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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