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ARSCLIST  November 2015

ARSCLIST November 2015

Subject:

Re: 78rpm replay speeds & pitch

From:

Eric Jacobs <[log in to unmask]>

Reply-To:

Association for Recorded Sound Discussion List <[log in to unmask]>

Date:

Fri, 27 Nov 2015 11:48:14 -0800

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (135 lines)

For those who donıt mind a physics/mathematical representation of
whatıs going onŠ

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.
>
>
>

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