Re: [ARSCLIST] Spoke Pattern

["Copeland, Peter" <Peter.Copeland@xxxxx>]

Dear All,
    As usual, I am replying late; but first I must say that I have a Scully
lathe, but it was given to me by MGM studios in London, which had been using
it for "instant" recording *and* playback, so performers could mime to the
song they had just recorded optically, while the set and the lighting were
still in place. For this reason, my Scully needs input from a 1000rpm
synchronous motor, and they don't grow on trees! And although I've looked, I
cannot see an obvious reason for it generating any spoking.
    However, I have met the effect on another lathe (a home-made one, since
you ask), in which the downwards pressure on the cutting-stylus was
counteracted by a tension-spring under screw control, so an appropriate
depth-of-cut could be preset. It also had variable groove-pitch, and the
stylus was heated. When the groove-pitch closed down (a metaphor meaning
that there were *more* grooves per inch), a situation could develop where
any small irregularity in the depth would affect the next turn, probably
because the lacquer was still warmer deeper. This could build up over
several revolutions until the groove became discontinuous! And in case
anyone asks, the vertical resonant frequency was very high - I removed all
possible mass from the floating system - and the depth variations from the
thermal cause were much lower in frequency. In any case I had a dashpot to
damp out all forms of mechanical resonances, but other machines with other
geometries might well have the same fault restricted in amplitude, which
would mean continuous "spoking". Amplitude-limiting would be more likely on
coarse-groove discs.
Peter Copeland

-----Original Message-----
From: Graham Newton [mailto:gn@xxxxxxxxxxxxxxxxxxxxx] 
Sent: 29 June 2004 21:39
To: ARSCLIST@xxxxxxxxxxxx
Subject: Re: [ARSCLIST] Spoke Pattern

This message was sent some days ago and apparently never got to the list due
to LOC's spam trap, a problem that now is fixed if you are reading this!

Kurt Nauck wrote:

> 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?

There are a number of reasons for a "spoke" or moiré pattern to appear on a
disc recording.  Most are due to mechanical problems in the cutting
turntable.  These are often traceable to idler wheels that have become
out-of-round due to wear or leaving the idler engaged for long periods when
the turntable is not running.

Varying large light and dark patterns will be often traced to warped discs,
bad or non-uniform lacquer coatings on the discs, or a warped turntable on
the cutting machine.

Varying small light and dark (herringbone) patterns will be often traced
to heavy hum being recorded on the disc due to defects in the cutting
head driver amplifier or other parts of the audio chain.

Another pattern is "groove bunching" where grouped variations in the spacing
of the grooves is caused by excessive play in the mechanism that carries the
cutting head across the disc surface.

... Graham Newton

--
Audio Restoration by Graham Newton, http://www.audio-restoration.com
World class professional services applied to tape or phonograph records for
consumers and re-releases, featuring CEDAR's new CAMBRIDGE processes.


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