Hello George,
Were all cutting lathes equipped with a centrifugal governor?
Andrew Hallifax¡¯s original question:
"More puzzling still are a couple of discs where the pitch drops
gradually by around a quarter of a tone (roughly 3%) during
playback of the side."
To what would you attribute such variation? The behavior is consistent
with a governor-less cutting lathe, if such a thing existed in the 1950s,
in Argentina.
It¡¯s true, a great deal of our work is with broadcast transcription
discs, where we see alternating inner-outer groove starts. The change
in audio quality is then a result of slower linear groove velocity at
the inner grooves, with a drop in frequency response roughly proportional
to groove velocity?
~ Eric
On 11/27/15, 5:00 PM, "Association for Recorded Sound Discussion List on
behalf of George Brock-Nannestad" <[log in to unmask] on behalf of
[log in to unmask]> wrote:
>From: Patent Tactics, George Brock-Nannestad
>
>
>Hello,
>
>mains hum is a good indicator under two circumstances: 1) that the mains
>frequency is known for the duration of the recording, and 2) that the
>motor
>driving the turntable is not a synchronous motor. For 78s, which are under
>discussion here, there is a fixed relationship: for 50 Hz drive the
>turntable
>runs at 77.92 rpm and for 60 Hz drive it runs at 78.26 rpm. But as the
>power
>frequency drops, so does the rpm, so the hum inscribed will always become
>the
>standardised hum if you run the turntable at 77.92 or 78.26 as the case
>may
>be. It is not an independent reference with a synchronous motor.
>
>Most of Argentina's electrical power before the formation of the national
>power grid in 1960 was hydroelectric, which is known to have large
>variations
>because the regulation of a water valve is slow. And as there was no
>national
>grid to form a flywheel, variations could be large. Obviously, on a 24hour
>basis (integration over 24 hours) the average was the intended with a very
>high precision. Some places had direct current!
>
>The alternative was to use a universal motor with a centrifugal governor,
>and
>they could be precise, certainly on a 5-minute basis.
>
>Eric Jacob's calculations are not relevant in the case of a centrifugal
>governor, because using torque as his variable, he is in effect using a
>constant current equivalent. But the relevant variable is constant
>rotational
>speed ("voltage"), and that is precisely what the centrifugal governor
>supplies. There needs to be sufficient power available (and a bucketful of
>nuts as used on the gravitational motors will supply that), and any power
>in
>excess of that needed to maintain the speed the motor is adjusted to is
>absorbed in the centrifugal governor. This means that if the turntable is
>acting as a variable brake, such as when cutting, then the centrifugal
>governor eases up to the necessary degree. It is is fast-reacting because
>the
>centrifugal governor is fast-rotating due to gearing.
>
>They are also not relevant in connection with a synchronous motor drive,
>because either the braking torque is too high, and the motor stops, or it
>is
>not, and it runs at constant speed. The variation can be in the phase, a
>very
>small angular displacement, because there will be some shifting of the
>poles
>on the rotor with respect to the stator that carries the coils.
>
>The conscecutive recording from the outside to the center on the first and
>from the center to the rim on the second, and repeating that sequence was
>essentially a broadcast technique. No commercial record sets were issued
>like
>that. And the reason was not variations in speed but variations in sound
>quality due to inner-groove distortion. The change-over was less
>noticeable
>-- on that we agree!
>
>Finally, as to clicks derived from a radial scratch from the center and
>outwards: as the rpm is constant it means that a click comes once per
>revolution on the dot, provided the rpm is absolutely constant. But also
>scratches shaped like an Archimedes spiral will give equally timed
>clicks, so
>a listener to a copy may be confused.
>
>Best wishes,
>
>
>George
>
>
>Eric Jacobs wrote:
>
>> For those who don©öt mind a physics/mathematical representation of
>> what©ös going onS
>>
>> The torque of the turntable motor is constant.
>>
>>
>> When cutting...
>>
>> The torque due to stylus friction while cutting a groove varies with
>> radius:
>>
>> Stylus Torque = Radius X Stylus Cutting Force
>> Radius outer groove > Radius inner groove
>> Stylus Torque (outer grooves) > Stylus Torque (inner grooves)
>>
>>
>> When playing...
>>
>> Stylus Cutting Force > Stylus Playback Force
>>
>>
>> When cutting...
