FM is Frequency Modulated.
That means the carrier of the signal is modulated up and down in frequency by
the audio. The carrier level stays the same at all times, unlike AM.
by Stanley Swanson
The younger generation probably knows little about the history of radio. Here
is a brief summary of the history of FM broadcasting.
Edwin Armstrong tested frequency modulation in 1933 and demonstrated it in
1935 to the Institute of Radio Engineers. FM modulation has a constant
amplitude, not like AM (amplitude modulation) and so is less susceptible to
interference from noise and from other FM stations. Frequency modulation can be
used at any radio frequency, but it is not practical at low radio frequencies.
It was first demonstrated to the FCC in 1937. The first construction permit for
an FM station was issued by the FCC in 1937 and regular broadcasts started in
Armstrong committed suicide in 1954 because of lawsuits resulting from his
inventions. His wife Marion finally won the lawsuits many years later.
FM was first allocated to 41 to 44 MHz and then assigned to 42 to 50 MHz in
1940. It was moved to 88 to 108 MHz in 1945 in order to permit more FM channels
and make frequencies available for television broadcasting. This has presented
problems to stations in the noncommercial band (88 to 92 MHz) because of FCC
requirements to protect channel 6 TV stations. (Channel 6 is just below the FM
band.) Apparently the FCC doesn’t care if TV interferes with FM.
In 1961 the FCC approved the GE/Zenith FM stereo system (which uses an AM
subchannel) instead of the Murray Crosby system. The Crosby system, demonstrated
in 1954, was superior to the AM subchannel system, but it was rejected because
it conflicted with the subcarriers used for background music, such as for Muzak.
(Remember - this was in the days before satellite.)
Originally, the FCC required that FM stations meet certain audio frequency
requirements, such as signal-to-noise radio, frequency response, and distortion.
These requirements have now been removed. It is now legally permissible to use
two tin cans and a string for audio. (The FCC inspector may laugh, but he won’t
give you a citation.)
In order to improve the overall signal-to-noise ratio, the FCC audio
requirements call for a transmitted high frequency pre-emphasis of 75
microseconds. This is then attenuated by the same amount in the FM receiver. The
specification actually says a MAXIMUM of 75 microseconds pre-emphasis. This
results in a boost of 17 dB at 15 KHz. This presented few noticeable problems in
the early days of FM broadcasting, because early recordings had little high
frequency content. But it does impose difficulties in audio processing because
of that large high frequency boost. European FM uses 50 microsecond
pre-emphasis. Some U.S. stations use 50 microsecond pre-emphasis; they sacrifice
a small amount of high frequency content in order to obtain better audio quality
with less compression. The FCC will probably never change the U.S. standard.