- published: 15 Jan 2014
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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, 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. 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 are broadcasting with this technology, and more than 1,000 HD2 and HD3 multicast channels are on the air. 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.
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. 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.
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").