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.
Brian

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From the Sofa of Brian Gaff Reply address is active

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

Not just cassette. My old Tandberg reel to reel could be slowed enough to
actually hear the tone.
However I think the real issue with cassette decks was that the mpx 19khz
was affecting the noise reduction circuitry and making it almost useless.
The fix was a low pass filter to clobber it above about 17Khz.
No this sound was definitely some kind of beat effect. Having played with
digital recording at lower sample rates this also causes a similar effect,
so it could well be it was made using a lowish bit rate, or its been
reprocessed badly.

PS The problem with tape recorders in the old days was with radio recorders
which might try to record medium wave. the nasty harmonics of the bias or
erase oscillator would give birdies on some frequencies that often wobbled
about with the audio, so a switch was often provided to move the frequency
of it to mitigate the problem.

Brian

--
From the Sofa of Brian Gaff Reply address is active
"Phil Allison" wrote in message
...
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