NEW VERSION in one piece instead of multiple parts, and with improved video & sound: https://www.youtube.com/watch?v=xn6lzrMJUDs more at http://scitech.quickfound.net/ FUNDAMENTALS OF AM AND FM RADIO COMMUNICATION, HOW FM ELIMINATES PROBLEM OF ELECTRICAL INTERFERENCE, FUNCTIONING OF FM RADIO EXAMINED IN DETAIL. US Army Training Film TF11-3482 Public domain film from the National Archives, slightly cropped to remove uneven edges, with the aspect ratio corrected, and mild video noise reduction applied. The soundtrack was also processed with volume normalization, noise reduction, clipping reduction, and equalization (the resulting sound, though not perfect, is far less noisy than the original). part 2: http://www.youtube.com/watch?v=JS8xlwyc6G4 Also see TF11-2069 "Basic Principles of Frequency Modulation" (1944) http://www.youtube.com/watch?v=jXp3k0FhMIw http://en.wikipedia.org/wiki/Frequency_modulation In telecommunications and signal processing, frequency modulation (FM) conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant. In analog applications, the difference between the instantaneous and the base frequency of the carrier is directly proportional to the instantaneous value of the input-signal amplitude. Digital data can be sent by shifting the carrier's frequency among a range of settings, a technique known as frequency-shift keying. FSK (digital FM) is widely used in data and fax modems. Morse code transmission has been sent this way, and FASK was used in early telephone-line modems. Radioteletype also uses FSK. FM modulation is also used in telemetry, radar, seismic prospecting and newborn EEG seizure monitoring. Frequency modulation is known as phase modulation when the carrier phase modulation is the time integral of the FM signal. FM is widely used for broadcasting music and speech, two-way radio systems, magnetic tape-recording systems and some video-transmission systems. In radio systems, frequency modulation with sufficient bandwidth provides an advantage in cancelling naturally-occurring noise. Radio Edwin Howard Armstrong (1890--1954) was an American electrical engineer who invented wideband frequency modulation (FM) radio. He patented the regenerative circuit in 1914, the superheterodyne receiver in 1918 and the super-regenerative circuit in 1922. Armstrong presented his paper, "A Method of Reducing Disturbances in Radio Signaling by a System of Frequency Modulation", (which first described FM radio) before the New York section of the Institute of Radio Engineers on November 6, 1935. The paper was published in 1936. As the name implies, wideband FM (WFM) requires a wider signal bandwidth than amplitude modulation by an equivalent modulating signal; this also makes the signal more robust against noise and interference. Frequency modulation is also more robust against signal-amplitude-fading phenomena. As a result, FM was chosen as the modulation standard for high frequency, high fidelity radio transmission, hence the term "FM radio" (although for many years the BBC called it "VHF radio" because commercial FM broadcasting uses part of the VHF band—the FM broadcast band). FM receivers employ a special detector for FM signals and exhibit a phenomenon known as the capture effect, in which the tuner "captures" the stronger of two stations on the same frequency while rejecting the other (compare this with a similar situation on an AM receiver, where both stations can be heard simultaneously). However, frequency drift or a lack of selectivity may cause one station to be overtaken by another on an adjacent channel. Frequency drift was a problem in early (or inexpensive) receivers; inadequate selectivity may affect any tuner. An FM signal can also be used to carry a stereo signal; this is done with multiplexing and demultiplexing before and after the FM process. The FM modulation and demodulation process is identical in stereo and monaural processes. A high-efficiency radio-frequency switching amplifier can be used to transmit FM signals (and other constant-amplitude signals). For a given signal strength (measured at the receiver antenna), switching amplifiers use less battery power and typically cost less than a linear amplifier. This gives FM another advantage over other modulation methods requiring linear amplifiers, such as AM and QAM. FM is commonly used at VHF radio frequencies for high-fidelity broadcasts of music and speech. Normal (analog) TV sound is also broadcast using FM. Narrowband FM is used for voice communications in commercial and amateur radio settings. In broadcast services, where audio fidelity is important, wideband FM is generally used...

• published: 28 Apr 2012
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