[linux-audio-dev] SMTPE/MTC Again

Martijn Sipkema msipkema at sipkema-digital.com
Mon Apr 14 10:00:01 UTC 2003

>  > SMPTE/MTC has a resolution of (typically) 1/30 second. it therefore
>  > cannot be used for sample-accurate positioning. this will still ensure
>  > that everything is in the same place, but it may not be exactly where
>  > you want them to be.
> This problem disappears when you forget about frames and start looking 
> at it just as an encoded form of time epressed in seconds. Depending
> on the signal quality (no problem with today's hardware), you get an
> accuracy of better than a microsecond.
> The only problem with SMPTE synchro is that you have to accept that
> the 'sample time' of your system is irrelevant w.r.t to the
> synchronised content you should deliver, and there is apparently
> fierce resistance against that idea.

I think SMPTE should be used to adjust the word clock, either
using external hardware, e.g. a MOTU MTP AV, or by adjusting the clock
of the audio device in software using varispeed. If the audio device
does not support varispeed one could best forget about SMPTE sync
IMHO (or do sample rate conversion to emulate varispeed).
Using SMPTE in this way means SMPTE time == 'sample time' (, the speed
that is..).


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