From: Patent Tactics, George Brock-Nannestad
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.
Eric Jacobs wrote:
> For those who donıt mind a physics/mathematical representation of
> whatıs going onS
> The torque of the turntable motor is constant.
> When cutting...
> The torque due to stylus friction while cutting a groove varies with
> 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
> ~ Eric
> Eric Jacobs, Principal
> The Audio Archive
> 1325 Howard Ave, #906, Burlingame, CA 94010
> Tel: 408-221-2128 | [log in to unmask]
> 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.
> > 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
> >and practices as you describe. We're working on the presumption that
> >standard pitch seems to have been adopted in Argentina sometime during
> >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
> >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
> >two sides of discs recorded on the same day and bearing adjacent
> >and also, why or how certain recordings of this period change pitch
> >the side.