MANUAL OF ANALOGUE SOUND RESTORATION ... - British Library

2.11 Relative phase
It is even possible to enlarge upon the above ideal, and insist on correct “relative phase”
as well. Please allow me to explain this, even though we shan’t encounter the problem
very often. Practical recording equipment (both analogue and digital) introduces relative
phase shifts between different components of the same sound, which may occur due to
acoustic effects, mechanical effects, or electronic effects. Any piece of equipment which
“rolls off” the extreme high frequencies, for example, also delays them with respect to
the low frequencies - admittedly by not very much, half a cycle at most. Since this
happens every time we shout through a wall (for example), our ears have evolved to
ignore this type of delay.
Many years ago at my Engineering Training School, our class was given a
demonstration which was supposed to prove we couldn’t hear relative phase distortion.
The test-generator comprised eight gearwheels on a common axle. The first had 100
teeth, the second 200, the third 300, etc. As the axle rotated, eight pickup coils detected
each tooth as it passed. The eight outputs were mixed together, displayed on an
oscilloscope, and reproduced on a loudspeaker. The pickup coils could be moved slightly
in relation to the gearwheels. As this was done, the relative phases of the components
changed, and the waveform displayed on the oscilloscope changed radically. The sound
heard from the loudspeaker wasn’t supposed to change; but of course there was one
sceptic in our class who insisted it did, and when the class had officially finished, we spent
some time in the lab blind-testing him - the result was that he could indeed hear a
difference.
But I don’t mention this for the small proportion of listeners who can hear a
difference. I mention it because the elimination of overload distortion may depend
critically upon the correct reproduction of “relative phase.” So I shall be insisting on
reproduction techniques which have this feature, and on using originals (since we usually
don’t know the relative-phase characteristics of any equipment making copies).
2.12 Scale distortion
The third consideration has had several names over the years. The controversy flared up
most brightly in the early 1960s, when it was called “scale distortion”. It arises from the
fact that we almost never hear a sound recording at the same volume as the original
sounds. Various psychoacoustic factors come into this, which I won’t expound now, but
which may be imagined by knowledgeable readers when I mention the “Fletcher-Munson
Curves.” Where does the controversy arise? Because it is not clear what we should do
when the sounds are reproduced at the wrong volume. I think everyone agrees that in the
ideal world we should reproduce the original volume. The trouble for archivists is that we
do not usually have objective knowledge of what this original volume was. A standard
sound-calibration would be needed at every location, and this would have to be included
on every recording. Such calibrations do occasionally appear on recordings of industrial
noises or historic musical instruments, but they are the exception rather than the rule. Yet
every time we have even a tiny scrap of such information we should preserve it.
The acoustic-recording system is again a case where this applies. It was not
possible to alter the sensitivity of an acoustic recording machine during a “take”, so it
would be silly to transfer such a recording without including a calibration signal to link the
original waveform with the transferred version. And I would point out that many early
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