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.

[Park Seward]

Yes, sounds like an issue with locking to the external clock. I don't think it has anything to do with the update. What sample rate to you use?

External Clock Not Supported at Higher Sample Rates with Mbox 2 Pro
Although Mbox 2 Pro supports sample rates of up to 96 kHz, its external clock sources (S/PDIF and Word Clock) are only supported in 44.1 and 48 kHz sample rate sessions.

(Taken from the "Read Me" for Pro Tools LE 7.3 for Mac and Windows, Item #79394, 80746, 80961, 76544)

[SeasonOfPain]

I generally use 44.1, 24-bit. I've confirmed that the ULN-2 is set at 44.1 as well. That should be a safe sample rate for either MBox1 or MB2Pro using external clock.

Any other known gotchas with MBox1 external clock sync? Are there any good troubleshooting techniques for isolating external clock sync issues?

[DigiTechSupt]

Just out of curiousity, what type of SPDIF cable are you using - a generic audio cable or an actual coax cable? How long is the cable? Have you tried a different one? What happens if you disconnect the cable going back the the ULN?

[SeasonOfPain]

I'm using a Hosa S/PDIF gold-plated RCA cable (from Guitar Center). The cable is about 1 meter.

I planned to pick up a replacement cable, but didn't want to waste the cash if the cable itself was unlikely to be a factor. Is there a better brand to go with than Hosa?

[Park Seward]

If the cable is only one meter, the cheaper cable will be fine. And yes, you want to use a 75 ohm "video" cable.