Subject: Re: [linux-audio-dev] MTC, SMPTE, etc.
From: Paul Winkler (slinkp23_AT_yahoo.com)
Date: Sat Jul 22 2000 - 00:05:30 EEST
Paul Barton-Davis wrote:
>
> In message <3978AB66.F0E9B7C9_AT_yahoo.com>you write:
> >Paul Barton-Davis wrote:
> >>
> >> >I'd rather say the solution is to not only decode the SMPTE frame,
> >> >but also keep track of it's exact position in the audio stream. Works
> >>
> >> who says its part of the same audio stream ? i would tend to have
> >> SMPTE streaming in through a cheap interface, and not taking up a
> >> channel on a good multichannel card.
> >
> >Sorry if this is a dumb question, as I've never done anything
> >with SMPTE... but wouldn't this have exactly the same problems as
> >trying to sync audio data from different cards?
>
> i'm not sure. SMPTE is not even close to frame-accurate (the finest
> resolution is 1/30 second or thereabouts). there is therefore a lot of
> room for sloppiness.
> the situation i was thinking of was not multiple external devices,
> both synced, with SMPTE coming in over one audio interface and audio
> coming over another. rather, i was thinking of a situation with an
> external audio source, and external SMPTE source (say, a video
> playback device).
Well, how much is "a lot of room for sloppiness"?
Let's see if I understand the situation (it's entirely possible I'm
being rather dense). Let's say you have some audio coming in through
your nice Hammerfall or whatever. Let's say the audio is from a
microphone: you're doing sound effects for a video, and you're
recording a foley artist. The foley artist is wathching video
playback on a device which sends SMPTE code to your computer, and
it's coming in through your Soundblaster or whatever, because you
have managed to use up all those Hammerfall inputs. Is this the kind
of situation you're thinking of?
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. In that
situation, if SMPTE comes in as audio data to be decoded, it's still
going to be WAY off much sooner than you'd like, regardless of SMPTE
resolution. So you'll never get your foley artist's sound effects
synced up to video this way, unless your recording software and / or
SMPTE decoder can compensate for the clock frequency difference
between your Hammerfall and Soundblaster. I believe people on this
list have mentioned tackling that problem at some point, but I
haven't heard anyone say they've done it yet.
................. paul winkler ..................
slinkP arts: music, sound, illustration, design, etc.
web page: http://www.ulster.net/~abigoo
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