• John Dunning: On the Air. The Encyclopedia of Old-Time Radio. New York; Oxford: Oxford University Press, 1998. ISBN 0-19-507678-8
• Henry Ewbank and Sherman P. Lawton: Broadcasting: Radio and Television (Harper & Brothers, 1952).
• Marc Fisher: Something In The Air: Radio, Rock, and the Revolution That Shaped A Generation (Random House, 2007).
• Leland I. Anderson (ed.), "John Stone Stone, Nikola Tesla's Priority in Radio and Continuous-Wave Radiofrequency Apparatus." The AWA Review, Vol. 1. 1986. 24 pages, illustrated.
• Tom Lewis: Empire of the Air: The Men Who Made Radio, 1st ed., New York : E. Burlingame Books, 1991. ISBN 0-06-018215-6. "Empire of the Air: The Men Who Made Radio" (1992) by Ken Burns was a PBS documentary based on the book.
• W. Rupert Maclaurin: Invention and Innovation in the Radio Industry (The Macmillan Company, 1949).
• William B. Ray: FCC: The Ups and Downs of Radio-TV Regulation (Iowa State University Press, 1990).
• Alexander Russo: Points on the Dial: Golden Age Radio Beyond the Networks (Duke University Press; 2010) 278 pages; discusses regional and local radio as forms that "complicate" the image of the medium as a national unifier from the 1920s to the 1950s.
• Scannell, Paddy, and Cardiff, David. A Social History of British Broadcasting, Volume One, 1922-1939 (Basil Blackwell, 1991).
• Schwoch James. The American Radio Industry and Its Latin American Activities, 1900-1939 (University of Illinois Press, 1990).
• Christopher H. Sterling with Michael C. Keith (ed.): Encyclopedia of Radio. New York; London: Fitzroy Dearborn, 2004 (three vols.)
• Llewellyn White: The American Radio (University of Chicago Press, 1947).
• Ulrich L. Rohde, Jerry Whitaker: Communications Receivers, Third Edition, McGraw Hill, New York, NY, 2001, ISBN 0-07-136121-9.

External links[link]

Wikimedia Commons has media related to: Radio

Look up radio in Wiktionary, the free dictionary.

General

• Radio at the Open Directory Project
• "It's Radi-O! Essay by Richard Rubin, The Atlantic Monthly, January 1998.

History

• U.S. Supreme Court, "Marconi Wireless Telegraph co. of America v. United States." 320 U.S. 1. Nos. 369, 373. Argued 9–12 April 1943. Decided 21 June 1943.
• "Who Invented Radio?" Buzzle.com Date unknown. retrieved 20 January 2011.
• Steven Schoenherr's History of Radio
• The Broadcast Archive - Radio History on the Web! Retrieved 20 January 2011.
• Canadian Communications Foundation - The History on Canadian Broadcasting - 1920 onward. Retrieved 20 January 2011.
• United States Early Radio History -1897 to 1927. Retrieved 20 January 2011.
• Historic Radios from Around the World at Kurrajong Radio Museum, Australia - a private collection. Retrieved 20 January 2011.

Antiques

• George H. Clark Radioana Collection, ca. 1880 - 1950 - Archives Center, National Museum of American History, Smithsonian Institution
• A gallery of Antiques from the 1920s to the 1960s

Technical

• Radio Frequency Chart National Telecommunications and Information Administration (NTIA).
• IAteacher: Interactive Explanation of Radio Receiver Construction
• How Stuff Works - Radio
• VOR Basic Information
• Dr. Phil's Receiver Designs Single-Triode and Single-Transistor Regenerative Radio Designs
• How to design a traditional radio by Natalian Zhai, Silicon Labs

DX

• The British DX Club
• World of Radio Glenn Hauser's internationally known DX radio show

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