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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 ˇ°forwardˇ± and
ˇ°backwardˇ± on those tracks requires the playback engineer to make a
decision as to how some tracks came to be recorded ˇ°backwardsˇ± 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 ˇ°Side 1ˇ± of the tape (top "half" of the
longitudinal dimension of the tape), the reels are interchanged AND
INVERTED, and ˇ°Side 2ˇ± 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
ˇ°backwardsˇ± 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 ˇ°Side 2ˇ± 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
ˇ°pre-distortingˇ± (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 - ˇ°phase compensation.ˇ±

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


On 3/11/15, 5:28 PM, "Matthew Sohn" <[log in to unmask]> wrote:

>If you play a tape that has been recorded on both sides with side 1
>playing forward, the part that was recorded on side 2 will be upside-down
>from side 1, as well as being backwards. It is the upside-down part that
>induces the polarity shift.
>
>
>On Wednesday, March 11, 2015 2:29 PM, Tom Fine
><[log in to unmask]> wrote:
>  
>
>
>Mark, so you're saying that full-track tapes DO invert polarity but
>2-tracks and 4-tracks DON'T? I'm
>not sure I understand this. If a tape is played backwards (ie what is
>supposed to be the head is on
>the inside and the tail is on the outside of the supply reel), all of the
>tracks are upside-down,
>no? So if all of the tracks are upside-down, why doesn't polarity reverse
>for all of them?
>
>-- Tom Fine
>
>----- Original Message -----
>From: "Hood, Mark" <[log in to unmask]>
>To: <[log in to unmask]>
>Sent: Wednesday, March 11, 2015 1:08 PM
>Subject: Re: [ARSCLIST] Tape dubbing backwards?
>
>
>>In our research for the Sound Directions project at the IU Archives of
>>Traditional Music, we discovered and documented the following:
>>
>>1.  The polarity of the reproduced signal is not affected by the
>>direction
>>of tape travel - it is the same whether the tape is traveling past the
>>playback head in the same direction it was recorded (©řforward playback©÷)
>>or the direction opposite that in which it was recorded (©řreverse
>>playback©÷).  You can observe this easily if you have a tape deck that
>>tolerates purposeful mis-threading of the tape path to play the tape in
>>reverse direction without interchanging and inverting the reels.  We used
>>to do this (threading the tape on the wrong side of the captan shaft and
>>backwards around the pinch roller) to back-cue machines a specific number
>>of seconds in radio program production.
>>
>>This seems to me to be analogous to playing a disc forward or backwards -
>>the stylus moves in the same direction when tracing the groove modulation
>>regardless of which direction the stylus is traveling along the length of
>>the groove.
>>
>>
>>2.  The polarity of the reproduced signal IS inverted if the tape is
>>played upside-down from the orientation in which it was recorded, i.e.,
>>what was the top edge of the tape is now closest to the top plate of the
>>deck.  If you take a full-track recording and ©řturn the tape over©÷ by
>>interchanging the supply and take-up reels AND inverting them, you will
>>then be playing the tape ©řbackwards©÷ (reverse playback) AND inverted
>>(original top edge of tape is now on the bottom boundary of the reproduce
>>head).  The reproduced waveform will be the opposite polarity of what you
>>observe in normal, forward playback configuration, but this is due ONLY
>>to
>>the inversion of the tape, not the change in playback direction.  Playing
>>the tape in either direction will produce the same signal polarity.
>>
>>Mark Hood
>>Indiana University
>>
>>
>>On 3/11/15, 12:03 AM, "Ellis Burman" <[log in to unmask]> wrote:
>>
>>>That is the right hand rule.  Your thumb points in the direction of the
>>>current, and your fingers curl in the direction of the magnetic field
>>>around the wire.
>>>
>>>True, if a magnet is moving towards a pickup, it'll produce voltage
>>>opposite that of when it is moving away from it.  But I see it like
>>>Jamie
>>>does - N is N, + is +.  I guess I'll have to try it and prove it to
>>>myself.
>>>
>>>Kinda reminds me of the reason I've heard was to why people store tape
>>>tails out - so that the print through is a post echo instead of a
>>>pre-echo.  I never understood that one either.  It's the same distance
>>>from
>>>oxide layer to oxide layer, so why would it matter?  Why would the
>>>magnetic
>>>field have a "preference" in any direction?
>>>
>>>Ellis
>>>
>>>On Tue, Mar 10, 2015 at 2:58 PM, DAVID BURNHAM <[log in to unmask]>
>>>wrote:
>>>
>>>>It's not analogous to playing a record backwards.  If you think of a
>>>>phono
>>>>cartridge, as a wave of the groove moves the stylus towards the
>>>>spindle,
>>>>the output will be of a certain polarity; if the stylus moves towards
>>>>the
>>>>edge of the record, the opposite polarity will be presented.  Whether
>>>>the
>>>>record is playing forwards or backwards the wave towards the spindle
>>>>will
>>>>always be towards the spindle and the polarity won't be reversed.
>>>>With tape, however, if the tape is playing backwards, the magnetic
>>>>signal
>>>>on the tape will be exciting the coils in the opposite direction,
>>>>causing a
>>>>reverse in the polarity.  You might remember from school, (if you took
>>>>the
>>>>same courses as I took), where they drop a magnet through a coil and an
>>>>electric signal is present at the terminals of the coil.  If the magnet
>>>>goes through the coil in the opposite direction the signal is also in
>>>>the
>>>>opposite direction, plus in one direction minus in the other.  I know
>>>>there
>>>>was a formula where if you hold up your hand with the fingers curled
>>>>and
>>>>the magnet travels in the direction of your thumb, the current will
>>>>flow in
>>>>the direction of your fingers, but unfortunately I've forgotten if it
>>>>was a
>>>>left hand rule or a right hand rule.
>>>>I'm sure somebody knows.
>>>>db
>>>>