MBox + firmware v22 + S/PDIF = clock sync problems (popping)

[SeasonOfPain]

Following up on the MBox noise bursts thread:

Since applying the v22 firmware update to my MBox 1, I no longer encounter the full-blown noise bursts. However, I'm still hearing intermitant "mini" bursts, basically pops and clicks that are similar in volume to the blasts I was getting but constrained to an extremely short duration (e.g. a few samples). The pops only occur on playback (they aren't recorded in the audio tracks, so I can't confirm exact duration). While not as horrible as the continuous blasts, they are still quite painful while wearing headphones.

I use a Metric Halo ULN-2 for a front-end, connecting to and from my MBox via S/PDIF. I use the MH as an external clock, and this type of popping/clicking has been reported as a common side-effect of problems syncing to an external clock. Both the MBox & MH are set up correctly, and audio playback otherwise works. Neither the MBox nor the MH manifest this issue when used on their own; only when connected via S/PDIF.

I'm curious whether anyone else with has encountered this issue, and whether this may have been caused by the v22 firmware update. I also wonder if the issue was there all along, but the full noise blast issue corrected by the v22 firmware simply masked it... the frequency with which is occurs seems to be about the same.