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Hi, Mark,

That is a very good distinction to make. However, in the context of 
quarter track recordings and two-track mono recordings where the reels 
were flipped over, the obvious indication that the tape was recorded 
with the edges inverted is that the audio is playing backwards. Yes you 
can play backwards without flipping, but in the case of tapes recorded 
in both directions, that was achieved by flipping in most cases in the 
consumer and professional audio domain.

Therefore, with most tapes since the edges were inverted during 
recording, the polarity is inverted.

In very rare instances (and this is a great outcome of your research), 
serpentine recordings were made on logging and instrumentation (IRIG) 
tapes where the transports would get near the end, reverse, and switch 
tracks. In fact, some 28-track machines could be set up to make 28 
passes recording on a different track each pass. Many IRIG recorders are 
symmetrical-ish transports that perform about the same in both 
directions. In an attempt to reduce wow and flutter, some versions had a 
large diameter capstan and the heads interfaced with the tape while it 
was on this elastomeric capstan.

Some logging recorders (which were generally not of the quality of IRIG 
or audio machines) also could record in a serpentine manner, although 
those were mostly specified for a single pass with multiple tracks.

Cheers,

Richard



On 2015-03-11 1:08 PM, Hood, Mark wrote:
> 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 (³forward playback²)
> or the direction opposite that in which it was recorded (³reverse
> playback²).  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 ³turn the tape over² by
> interchanging the supply and take-up reels AND inverting them, you will
> then be playing the tape ³backwards² (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
>>>
>>>       On Tuesday, March 10, 2015 4:45 PM, Ellis Burman <
>>> [log in to unmask]> wrote:
>>>
>>>
>>>   So, the same program was recorded on tracks 1 and 4?
>>>
>>> Ellis
>>>
>>> On Tue, Mar 10, 2015 at 12:47 PM, Tom Fine <[log in to unmask]>
>>> wrote:
>>>
>>>> Hi Ellis:
>>>>
>>>> No, I was just looking at a scope with spoken-word material on the top
>>> and
>>>> bottom track. They were also recordings from transcription records, so
>>>> there was quite a bit of record noise. I was surprised how many times
>>> there
>>>> would be a loud sound at the same time for both tracks, often enough
>>> to
>>>> verify that azimuth wasn't way off. This is inexact, to say the least,
>>> but
>>>> everything sounded surprisingly good.
>>>>
>>>> The whole 4 tracks at once thing gets into craft vs. science.
>>>>
>>>> -- Tom Fine
>>>>
>>>> ----- Original Message ----- From: "Ellis Burman" <
>>>> [log in to unmask]>
>>>> To: <[log in to unmask]>
>>>> Sent: Tuesday, March 10, 2015 3:18 PM
>>>>
>>>> Subject: Re: [ARSCLIST] Tape dubbing backwards?
>>>>
>>>>
>>>>   Hi Tom.  If is was a 4-track mono tape, how did you check the
>>> azimuth?
>>>>> Was
>>>>> there phase coherent tones or pink noise on all four tracks?  That
>>> seems
>>>>> highly unlikely to me.
>>>>>
>>>>> Best,
>>>>>
>>>>> Ellis Burman
>>>>>
>>>>> On Tue, Mar 10, 2015 at 8:59 AM, Tom Fine
>>> <[log in to unmask]>
>>>>> wrote:
>>>>>
>>>>>   I've never tried doing all four tracks of a quarter-track stereo of
>>>>>> high-fidelity music all at once using my Tascam 44-OB, but I have
>>> had
>>> no
>>>>>> problems doing some OTR (4-track mono) tapes. The quality going in
>>>>>> sucked,
>>>>>> so the client was very happy to save money not paying for 4 passes
>>> across
>>>>>> the heads. I was actually surprised at how good it did sound. He
>>> told
>>> me
>>>>>> his dubs were second-generation from transfers of transcriptions (so
>>>>>> either
>>>>>> 3rd or 4th generation from the transmission line). He had been smart
>>>>>> enough
>>>>>> to use a good quality deck (I forgot he told me it was Pioneer or
>>> Teac,
>>>>>> later-generation so with direct drive capstan and decent azimuth
>>>>>> stability). On my scope, the azimuth looked OK between tracks 1 and
>>> 4,
>>>>>> so I
>>>>>> figured I was probably getting pretty good fidelity out of all 4
>>> tracks,
>>>>>> especially considering the relatively lo-fi source. His smartest
>>> moves
>>> in
>>>>>> making the tapes were doing them at 7.5IPS and using well-slit
>>> Maxell
>>> UD
>>>>>> tape. We also transferred at double speed (7.5IPS of 3.75IPS
>>> material),
>>>>>> and
>>>>>> again this did not effect the sound quality of OTR source material
>>> very
>>>>>> negatively. The guy got 4 hours of transfer material for every half
>>> hour
>>>>>> of
>>>>>> tape machine on the clock time. As I said up front, I would never do
>>> this
>>>>>> for high-fidelity musical recordings.
>>>>>>
>>>>>> -- Tom Fine
>>>>>>
>>>>>> ----- Original Message ----- From: "Richard L. Hess" <
>>>>>> [log in to unmask]>
>>>>>> To: <[log in to unmask]>
>>>>>> Sent: Tuesday, March 10, 2015 11:49 AM
>>>>>> Subject: Re: [ARSCLIST] Tape dubbing backwards?
>>>>>>
>>>>>>
>>>>>>
>>>>>>   Some of the four-track in-line heads cheated down a little from the
>>>>>>
>>>>>>> standard 43 mil track width, but I think it was down to 38 mils to
>>> allow
>>>>>>> for better crosstalk. This is not well documented...but then again
>>> we
>>>>>>> have
>>>>>>> a variation of at least 75-82 mils in "NAB" two track heads. At
>>> this
>>>>>>> point,
>>>>>>> if one is dealing with more than three tracks on 1/4-inch tapes
>>> there
>>>>>>> are
>>>>>>> usually larger issues than this.
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> On 2015-03-09 9:09 PM, Dave Radlauer wrote:
>>>>>>>
>>>>>>>   Careful there, I don't think there's a one to one relation between
>>>>>>>> 4-track
>>>>>>>> and 1/4 track formats, but I'm sure more knowledgeable voices will
>>>>>>>> chime
>>>>>>>> in.
>>>>>>>>
>>>>>>>> Dave R
>>>>>>>>
>>>>>>>>   --
>>>>>>>>
>>>>>>> Richard L. Hess                  email: [log in to unmask]
>>>>>>> Aurora, Ontario, Canada                            647 479 2800
>>>>>>> http://www.richardhess.com/tape/contact.htm
>>>>>>> Quality tape transfers -- even from hard-to-play tapes.
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>
>>>>> --
>>>>> Ellis
>>>>> [log in to unmask]
>>>>> 818-846-5525
>>>>>
>>>>>
>>>>>
>>>
>>>
>>> --
>>> Ellis
>>> [log in to unmask]
>>> 818-846-5525
>>>
>>>
>>>
>>>
>>
>>
>> --
>> Ellis
>> [log in to unmask]
>> 818-846-5525
>
-- 
Richard L. Hess                   email: [log in to unmask]
Aurora, Ontario, Canada                             647 479 2800
http://www.richardhess.com/tape/contact.htm
Quality tape transfers -- even from hard-to-play tapes.