Brian Gaff wrote:

I was listening to a piece on Inside Science yesterday, and they were
speeding up and slowing down animal sounds. When they played one at normal
speed it seemed to me it had a kind of beat effect that would not be heard
in a real bird. Indeed when they slowed it down it was even more obvious.
Nobody commented on that at thetime, but it reminded me of the old days of
biaoscillators that were less than clean beating with the audio being
recorded even though they were outside the ears passband.


** A bias oscillator needs to run at a frequency at least 20kHz higher than the highest audio signal to be recorded - or beat tones at the difference frequency will appear on playback. This never amounted to much of a problem until stereo FM came along and folk tried to record programs on cassette decks.

Stereo FM's 19kHz "pilot tone" is often not filtered from the output of tuners and is at a constant level of about 10% of full modulation. Few could hear this but it was an issue for any cassette deck with its bias oscillator operating at less than 40kHz.

Some makers fitted a switch on the back of their decks to shift the bias to a higher frequency as a fix for the problem.



..... Phil