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Alesis ADAT – Affordable Digital Multitrack (part 14)

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Phase Distortion


If you throw a bundle of different frequencies (which could be music) at a
piece of equipment you hope that it will throw them back at you with the lows,
mids and highs all in the same proportion. If it does then we say it has a good
frequency response. One might also hope that they would all come back with the
same relative timings, but in practice the lows or highs may be advanced or
delayed with respect to the mid frequencies. If this happens more than the minimum
amount that corresponds with the frequency response of the equipment (frequency
and phase response are closely linked) then there is phase distortion. Analogue
tape recorders are the worst offenders in this respect. Try recording a square
wave and view it on an oscilloscope – it will come back anything but square.
It used to be thought that this didn’t matter since it still sounds pretty
much, in fact almost exactly, like a square wave. But almost isn’t enough,
and the phase response of digital recorders is much much better.


Drop outs


Analogue tape and digital tape can both suffer from drop outs due to faults
in the tape. The difference is that on analogue the effects can be unnoticeable,
irritating, annoying or disastrous, depending on the drop out’s length
and depth. A digital system can be designed so that its error correction system
can cover up faultlessly the worst drop outs that can be expected in everyday
use. However, really bad drop outs will cause the system to mute, or even if
the muting can be switched off there may be a loud digital splat. Some would
argue that the advantage is in analogue’s favour since you will get at
least something from the tape and drop outs never cause additional noises.


Analogue advantages


It wouldn’t be fair not to mention a couple of things that analogue can
do that digital can’t. Spot erasing a click in a digital recording can
be impossible, difficult or time consuming depending on the system. On an analogue
recorder with a specific spot erase facility it’s straightforward enough.
Even if the recorder doesn’t have the facility it may be possible to do
it by rethreading the tape so that it doesn’t pass between the capstan
and pinch roller and moving the tape by hand. Another analogue trick which used
to be very popular before we had such an array of effects units is to turn the
tape over and record backwards, and even now it’s the only way to get true
reverse reverb. Now how are we going to do it?

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Amendment


In fact the digital clone will play back approximately 20µs later than
the original – I spotted this accidentally and did a rule of thumb measurement
on my oscilloscope. If you did by chance split up a stereo pair of tracks by
cloning one of them and then mixed them into mono you might notice a loss of
very high frequencies, but this would be a very unlikely chain of events. With
all normal copying and track bouncing operations everything else is perfect
(and a 20µs discrepancy is in fact within Alesis’ claim of single
sample synchronisation accuracy).

David Mellor

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David Mellor