Proper way to transfer digital signal via RCA SPDIF? Why am I getting "clicks"?

[666666]

I needed to record audio into Pro Tools LE (001) from a Tascam HD-P2 portable digital deck. Both have RCA SPDIF ports.

I had PTLE running as master clock and I set the Tascam to lock to the clock coming from the SPDIF port. I connected one SPDIF cable from the Tascam SPDIF out to the PTLE SPDIF in... the Tascam would not see the clock at that point... I had to run a second SPDIF cable from PTLE SPDIF out to the Tascam SPDIF input to get the clock in. At this point the Tascam indicated that it DID see the clock coming from PTLE SPDIF, the audio played from the Tascam into PTLE just fine, sounded great.

Both PTLE and the Tascam were set to 24-bit, 44.1k. Proper Apogee digital SPDIF cables were used.

After recording say an hour of music, upon listening back to the recorded program in PTLE, tiny digital "click" noises were observed in the program that did not exist in the original. Maybe once every minute or so you'll hear one low level high pitched "click". I zoomed in on a few of the "clicks" in the DAW and indeed, in the waveform at the sample level, you can see a sharp "spike" at the click points on both stereo channels... the click noises are in the audio.

So, somehow these "clicks" occured during this SPDIF transfer.

I have had similar problems in the past with digital transfers via SPDIF. What am I doing wrong?

Perhaps I should add an addtional word clock cable and run clock through there? Is the word clock path more reliable perhaps? Only thing though, PT LE (Digi-001) does not have a WC I/O.

What other issues might I be having?

One friend speculated that it might be an issue with the computer / DAW itself, because we were maxing out the computer, running lots of tracks etc, computer was darned near choking, but it WAS running fine... just maxed out. I wouldn't think this would be an issue, but.....?

One thing I noted... the two SPDIF cables used were of slightly different lengths... not sure if that would make any difference. Of course one SPDIF cable was bringing clock from PTLE to the Tascam... and the other SPDIF cable was bringing the audio from the Tascam to PTLE.

Any help, ideas, tips etc would be appreciated... thanks!

[seaneldon]

I'd try it again running at a higher buffer rate. Also, how long was the SPDIF cable you used? I've found that running over 6ft with a SPDIF signal can bring these kinds of clicks into the audio...

[balanceman]

I would make the Tascam the master clock and slave your le rig to that.
it seems more correct to have the recording machine slave to the playback machine.

[John Willett]

Originally Posted by balanceman

I would make the Tascam the master clock and slave your le rig to that.
it seems more correct to have the recording machine slave to the playback machine.

Yes, the recording machine needs to be the slave and the playback the master.

It sounds like the glitches are occurring because the recording machine is trying to make the playback machine play back at a slightly different clock-time from what it recorded at.

Set PTLE as slave and Tascam as master when dubbing from Tascam to PTLE.

[666666]

Guys, thanks! Seems like some really good info here. Sorry to be such a digital-dummy

Edit: the SPDIF cables I used were not longer than 6 feet... and as for the buffer in the DAW... the person I was working with DID suggest that the buffer may have been an issue the way it was set... I did not check it personally... again, I'm a bit less familiar with all this modern digital stuff.... but I'll now know what to be looking at next time. Thanks!

[Baithak]

I have had similar problems with Spdif transfers mainly Tascam DAT machines and occasionally the SD 744 in transferring to the Masterlink. I have had success if one of the machines does not operate on mains power and is operating on batteries. I don't exactly know why this has been the case but once the mains power (wall wart) is disconnected the clicks have disappeared.

Good luck,
Baithak