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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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