[ProAudio] FeralA - Recordings released encoded with Dolby A

Bob Katz bobkatz at digido.com
Tue Feb 11 10:52:45 EST 2020

Dear John:

In other words, in Plangent, first you adjust the angle of the repro 
head to get the best bias recovery --- which should match the angle of 
the record head. BUT, due to several factors, including possible gap 
scatter (non-colinear gaps), this might not fix the time delay between 
the two channels.

So what happens in post, do you adjust the time delay between the two 
channels listening in mono?

An interesting dilemma is a stereo recording made with spaced mikes. If 
you correct the time delay between two audio impulses seen on two 
channels, you then mess up the intended stereo imaging. In that case, I 
think that examining the time delay between the two bias signals would 
be far more accurate than trying to fix it based on audio impulses. What 
about that dilemma?

When I have a recording made with two spaced omnis, first I check that 
the polarity of one is not messed up by checking for maximum bass 
response in mono, inverting the polarity of one of them. If I was doing 
an analog tape transfer of this recording, if I had access to the bias 
signal like you that's what I'd use to adjust the angle of the repro 
head. In the absence of that I listen and do my best.


On 2/11/20 1:12 PM, John Chester wrote:

> On 2/11/20 12:21 PM, Bob Katz via ProAudio wrote:
>> III. Dear John:
>> So you set azimuth by looking at the bias. Are you looking at two 
>> channels of bias? And how is this superior to the tried and true 
>> method of mono-summing the left and right audio channel and adjusting 
>> for maximum high frequency response, also checking by inverting the 
>> polarity of one and going for a minimum as a cross check?
> Adjusting for precisely correct azimuth usually does not produce 
> minimum time difference between the channels, and vice-versa.  The 
> usual methods of azimuth adjustment -- either the ones you describe or 
> using a phase scope -- give minimum interchannel time difference.  
> This is the correct approach when the output of the tape playback 
> machine will be used without any further adjustment of interchannel 
> time difference.
> I do look at bias on both channels of a stereo tape.  Sometimes bias 
> on one channel has much better signal to noise, and I will choose to 
> optimize azimuth for that channel.   If s/n is similar on both 
> channels, I may use an azimuth which gives equal bias level stability 
> on both channels.   Azimuth is rarely perfectly identical on both 
> channels, but the maximum difference I've seen has never been large 
> enough to cause any measurable change in level at 15 kHz.
> Why does perfect azimuth adjustment usually not produce minimum 
> interchannel time difference?  The most obvious answer is gap scatter 
> in the record head, but there are others.  Record EQ may not be 
> precisely matched, and bias setting may be different. Recording occurs 
> at a point slightly past the trailing edge of the record head gap, and 
> that distance varies as bias level changes.
> To get the best possible bias recovery, I have to use precisely 
> correct azimuth.  Plangent processing includes digital correction of 
> interchannel time difference.
> -- John Chester

If you want good sound on your album, come to Bob Katz 407-831-0233 
DIGITAL DOMAIN MASTERING STUDIO Author: *Mastering Audio* Digital Domain 
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