Walter Welch demonstrated the mechanical synchronization of two Edison Diamond Disc machines to every visitor to the Syracuse Belford labs. He took one electric motor coupled to the turntables of two motorless machines. His stated purpose was doubling the effective horn length to lower the lowest frequency of the reproduction, but another result was allowing the brain to fuse the audio midway between the two machines which also decouples the music in the center from the differing noises of the two discs. This was a trick which the WW II internal broadcast monitors did with noisy shortwave reception. Using headphones and two receivers with antennas at different ends of the building, the noise is decoupled from the audio which fuses in the center. Sometimes they had the further advantage of the same station on two frequencies, but differing signal paths due to ionospheric reflections lowered the fusing of the audio. Considering the discussion so far, this only works if the signals are NOT summed to mono.
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From: Association for Recorded Sound Discussion List <[log in to unmask]> on behalf of Ted Kendall <[log in to unmask]>
Sent: Sunday, December 20, 2020 1:58:42 AM
To: [log in to unmask] <[log in to unmask]>
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.
> 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
> Has anyone tried this technique with 78 RPM shellac records or have
> any comments?
> Cheers Tim.
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