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These contributions from Ted Kendall and Doug Pomeroy have convinced me that
all attempts at reconstructing "accidental stereo" from two
simultaneously-recorded 78's with separate mic sources will be futile ---
that is, unless someone can construct an algorithm to keep the separate
sources in a continuously stable phase relationship. I've already looked at
a couple of these reconstruction attempts and observed that even short-term
phase stability is completely absent. The net result is pseudo stereo even
if there's genuine spatial information shared between the two sources. I
take no pleasure at all in coming to this conclusion, but that's reality.

Aaron Z.

> -----Original Message-----
> From: Association for Recorded Sound Discussion List
> [mailto:[log in to unmask]] On Behalf Of Doug Pomeroy
> Sent: Friday, June 22, 2012 10:30 AM
> To: [log in to unmask]
> Subject: [ARSCLIST] multiple copies for surface noise reduction, was Re:
audio
> from pictures
> 
> >
> >> Date:    Wed, 20 Jun 2012 10:44:42 +0100
> >> From:    Ted Kendall <[log in to unmask]>
> >> Subject: Re: audio from pictures
> >>
> >> <<SNIP>>
> >>
> >> Chris Hicks of Cedar did his doctorate on just this question of
> >> multiple copies. When the algorithm could be persuaded to work, the
> >> results were much as you would expect - enhancement of the correlated
> >> wanted signal and reduction of the random noise. Unfortunately,
> >> pulling the recordings into good enough sync for artifacts to be
> >> negligible was a frustrating and difficult business, even with
> >> recordings made one after the other (by me, as it happens) on the
> >> same kit on the same settings on the same afternoon, with meticulous
> >> centring. The minute geometrical differences between different
> >> laminated pressings were enough to throw things continually out of
> >> register. The example produced, however, is a tantalising glimpse of
> >> what might one day be possible.
> >>
> >> Whenever I do a master of the QHCF Decca of "Souvenirs", I use three
> >> copies from the shelf - one for the very start, one for most of the
> >> side and one for the last few turns where the others are afflicted
> >> with "Decca scrunch".
> 
> I gave a talk on this subject at the ARSC conference in Nashville in 1997.
> Two perfectly synchronized copies would provide a theoretical signal-
to-noise
> improvement of 3 dB, four copies an improvement of 6 dB and eight copies
an
> improvement of 9 dB. The first problem is finding so many copies in E
condition.
> 
> But the bigger problem is how to achieve perfect synchronization.
> John S Allen of the Boston Audio Society discusses this matter in an
article he
> wrote for the Spring 1990 issue of the ARSC Journal.  He says the accuracy
of
> sync must be "about ten microseconds (72 degrees of phase shift at 20
kHz)".
> No real-world turntable is stable enough to achieve this result on
successive
> plays, and I proposed a stack of tables all powered by a single motor for
playing the
> discs simultaneously, with use of digital delay lines to align the audio
from them
> after transferring.
> 
> Discs which are even slightly out-of-round would need to be matched
perfectly
> with others of the same shape (not impossible, since these discs reflect
the shape
> of the metal stamper used to press them), and digital "azimuth correcting"
software
> might be used to help deal with alignment variables such as this.
> 
> Doug Pomeroy
> Audio Restoration & Mastering Services
> Transfers of metal parts, lacquers,
> shellac and vinyl discs & tapes.
> 193 Baltic St
> Brooklyn, NY 11201-6173
> (718) 855-2650
> [log in to unmask]