Hi, Mark,

Another great post on this subject. From a practical standpoint, the 
tapes I'm seeing recorded in this way were not recorded on a machine 
that used any "pre-distortion" to help improve overall square-wave 
response (such as those you mentioned). The names Webcor and Wollensak 
come to mind, along with Lafayette, Realistic, and Knight. And then 
there is the venerable Sony TC-355 and the Tandberg machines.

As you said, the transfer engineer has to make a decision.

However, you raise an additional point about absolute polarity. Since it 
was not specified in any of the original standards, when proceeding in 
this direction the choice of which track to flip may reside in the punch 
of the kick drum or the asymmetry of a vocal or wind instrument waveform.

I would like to suggest that the use of an ATR100 or A810/A820 to record 
a tape which is flipped to record a second program is very rare. This 
was a practice which all professional standards urged not be done. 
"Record in one direction" was the rule.

The less well-enforced rule, but was at least partially alive in the 
background was "fill all the tape with one program" which, if you were 
still recording mono implied full-track and at least Ampex/NAB (75/82 
mil) stereo. The Europeans took it a bit farther with the DIN stereo 
(wider tracks, narrower guard band).



On 2015-03-12 12:12 AM, Hood, Mark wrote:
> Hi Tom -
> I used the full-track mono recording as the simplest example.  Any analog
> magnetic tape track will reproduce the same signal polarity when played
> back in either a forward or reverse direction.  Inverting that same track
> for playback will result in an inverted signal polarity when played back
> in either a forward or reverse direction.
> The proliferation of track formats and methods of recording “forward” and
> “backward” on those tracks requires the playback engineer to make a
> decision as to how some tracks came to be recorded “backwards” and whether
> or not inverting the signal polarity of those tracks is the best practice
> when preserving that audio signal.
> A simple example might be a dual half-track monaural field recording made
> by a researcher.  After recording “Side 1” of the tape (top "half" of the
> longitudinal dimension of the tape), the reels are interchanged AND
> INVERTED, and “Side 2” is recorded (same record head domain, acting upon
> the virgin region of the tape).  If a preservation audio engineer captures
> both of these tracks in one pass - top track (left on a stereo machine)
> playing in a forward direction and bottom track (right) playing
> “backwards” which is then reversed in the time domain with DSP - the two
> analog audio streams will be of opposite polarity, when they obviously
> were not recorded that way.  Research has indicated that absolute polarity
> is audible to trained listeners, so best practices would seem to be to
> invert the polarity of the “Side 2” audio stream as part of the
> preservation process.
> The magnetic recording/reproduction of analog audio signals also
> introduces unavoidable frequency-dependent phase shift (time delay).  This
> was made obvious by examining the square wave response of the
> record/playback process - it looked terrible for decades.  Towards the end
> of the analog tape machine era, manufacturers developed various methods of
> “pre-distorting” (my term) the signal to the record head to compensate for
> this unavoidable phase distortion, and were able to show much better
> square wave response through the full record/play process.  Examples -
> Otari MTR, Ampex ATR 100, Studer A810 - “phase compensation.”
> Unlike signal polarity, the mathematics governing phase response of the
> record/play process are dependent on time and its direction - moving
> forward or backward.  I believe these phase compensation circuits were
> predicated on the reasonable assumption that the tapes that were recorded
> with these features would be played back in the forward direction, and
> phase distortion would be reduced under those conditions and square wave
> response was much improved, as touted in advertising of the day.  But it
> seems likely that if those tapes/tracks recorded with this feature are
> reproduced in negative time (played backwards), the phase correction
> pre-distortion introduced during recording might now lead to excessively
> degraded phase response when compared to the results obtained if that
> track were reproduced in the forward direction, as recorded.  Our research
> at IU implied this, but we did not have the resources, expertise or time
> to pursue the topic to the appropriate depth.
> Mark Hood
Richard L. Hess                   email: [log in to unmask]
Aurora, Ontario, Canada                             647 479 2800
Quality tape transfers -- even from hard-to-play tapes.