Don Pearce wrote:
Adrian Tuddenham wrote:
Paul Stamler wrote:
Hi folks:
Here's a weird one, for those interested in 78s....
[...]
you can see that in fact it's a tone which alternates pitch between high
and
low, once per revolution. (It may help to click on the image for a cleaner
view.) At the start of the recording, the lower pitch is at about 20.5kHz,
while the upper pitch is above the Nyquist frequency of this 44.1k sample
rate and so is not recorded. By the end of the tune, the high pitch is
about
12kHz and the low is about 10kHz, both quite audible despite the record's
surface noise.
It often occurs very noticeably at the end of 8" 'Broadcast" records
made by the UK branch of the Vocalian Co. For a long while I thought it
was because the Marconi recording process, which is what they used,
might have been equipped with a long whippy cutting tool which began to
chatter at slow surface speeds near the end of the disc.
Some time ago, I discussed this matter with two disc cutting engineers,
the late Peter Copeland and (the still very much alive) Sean Davies.
They came to the conclusion that it was mainly due to the wax cooling on
the lathe turntable during the cutting session (no nitrate in those
days).
It would be useful to keep a note of these examples and try to relate
them back to the recording conditions. Were any know to occur during
summer or were they all recorded in winter? Were they more common on
location and/or in a recording van, where the air temperature would be
lower or the wax ovens less effective?
An interesting project for anyone with enough time to spare.
How does this work? Is it that the wax forms tiny ripples or wrinkles as
it cools? Assuming they formed in lines across the disc, that would
explain both the variation in frequency with each turn and the drop in
frequency towards the centre.
I think the wax stays smooth but becomes too hard for the cutting tool
to work smoothly. The tool chatters as it tries to cut the hard wax.
If it has been heated or cooled unevenly, the effect will vary around
the axis of rotation.
If you have ever tried to machine a very hard material in a conventional
metalwork lathe, or machined something that was running too slowly with
an incorrectly set tool, you will have experienced the effect. The
drop in frequency relates to the surface speed and the changing
mechanical impedance of the mechanical resonant circuit, which includes
the wax.
The biggest component on the disc is probably longitudinal ('Z'-axis)
modulation which would give a frequency-modulated effect, but this won't
be visible or strongly audible. Any lateral component of the chatter
will be much more audible. and may be visible under the right
conditions.
--
~ Adrian Tuddenham ~
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