Re: [ARSCLIST] Audograph disc

[Eric Jacobs <EricJ@xxxxxxxxxxxxxxxxxxx>]

On Thursday, October 16, 2008 5:40 AM, Stewart Adam wrote:

> Also I know that the speed changes during playback on a turntable. What
> is the amount of speed correction needed? Any assistance would be greatly
> appreciated.

The Gray Audograph discs are CLV discs - constant linear velocity.  We
actually purchased a machine and reverse engineered the speed compensation
required to map a CLV disc played back on a typical CAV (constant angular
velocity) turntable.  We've also gone through this exercise with
Soundscriber
and Edison Voicewriter machines.

The mapping process involves a series of equations that are best represented
in a spreadsheet, and are a function of groove diameter.  The mapping is
multi-step - there is a linearization of the sound step, and there is a
pitch adjustment step.  There is also a calibration step needed to adjust
for the particular machine that was used to make the recording.  If these
recordings were made on multiple machines, this will need to be accounted
for because we have found that there is some (audible) speed variability
among machines.  The speed variability is a function of manufacturing
tolerances of the machine, machine wear (idler), and maintenance
(lubrication
and friction).  To get the job done right (for best audio quality) with a
CAV turntable, you will need a turntable that can run at 20-25 RPM (we use
the Simon Yorke S7 modified for archival work).  You will also need a custom
splined spindle to match the disc spindle hole.

You probably know this, but it bears repeating - the transfer should be done
with FLAT (no) EQ.  All speed compensation should be performed next.  Noise
reduction follows, and the very last step would be applying EQ.

Maybe worth noting is that the best results are obtained with a wide
bandwidth transducer and a wide bandwidth preamplifier.  You need the
extra bandwidth with the CAV playback, otherwise you will loose some
frequency response in the linearization process.  In order to achieve the
best sound possible from these discs, we use moving coil cartridges and
the Millennia Media LPE-2 phono preamp - both are wide bandwidth devices.

As for stylus, you may find that a truncated elliptical stylus will perform
better than a typical "line contact" LP stylus.  We have a custom 1.2 mil
TE moving coil that we often find works well with the Gray Audograph
format.

We also have special equipment needed to handle badly creased discs as
people were fond of paperclipping notes to these discs (!) ...leaving
behind wrinkles in the disc that are nearly impossible to track without
the right tools.

The Gray Audograph discs are one of the formats that we specialize in, and
we have transferred and restored several hundred of these.  The learning
curve is a bit steep, but not impossibly so.

Hope this helps,

Eric Jacobs

The Audio Archive, Inc.
tel: 408.221.2128
fax: 408.549.9867
mailto:EricJ@xxxxxxxxxxxxxxxxxxx
http://www.TheAudioArchive.com
Disc and Tape Audio Transfer Services and Preservation Consulting