>>
>> Net Torque = Turntable Motor Torque - Stylus Torque
>>
>> Net Torque (outer grooves) < Net Torque (inner grooves)
>>
>> RPM (outer grooves) < RPM (inner grooves)
>>
>> Because there is more net torque when cutting the inner grooves, the
>> turntable spins a bit faster when cutting the inner grooves. Or you
>> can think of it the other way - that the turntable spins slower when
>> cutting the outer grooves because there is less net torque.
>>
>>
>>
>> When playing...
>>
>> Net Torque is more constant because the Stylus Torque during
>> playback is so much smaller than during cutting.
>>
>> Net Torque = Turntable Motor Torque - Stylus Torque (very small)
>>
>> Net Torque (outer grooves) ~= Net Torque (inner grooves)
>>
>>
>> RPM (outer grooves) ~= RPM (inner grooves)
>>
>>
>> Pitch variation is a function of playback speed variation
>>
>> IMPORTANT: When playback speed is faster than the recording speed,
>> the pitch is higher, and vice versa. This is the essence of why
>> there is pitch variation.
>>
>>
>> Recall from above:
>>
>> When cutting: RPM (outer grooves) < RPM (inner grooves)
>> When playing: RPM (outer grooves) ~= RPM (inner grooves)
>>
>> and therefore
>>
>> RPM cutting (outer grooves) < RPM playback (outer grooves)
>>
>>
>>
>> The playback RPM on the outer grooves is faster than the original
>> cutting speed. Therefore the outer groove pitch is higher than the
>> pitch on the inner grooves (or vice versa, the pitch on the inner
>> grooves is lower than the outer grooves).
>>
>>
>> To account for this variation in speed during recording, the
>> recording engineer would cut the first disc in a series starting
>> with the outer groove. The second disc in a series would start
>> on the inner groove, so that the speeds would more closely match
>> between the first and second discs. Inner and outer groove start
>> would continue to alternate during the recording session.
>>
>>
>> Hopefully this somewhat long-winded mathematical explanation is
>> helpful.
>>
>> ~ Eric
>>
>> Eric Jacobs, Principal
>> The Audio Archive
>> 1325 Howard Ave, #906, Burlingame, CA 94010
>> Tel: 408-221-2128 | [log in to unmask]
>> http://www.theaudioarchive.com/
>>
>>
>>
>>
>> On 11/27/15, 9:09 AM, "Association for Recorded Sound Discussion List on
>> behalf of DAVID BURNHAM" <[log in to unmask] on behalf of
>> [log in to unmask]> wrote:
>>
>> >I think the obvious answer to your second question is insufficient
>>torque
>> >on the recording turntable. This happened on many recorded sides, one
>> >example that comes to mind is the Weingartner "Les Preludes" by Liszt.
>> >The cutting stylus puts considerable drag on the turntable and that
>>drag
>> >increases towards the centre of the disc, dragging the speed down. If
>> >the recording turntable motor is not VERY strong and is unable to
>> >maintain the corrrect speed throughout the cut, the resulting slower
>> >speed towards the end of the side gradually raises the pitch on
>>playback.
>> > With modern digital workstations, this error is easy to fix, but back
>>in
>> >the days of reel to reel tape, trying to rejoin the sides on the
>> >aforementioned "Les Preludes" was a nightmare.
>> >db
>> >
>> >
>> > On Friday, November 27, 2015 11:48 AM, Andrew Hallifax
>> ><[log in to unmask]> wrote:
>> >
>> >
>> > Thanks for your contribution Jolyon. I am in fact following all such
>> >advice
>> >and practices as you describe. We're working on the presumption that
>> >standard pitch seems to have been adopted in Argentina sometime during
>> the
>> >late 30s. Regardless of how true that is, our presumption is supported
>>by
>> >most other discs in the series produced during the 1950's which render
>> >more
>> >or less reliably A442ish at nominal 78rpm.
>> >However, my question to the list was aimed not so much at resolving the
>> >pitching/speed conundrum per se, but in the hope of discovering whether
>> >anyone might offer an insight into why speeds were inconsistent across
>> the
>> >two sides of discs recorded on the same day and bearing adjacent
>> matrices,
>> >and also, why or how certain recordings of this period change pitch
>> during
>> >the side.
>> >
>> >
>> >
>
|
