Chapter 7 - Digital Connections

Digital Clock Synchronization

Before talking about digital clocks, itÕs important to understand the Òone clock theoryÓ. This simply states that when you synchronize multiple machines (in this case digital audio recorders, processors, mixers, etc.) there must be one master clock which they all sync to. When there is more than one master clock, these two clocks will eventually drift apart and you will hear clicks and pops in your digital signal.

Digital signals are self-clocking; they carry a word clock along with them. Usually the receiving unit automatically adjusts its sample clock to follow the rate of the incoming digital signal. When the receiving unit must synchronize to another clock--for example, an ADAT synchronized to SMPTE time code--a conflict may occur between the clocks that will result in audible "clicks" in the signal. For this reason, in more complex setups, the Q20 features a 48 kHz IN jack. This is to synchronize the sample clock of the Q20's analog-to-digital converter to the BRC or M20. If you're using the Q20 only to process signals that come via the optical input from the ADATs, this connection is not necessary because the optical clock coming in is already indirectly synchronized by the BRC. The 48 kHz connection IS necessary if the Q20 is the originator (master clock) of the optical for the rest of the system, from its analog inputs, while the rest of the system is seeing the BRC as the master.

A complete diagram of every single possible setup with every possible input would be needlessly complex. Just remember that there can only be one master clock. If the BRC is the master, the Q20 must be synchronized to its clock via the optical or the 48 kHz in. If you have a single ADAT with no BRC, the Q20 becomes the master clock source whenever the ADAT is set to DIGITAL IN and the Q20's Input Audio Source is set to ANALOG.

✪Note: If you experience problems (clicks, etc.) with an ADAT system set to digital input, it may be that the ADATÕs clock source is set to internal, even though its Digital In is pressed. (This will only happen if youÕve set it this way for making tape copies) See your ADAT manual for more information.

Note that the front panel level controls have no effect on a digital signal. The [INPUT LEVEL] controls will affect an analog input being converted to digital, and the [OUTPUT LEVEL] control affects the analog outputs only. If you need to change the levels in all-digital mode, you must do so by editing the program using page 2 of the Routing function.

When the input is set to Digital, the meters will show only the level of the signals on Channels 1 and 2 of the ADAT Optical interface, regardless of which channels are selected by the Q20. However, internal clipping within a program will still be shown by the Clip LEDs.

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Q20 Reference Manual