RE: arsclist Transfer of multiple copies, was: Full 3-D mapping o f groove?

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

Dear All,
    Once again I find myself apologising for being late to enter this
discussion. I believe I have read all its pages, but here is a technical
comment.
    Three or four years ago, I did some practical work using two shellac 78s
with plain grooves (usually the *backs* of "single-sided" discs which
happened to carry unmodulated grooves), and *measured* the degree of
noise-reduction in the absence of wanted sound using Packburn/Mousetrap
"groove-wall selection" with the two copies plugged to the two channels. It
was considerably greater than the theoretical 3dBs, actually around 7 or
8dBs, which made the principle worth pursuing.
    I then devised two ways of synchronising (besides a "cakestand model",
which used a redundant Scully disc-cutting lathe!) One method "looked at"
the stroboscope on the outside edge of a Garrard 401 turntable, giving (in
Europe) a 100Hz tone, and *multiplying* it by 441 to get 44100Hz, and
clocking an analogue-to-digital converter from it so the two discs would
comprise the same number of samples. The other used a Technics SL1200
turntable, and *divided* its internal clock (different factors for the
national standards on either side of the pond so stroboscopes would be
accurate, and more factors for 80rpm originals, etc. etc.). This was found
better, because in electronics it is easier to *divide* rather than
*multiply*. I took the usual precautions about centering the discs on the
turntable. Using a parallel-tracking pickup with its vertical pivots located
in the plane of the disc surface (I don't know a way of *buying* this
mechanism, I had to adapt one), the synchronism was outside the capability
of the CEDAR azimuth corrector, which fails when the synchronism exceeds 99
samples. This seemed to be due to minute disc warpages pushing the pickup up
and down, which (because it had a cantilever) became translated into motion
*along* the groove.
    It would of couse be possible to fudge this problem by various means,
but I lacked the time and resources to do more. In any case, for practical
work the CEDAR units used fewer person-hours, and I wasn't atempting
archival standards of reproduction.
    The plan of 3d-mapping a groove lacks an anti-aliassing filter of
course, so is bound to fail in the digital domain.
Peter Copeland

-----Original Message-----
From: Don Cox [mailto:doncox@xxxxxxxxxxxxxx]
Sent: 11 December 2002 15:00
To: ARSCLIST@xxxxxxxxxxxxxxxx
Subject: Re: arsclist Transfer of multiple copies, was: Full 3-D mapping
of groove?

On 10/12/02, Mwcpc6@xxxxxxx wrote:

> This discussion started with mention of the problems of accurately
> replacing the lost data during the removal of pops and clicks. It
> seems to me that the major advantage of synchronizing and
> mathematically analyzing the data from multiple copies would be the
> total removal of such defects without affecting the original music.
> 
> It is unlikely that a pop or click would occur at the same point on
> three or more copies. By completely rejecting data that deviates by a
> significant amount from the average, the defect would be removed and
> the "good" data from the other copies used.
> 
> This same technique might also remove distortion products due to
> record wear to a greater extent than simple averaging.
> 
> Beyond this, averaging of the random noise of the pressings would give
> the 3 dB benefit per data doubling as above, but being able to reach
> through the impulse noise and distortion should be a great value.
> CEDAR or other processing would then be applied to the noise
> components from the original master, common to all copies available.

That is exactly how I understand it.

Wow is the only thing that makes it difficult.

Regards
-- 
Don Cox
doncox@xxxxxxxxxxxxxx



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