I'm testing a few very cheap/small USB interfaces for a non-critical listening situation. In other words, it can kinda suck...but not completely suck. I'm using my MOTU 16A as a control, because I know it's a pretty decent unit.

My first test (testing the DAC) accidentally revealed that when I play the same bit of audio through the cheap interface twice (and record the outputs with the 16A), the two tracks phase/flange with each other...almost as if the audio is slightly drifting and moving around. When the two tracks are recorded from the MOTU 16A outputs, they're locked in with each other and this can't be heard at all.

So does anyone know what the flaw is here? Is it a clock issue with the cheap interface? Something to ignore?

How are you measuring this?

I used a laptop with the cheap interface connected to play a test file, and ran TRS cables from the line outputs of the cheap interface into the line inputs of the MOTU 16A (hooked up to a separate computer).

And when I capture the output of the cheap interface twice (same exact test file), the two recordings phase/flange when lined up and played together. When I invert the phase of one of the files, they come close to nulling for a second or two, then drift away and don't null at all, then it sometimes drifts back and almost nulls again.

When I compare the playback capture file from the cheap interface to the original test file, it also phases and flanges.

When I capture the line outputs of the MOTU 16A in the same manner, the two files almost null and don't drift around and phase/flange like the two files from the cheap interface outputs.

And when I compare the capture of the MOTU 16A to the original test file, it's almost nulls without any phasing/flanging. Even 5 passes through the MOTU DAC almost nulls with the original test file.

Apologies, hopefully I'm being clear.

Clocking!
Even if you'd captured audio two times from a good interface — it would show the same behavior. That's why we have clocks to synchronise converters or interfaces.

But the MOTU 16A doesn’t exhibit this at all. When its captured playback is played against the original test file, or played against another test pass, it syncs perfectly. Almost nulls without any drift. I ran 5 passes through the 16A, and it still (nearly) nulls without any phasing.

I know clock syncing is critical when using two interfaces at the same time in the same project…but since my test file wasn’t recorded on either system, I thought I’d hear the same type of imperfections (noise, frequency response discrepancies) from both DACs. And I was surprised when one was so phase/drifty. You know what I mean? Maybe I’m wrong about that.