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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]