When connecting the TC unit on its S/PDIF input, it will slave automatically to the incoming digital signal (44.1 or 48 khz) from your audio interface. Although the audio interface can be set to master clock (which it has to, in this case), its S/PDIF input (the input for the return signal) will work only when the clock is set to slave. This means: if both units are connected digitally, the TC unit will receive signal from the audio interface, but the audio interface will not receive the return signal. Some audio interfaces simply aren't capable of providing master clock AND locking to digital input at the same time, which you will need in this case. The only workarounds are to either:
- Use another audio interface that is capable of doing this (as it is an issue with only some audio interfaces, not all),
- Set up the TC unit in an analog I/O loop instead.
- Swap the roles, so the TC unit generates master clock, and the audio interfaces slaves to incoming clock.