FM broadcasting
From Biocrawler, the free encyclopedia.
FM radio is a broadcast technology invented by Edwin Howard Armstrong that uses frequency modulation to provide high-fidelity sound over broadcast radio. FM radio is distributed primarily through broadcast reception of FM radio signals, although it is also possible to distribute FM signals via cable FM, either by using an adapter to plug analogue cable wires directly into an FM receiver, or through the use of television channel allocations on a digital cable service.
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FM stereo technology
New technology was added to FM radio in the early 1960s to allow FM stereo transmissions, where the frequency modulated radio signal is used to carry stereophonic sound, using the pilot-tone multiplex system.
This multiplexes the left and right audio signal channels in a manner that is compatible with mono sound, using a sum-and-difference technique to produce a single "mono-compatible" composite signal, which has a baseband part that is equal to the sum of the left and right channels (L+R), and a higher-frequency part that is the difference of the left and right channels (L-R) amplitude modulated on a 38 kHz suppressed subcarrier (producing a double sideband suppressed carrier signal). A 19 kHz pilot tone is then added. This 19kHz tone is used by the receiver to re-create the original 38kHz subcarrier to allow the detection of the (L-R) audio signal.
This composite signal can then be passed through the FM modulation and demodulation process as if it was a monophonic signal, and the stereo signals extracted from the demodulated FM signal by reversing the multiplexing process.
Simple mono FM receivers will not extract the left and right signals, but simply reproduce the baseband part of the "mono-compatible" signal. (This relies on the fact that the subcarrier-modulated part of the mono-compatible signal is in a part of the audio spectrum that is inaudible to people, and the pilot tone is a low-intensity tone in a part of the audio spectrum that is inaudible to most people).
This backwards compatibility was important, as when the FM stereo system was introduced in the U.S. in the 1960s, mono FM transmissions had been in service since the 1940s, and there was a large installed base of mono receivers that needed to be able to receive stereo broadcasts without any modification.
Stereo receivers could automatically switch between "mono" and "stereo" modes based on the presence of the pilot tone. They were also equipped with a notch filter to remove the pilot tone. In poor signal conditions, stereo receivers could also fall back to mono mode, even on a stereo signal, allowing improved signal-to-noise performance in these conditions. The noisy quality of stereo signals compared to mono signals can be attributed to the FCC's 1961 selection of the AM-subcarrier GE/Zenith standard over the high-fidelity FM-subcarrier Crosby system.
Radio Data System
The stereo multiplexing system has been further extended to add an extra higher-frequency 57 kHz subcarrier. This subcarrier is used to carry a low-bandwidth digital Radio Data System signal, providing extra features such as Alternate Frequency (AF) and Network (NN).
See also
- FM_broadcasting in the USA
- FM band
- AM broadcasting
- AM stereo
- List of broadcast station classes
- History of radio
- List of radio stations
- RDS (Radio Data System)
- Long distance FM reception (FM DX)
External links
- U.S. Patent 1941066 (http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=/netahtml/srchnum.htm&r=1&f=G&l=50&s1=1941066.WKU.&OS=PN/1941066&RS=PN/1941066)
- An Introduction to FM MPX (http://www.smoke.com.au/~ic/mpx.html)
- Some history of the FM multiplex system (http://www.airwaves.com/archive3/v9_76.html) (search down the page for "pilot-tone multiplex system")
- "Table of Voltage, Frequency, TV Broadcasting system, Radio Broadcasting, by Country (http://www.salestores1.com/woreltab.html)".
- Stereo for Dummies (http://transmitters.tripod.com/stereo.htm) Many graphs that show waveforms at different points in the FM Multiplex process
- Listen to Live FM Radio (http://www.dxtuners.com) Tune a number of FM radios around the world and get live audioda:FM via VHF bånd IIfr:Radio FM
