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Question from a Audio Masterclass visitor...
"At school we have a digital recording studio. We have a Behringer digital desk, which goes to a Digidesign ADAT converter (not sure what the proper name for it is), which then goes to the computer. Between the desk and the Digidesign, we use optical cables. When we are doing a recording, we get a 'tick' sound at regular intervals. Does anyone know what is causing this? We have deduced that it is created between the optical ports of the desk (it is not the actual desk as the analog outputs do not have this tick) and the ADAT converter, and it is not this as we have put mics directly into this and here is no tick. Could it be the optical cables?"
It's nice to get such a classic question for which there is a classic answer.
Digital audio has a regular structure. If you sample at 44.1 kHz then the data flows at a rate of 44,100 samples every second. Each sample is a measurement of the voltage of the original analog signal.
If you mix (i.e. add) two digital data streams they have to be synchronized together. Otherwise one might change halfway through the sample period of the other. How would the equipment know which samples to add?
More than that, although the sampling rate is nominally 44.1 kHz, it reality it will always be slightly different, either faster or slower. Even if the error is down to a few parts per million, it still adds up over time.
So if you mix two digital signals that are not synchronized, all might go well for a time, until one becomes so far out of sync that it skips a sample.
That causes a 'tick' in the output.
When you send a signal from a digital mixing console to a digital recorder of any kind, then the recorder must be synchronized to the console. It is usually possible to select external sync, and where this is not possible it happens automatically.
When you have more than two pieces of digital equipment, it becomes more complex to decide what to synchronize to what. So the common solution is to use a master clock generator and synchronize everything to that.
When your studio is synchronized in this way, your ticks will disappear forever!