(Redirected from FM radio
FM radio is a broadcast technology invented by Edwin Howard Armstrong that uses frequency modulation to provide high-fidelity broadcast radio sound. 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.
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" 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 subcarrier. A 19 kHz pilot tone is then added, to allow receivers to detect the presence of a stereo-encoded signal.
This 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 which is used to carry a low bandwidth digital Radio Data System information. This information allows digitally controlled radios to provide extra features such as Alternate Frequency (AF), Network (NN) etc...
Visit http://transmitters.tripod.com/stereo.htm for an article that explains the concepts of FM Stereo, with SPICE analysis and waveforms at different points of the multiplexing process.
Last updated: 06-02-2005 05:00:31
Last updated: 10-29-2005 02:13:46