On Thu, 12 Mar 2015 21:21:34 -0700 (PDT), Phil Allison
wrote:

Johny B Good wrote:

I was going to mention the effects of phase shift on the harmonics of
a 500Hz squarewave played back from analogue magnetic tape where,
despite the oscilloscope display showing it looking nothing like a
square wave, it still sounds just like the original squarewave.


** That is not true of the vast majority tape machines which are well capable of reproducing a good square wave at 500Hz.

True of any decent cassette deck or R to R and includes Roland tape echo machines as well.

My standard test for a Roland 201, 303 and 501 models involves recording a 500Hz square wave and I except to see a good looking result on the scope.

Sure you were not thinking of 50Hz?

I can't recall the actual test frequencies used, possibly but 50Hz
seems rather on the low side to me. I reckoned 500Hz as being not to
high to limit the odd harmonic content unduly in the tape playback.

Certainly when it came to preserving the waveshape when recording
such mid frequency (circa 2.4KHz at standard cassette speed) 'squarish
waveforms' in a Bi-Phase Manchester coded digital stream, a phase
compensation filter was required before the recording amp to preserve
some modicum of the original waveshape in order to aid reliable
detection of the zero crossings required to successfully decode the
data stream.




Also
the trick, when dubbing copies from a master tape, of playing the
master and slave recorders in reverse to cancel the effect of the
phase shifts, largely restoring the waveshape back to a semblence of
'squareness'.

** Very hard tobeiev it worked or was in any way necessary.

Well, I guess the concern was that dubbing a source already
"afflicted" by such phase 'distortion' would accumulate to the point
of becoming an audible, if subtle, effect. The idea being that using
such a technique would improve the second generation copies by
cancelling out this particular form of 'distortion'.

As to whether it was really necessary seems to be a moot point other
than to dedicated "Hi-Fi Purists" looking to minimise any and _all_
distortions in their recordings.

Since you needed two machines capable of such reversed direction
playback and recording (or machines that could be modified to do so
with bulk erased tape allowing the erase head to be defeated), this
was yet another experiment I (and doubtless the majority of others
similarly interested) didn't try to repeat.



I guess the phase shift sensitivity reference Trevor is thinking of
is in relation to stereo detection of the direction of sound sources
rather than the percieved quality of the sound by each individual ear.


** For certain.


My thought too but I thought it would be a little presumptious of me
to claim absolute knowledge of what Trevor was thinking. :-)
--
J B Good