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<p>Dear John: <br>
</p>
<p>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. <br>
</p>
<p>So what happens in post, do you adjust the time delay between the
two channels listening in mono?</p>
<p>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?<br>
</p>
<p>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. <br>
</p>
<p>Bob</p>
<p><br>
</p>
<p><br>
</p>
<p><br>
</p>
<p>On 2/11/20 1:12 PM, John Chester wrote:</p>
<blockquote type="cite"
cite="mid:3432521e-d7fb-abee-0a46-841a70439080@jkc-lab.com">On
2/11/20 12:21 PM, Bob Katz via ProAudio wrote:
<br>
<br>
<blockquote type="cite">III. Dear John:
<br>
<br>
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?
<br>
<br>
</blockquote>
<br>
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.
<br>
<br>
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.
<br>
<br>
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.
<br>
<br>
To get the best possible bias recovery, I have to use precisely
correct azimuth. Plangent processing includes digital correction
of interchannel time difference.
<br>
<br>
-- John Chester
<br>
<br>
<br>
</blockquote>
<div class="moz-signature">-- <br>
<pre><font class="3D""" face="3D"Courier"">
If you want good sound on your album, come to
Bob Katz 407-831-0233 DIGITAL DOMAIN MASTERING STUDIO
Author: <b>Mastering Audio</b>
<a href="https://www.digido.com/">Digital Domain Website</a>
No trees were killed in the sending of this message. However a large number
of electrons were terribly inconvenienced.</font>
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