14 Apr
2003
14 Apr
'03
12:26 p.m.
Martijn Sipkema writes:
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..).
I wonder, do there exist cards that allow you to fine tune the word
clock by software ? Given such a card you could have an SMPTE decoder
that adjusts word clock to keep the received SMPTE at nominal speed,
thereby syncing (the speed) of the whole system...
FA
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.