At 04:30 PM 9/10/2014, Tom Fine wrote:
>Isn't there some technique for full-track azimuth adjustment where you
>take two feeds of the output, polarity-reverse one of them, input into two
>console positions at unity gain and then adjust azimuth for greatest net null?

That technique, and many of the other techniques described in this thread,
will give you the best phases match between the channels. However, this is
often *not* the azimuth adjustment that gives the best HF response. When I
do Plangent transfers, I can of course adjust the azimuth for best HF
response, because I'm looking at a real time display of recovered bias --
but this means that the phase match between the channels is sometimes not
perfect.

What causes this? Two things that I know of: 1) the gaps in the record
head were slightly staggered; and 2 (probably more common) the recording
was made with different bias on the 2 channels. The recording is made at a
point slightly past the trailing edge of the record head gap, and changing
the bias changes this distance. This is why bias should be adjusted before
adjusting record head azimuth, or record head azimuth should be adjusted in
sync playback mode, on machines that have that feature.

(And, BTW, my version of 20-20 hindsight is that record head azimuth should
have always been fixed, and playback head azimuth should always be adjustable.)

Given that these days everyone is transferring to digital, and analog tape
copies are very seldom made, phase alignment can easily be done with a
digital tool, if it's not good enough when azimuth is set for best
frequency response.

-- John Chester