HI Tim et al 
The late Roger Nichols talked about a zillion track multitrack head for mono or stereo whereby the correlated audio could be made distinct from each track's noise and AM distortion and that probably could work, but it's not as easy as it looks because the tape isn't dimensionally that stable, dynamically - local momentary stretching occurs. 

We find that there are local skewing effects in not only the azimuth but in the wow and flutter profiles of a stereo pair, which seems odd until the ductile stretch-rebound of the tape is imagined into the visualization of the problem. 
Odd flutters that flip sides for example due to the idler roller of a 3M isoloop was a bit of a head scratcher. 
But in theory if the individual control signals were accurately obtained or if Patrick was pretty certain what the sync variations were then it would be easy to use Plangent to do some of this, and in fact the ability to individually edit the speed control tracks, correlate them etc would probably make this feasible. But the trouble will be the signal to noise ratio of the sampled "sync" - we find that just the hiss on the bias is enough to cause a stereo pair to diverge, and I'm sure those oddball differentials as described above don't help. At this point we pick the stronger of the two tracks and use it for both, with excellent results, tho one channel is ever so slightly less accurate (again because of things like oblate rollers causing momentary velocity differentials between the two tracks at the head. 

Don't know if this helps but I bet there's some thought experiments that would make it possible to be adapted for use as desired here. 

Jamie Howarth

+1 845.652.0277

> On Dec 20, 2020, at 4:47 AM, Tim Gillett <[log in to unmask]> wrote:
> Thanks Ted, 
> I'd thought I'd read  Peter Copeland's BL paper thoroughly enough but
> must have missed where he talks about this.  I  thought of the cake
> stand idea but then thought a conventional turntable with a high
> sample rate readout of time base (platter rotational angle at any
> given moment) would be more practical as it seems mechanically easier
> and requires the least mechanical mods to the turntable. Even though
> the turntable's speed mightnt be entirely accurate it seems possible
> to  correct it in post using the turntable's time base information
> once the files are digitised using a Plangent bias type process.  
> I'll try and chase up Hicks'  paper.
> Tim.  
> ----- 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.
>> 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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