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FM recording on Audio Cassettes
In , on 01/06/04
at 06:56 PM, (Curious) said:
Would heterodynes affect the audio in "AM cassettes"?
"AM" should not be associated with the cassette tape process. Are you
thinking that AM implies noise and FM does not; therefore, since
cassettes are noisy, they must be AM? This is incorrect thinking.
Answering your question: Heterodynes can be an issue with cassette
decks. The bias oscillator can mix with the music you are trying to
record, leakage from the FM Stereo subcarriers, or leakage from the AM
tuner in your receiver. (Some combination radio/cassette units have a
switch that will shift the tape bias frequency slightly to avoid
heterodynes with the AM radio section) It's not a heterodyne problem,
but most cassette decks that include Dolby will include a filter to
remove the FM Stereo pilot tone.
---
Home cassettes use a direct recording process. Air pressure changes
(sound) generate a changing current in the microphone. This current is
amplified and passed through the record head (a coil of wire wound on a
c-shaped metal core), which impresses a magnetic pattern on the tape
that is an analog of the air pressure changes. Records also use a
direct process, but the result is a mechanical analog cut into the
surface of the master recording.
Direct processes are simple and useful, but are not always adequate.
For example, if you are running a logging operation, it is usually not
practical to launch a tree directly into a remote saw mill, you enlist
the aid of a "carrier". This carrier could be a river, team of mules,
or possibly a truck. Ultimately you will throw away the carrier and use
the log.
Sound waves don't travel well in the air over long distances, but radio
waves do. If we could somehow attach sound to a radio wave, we could
send the sound great distances. This is where "AM" and "FM" enter the
picture. They are methods of attaching the sound to the radio frequency
carrier. The AM method is similar to throwing our logs into the river.
It's straight forward and inexpensive, but the log might pick-up some
pollutants along the way. FM is a bit more complicated, but is more
robust with respect to noise. (Think of shipping our logs in a boat.)
Yes, there are other applications for AM and FM. Yes, there are a
relatively few tape recorders that use an FM process in industrial
applications, but home audio cassettes are recorded by a direct
process.
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