Subject: Re: [linux-audio-dev] MTC, SMPTE, etc.
From: Paul Barton-Davis (pbd_AT_Op.Net)
Date: Sat Jul 22 2000 - 04:25:23 EEST
>Let's see if I understand the situation (it's entirely possible I'm
>being rather dense).
No, I think you understand it precisely.
>Last time I tried to get 4 tracks into my computer by using 2
>soundcards, I was using Multitrack (it was the only program for that
>purpose that even remotely worked on my system at the time). I
>fiddled with the buffer delays until I got the tracks to start in
>sync. The cards' clock frequencies were so far off that the tracks
>were at least 1/4 second out of sync after 1 minute. It only takes a
>couple minutes of that to get a full second out of sync.
This seems clear, but it leaves me puzzled. You're feeding analog
signals to both cards "in sync". They only buffer, say, 100ms of
data. How can they ever drift this far apart ? This is a rhetorical
question: I'm not doubting for a moment that they do. I just don't
quite understand how this can happen. I can understand that they are
not using the same sample clock, and so 44100 samples from one card
might represent an elapsed "wall clock" time of 1.02 seconds and 0.98
seconds on the other. But what has actually happened here ? Stretch
this out until we have, say, a full 1 second out-of-sync. Ah, I
see. We've read, say, 441,000 samples from both cards, but the last
sample (in particular) from each card is derived from the analog
signal sampled at a completely different time.
Note, interestingly, how bad this is for the idea of using audio
devices as a timer for, say, the ALSA sequencer ...
OK, I take it back - I guess this cannot work. the SMPTE signal has to
be processed by the same sample clock as the rest of the audio input
(and output, I would guess).
--p
This archive was generated by hypermail 2b28 : Sat Jul 22 2000 - 04:55:05 EEST