[ProAudio] FeralA - Recordings released encoded with Dolby A

John Chester jkc at jkc-lab.com
Tue Feb 11 10:12:03 EST 2020

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

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