[78-L] shifting phases

[Eric Jacobs]

On Sunday, March 22, 2009 11:23 PM, William A. Brent wrote:

> but I was wondering - is there some reason to have recorded these 
> "out-of-phase" in preparation for restoration?

Absolutely.  If the transfer set-up allowed for adjusting the
balance control ahead of a mono mixer, the L-R channels could
be nulled (cancelled out perfectly) using the balance control in
order to get the optimal mono mix later after restoration.

Naturally, after nulling the mono mix would have to be removed 
before making the transfer, but at this point you know you have 
the L-R channel balance set optimally for restoration work.

At least this is the process I follow, and perhaps your person
was doing the same thing:

   - reverse phase of one channel
   - insert a mono mixer after a balance control or L-R trim
   - mix to mono
   - adjust/trim the L-R balance until L-R channels cancel
     each other out
   - turn off the mono mix
   - set the phase of the channels correctly (ie. undo phase
     reversal)
   - playback and digitize

Your guy skipped the "set the phase of the channels correctly",
but no harm done otherwise.  As you point out, you can always 
reverse the phase of one of the channels (you'll need to figure 
out which of the two channels is out of phase, though).

If you do mix to mono (without correcting the phase) and you 
have near silence, then your transfers were done correctly with 
the L-R channels nicely balanced and nulled.

Eric Jacobs

The Audio Archive, Inc.
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