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ARSCLIST  June 2004

ARSCLIST June 2004

Subject:

Re: Spoke Pattern

From:

George Brock-Nannestad <[log in to unmask]>

Reply-To:

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

Date:

Sat, 26 Jun 2004 10:40:46 +0200

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (86 lines)

From: Patent Tactics, George Brock-Nannestad

----- the below with 2 arrows is actually a quote from an e-mail that I sent
to Kurt off-list - I shall repeat the contents of that e-mail at the bottom
of the present mailing.

> >As to 78 rpm: regular spokes I have only seen on lacquer recordings, whereas
> >a similar phenomenon is not unknown even on acoustic recordings. The surface
> >then looks like moiré silk, and it is actually a constant number of vertical
> >vibrations per second not tied precisely to the rpm, so the spokes are
> >"crooked". This was usually a cause for discarding a take, so they are
> >uncommon.

----- and then Kurt:

>
> I suppose another possibility might be a burr in a gear or some other
> deficiency in the recording mechanism that would result in a frequency that
> becomes visible in the recording.

----- a burr would generate wow or flutter - here we would need a vertical
vibration.
>
> Occasionally the music itself might have a rhythm or repeated phrase that
> results in a radial pattern of some sort (especially in audio test records),
> but this looks different from the spoking mentioned above.

----- it is different, and it is actually a good control of the precision of
the test record: if the signal generator has a stable frequency and if the
recording turntable was rock steady, then the waveforms will be "parallel" on
the record surface (actually distributed on Archimedes' spirals that
degenerate to radial lines if the frequency is harmonically related to the
rpm). The earliest such record that I have seen is an Edison speed control
Diamond Disc test record where this "Archimedes" test demonstrates that there
was nothing rock steady about that recording.

And then myself in my first response to Kurt's comment on John Hall's remark:



QUOTE:
From: Patent Tactics, George Brock-Nannestad

(JOHN HALL):
> >The lathe had a vertical motion that made a 54 spoke pattern on the
record.


(Kurt Nauck):
>
> Does this explain the spoke patterns sometimes seen on 78s? I have always
> wondered what caused this phenomenon. Was it a vibration in the cutting
> machine which caused the stylus to dig a little deeper into the wax?


(George):
----- essentially you are right, however the "vibration" is usually of a very
low frequency. And for spokes to appear it must be locked to the rpm of the
turntable.

 I was not aware of any "vertical motion" of the Scully lathe as such; the
phenomenon is related to the suspension of the cutterhead. There are two
basic ways to do this, and it is related to the adjustment of depth of cut.
In one, the cutting head is partly carried by an "advance ball", a spherical
surface that rides on the record material and so defines a "lowest vertical
position" of the cutterhead. This type would not display spokes with
variation of depth. The other type has a spring mount and/or a counterweight,
and that may be set in oscillation, sometimes when the vertical cutting angle
of the stylus is incorrect with respect to the pivot of the suspension. To
prevent such vertical oscillations, a "dashpot" is placed between the moving
components: a piston in a fluid or air dampens the movement. Or an
electromagnet may be used.

If there are 54 spokes on 33 1/3 rpm (this is implied by John Hall's
response), the frequency would be 30 Hz, but I do not see any simple
mechanical explanation for that precise frequency.

As to 78 rpm: regular spokes I have only seen on lacquer recordings, whereas
a similar phenomenon is not unknown even on acoustic recordings. The surface
then looks like moiré silk, and it is actually a constant number of vertical
vibrations per second not tied precisely to the rpm, so the spokes are
"crooked". This was usually a cause for discarding a take, so they are
uncommon.

QUOTE end

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