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The trouble with this idea is that everything has to be just so for the 
sync to be good enough, by which I mean something less than 10 degrees 
phase shift at 20k. Chris's subject recording, which incidentally I 
supplied, was Swing 23, Minor Swing by Django Reinhardt. Swing pressings 
are laminates and, whilst they have low surface noise, they are also 
subject to odd lateral shifts, apart from the usual centring problems. 
Warpage, and how a particular pickup rides it, also twists the timebase 
slightly - not enough to matter ordinarily, of course, but enough to 
knock the waveforms out of sync for this purpose. Even the tightness of 
the servo lock on the turntable may differ enough between passes. The 
common spindle of the cakestand would have to be quite thin for driving 
the mass of the upper turntable and thus whip may enter the equation 
too. This is such an attractive idea that it is worth considerable 
effort to make it work, but then the chances of finding two relatively 
unworn copies of a popular recording are pretty slim. People would play 
these things, having bought them...!




On 20/12/2020 09:56, Tim Gillett wrote:

> 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"
> <[log in to unmask]>
> To:<[log in to unmask]>
> Cc:
> 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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