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Alesis ADAT - Affordable Digital Multitrack (part 14)

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.

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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?

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).

By David Mellor Thursday January 1, 2004
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