Which clock is running my system?

I have an M2tech Evo usb to spdif converter. The Evo has two clocks inside the unit. If I have the digital signal running through the Evo and into my dec, which also has clocks inside, which clock is controlling the D/A conversion? Is the dac slaved to the clock from the Evo, or is the Evo simply passing the clock information onto the dac clock, which is acting as the master clock for the digital front end. This is very confusing. Thanks in advance for your help.
A S/PDIF signal combines clock, data, and various other information into a single signal. In the case of the Evo, the clock information embedded ("multiplexed") in that signal is generated internally within the Evo (assuming you are not using its external clock input). You didn't say what DAC you are using, but chances are that your DAC controls the timing of D/A conversion by using a phase locked loop to generate a clock that is synchronous to (i.e., slaved to) that embedded S/PDIF clock. That will not be the case with some DACs, however, which time D/A conversion independently of the S/PDIF clock.

All of that occurs asynchronously (i.e., with timing NOT being slaved) to the transfer of USB information between the computer and the Evo.

-- Al
The Clock in your EVO is the Master Clock. However, there may be ASRC or asynchronous upsampling hardware inside your DAC that re-clocks the data. In that case, both EVO and DAC ASRC clocks are important. You didn't give any details about your DAC, so we can only guess.

Steve N.
Empirical Audio
The clock in the DAC controls the D/A process.

Both clocks are important, but only the one in the DAC controls the D/A process