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 intevals. 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?