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Just to add, the higher the time base accuracy, the better to preserve
higher frequency musical information. The Eileen Joyce Berceuse
recording linked has little in the highs department  above 3 to 5
kHz, so in that sense the timebase accuracy wouldnt have to be as high
as with a higher frequency upper limit recording.  

----- Original Message -----
From: "Association for Recorded Sound Discussion List"
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Sent:Sun, 20 Dec 2020 06:58:42 +0000
Subject:Re: [ARSCLIST] Noise reduction on mono records using two
separate coherent sources

 This was a favourite thought experiment of Peter Copeland, late 
 conservation manager at the British Library National Sound Archive.
His 
 idea was the "cake stand", where two platters were mounted in the
same 
 spindle. Even with this (presumably) rigid coupling between the two 
 transfers, warpage, off centre pressings and differing pickup 
 compliances would throw synchronism off enough to defeat the object
of 
 the exercise. Christopher Hicks of CEDAR did his doctoral thesis on
this 
 problem as well, and managed by some cunning DSP to hold five copies
of 
 the same recording in sync long enough for the theoretical benefit to

 confirmed. As things stand, though, as a practical technique it's
still 
 a non-starter. One day, perhaps...

 On 20/12/2020 05:20, Tim Gillett wrote:
 > Hi all,
 >
 > I've  familiar with the principle of summing two audio recordings
of
 > the same programme where the wanted programmes are time coherent
but
 > the unwanted background noises arent, making it possible in theory
to
 > realize a 3db reduction of the background noise. I've used it  on
 > cassette and  1/4 track "stereo" recordings which are actually
dual
 > mono, although time alignment can be a little tricky and an
"azimuth"
 > tool can really help.
 >
 > Earlier 78 RPM recordings  are understandably noisy especially as
I
 > believe was customary, the original metal parts were usually
recycled
 > for cost reasons  so all transfer engineers have to work with may
be
 > the best shellac consumer pressings that have survived.
 >
 >  The other day while browsing I chanced upon a seemingly mint
 > condition 10"  78 shellac  record of  Eileen Joyce performing
 > "Berceuse" in 1939.  Here's a modern CD release of it.
 >  https://youtu.be/JybH6wxFUrs
 >
 > Quite noisy and limited bandwidth, even for 1939 I thought.   I
 > cleaned and transferred the disc I'd found and after declicking
 > compared the sound to the version linked to. The two versions
sounded
 > close. Signal to noise was about the same.
 >
 > I believe that compared to vinyl, shellac was  a relatively noise
 > medium so it occured to me that assuming the stamper had less noise
 > than the shellac copies, it should be possible to time align the
audio
 > from the two discs, sum to mono and  yield a potential background
 > noise reduction of 3db. Not huge but with an already noisy
recording,
 > not to be sneezed at!
 >
 > So far I've not had much success. I'm having problems trying to
 > accurately time align my disc transfer with the  commercial
release.
 > I can get them within maybe a few milliseconds but it's not enough.
 > The two versions drift in and out of sync causing comb filtering.
 >  My next step might be to find another mint disc of the same
 > performance and make my own transfer of both on the same gear under
 > the same conditions so at least there is a fighting chance of an
 > accurate  alignment. Maybe a better turntable with more precise
speed
 > regulation?
 >
 >  Has anyone tried this technique with 78 RPM shellac records or
have
 > any comments?
 >
 > Cheers Tim.
 >
 > 
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