Subject: Re: [linux-audio-dev] MTC, SMPTE, etc.
From: David Olofson (david_AT_gardena.net)
Date: Fri Jul 21 2000 - 19:27:37 EEST
On Fri, 21 Jul 2000, Benno Senoner wrote:
> On Fri, 21 Jul 2000, Maarten de Boer wrote:
> > >
> > > I don't think you necessarily mean jitter, do you ? More like latency?
> > > Unless you're referring to times when the SMPTE signal crosses the
> > > fragment (interrupt) boundaries ?
> >
> > Yes, that is exactly what I am refering to. Theoratically, when the SMPTE
> > would have a constant speed, the fragment size could be set to always
> > contain exactly one SMPTE frame. But SMPTE speed is not fixed, and also
> > not all sound cards allow sample-accurate setting of the fragment size.
> >
> > Imagine we use a fragment size of 256, at 22050 Hz (which I found sufficient
> > to recognize the SMPTE), we can be 0.012 sec off. At 30 SMPTE frame/sec, this
> > is more than a third of the SMPTE frame duration.
> >
> > Maarten
>
> So you mean that the application will get new SMPTE frames with up to
> 10msec jitter because the relatively big fragment size/low frequency.
>
> Seems that the solution lies (again) in using small fragmentsizes.
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
for MIDI (see my somewhat irritated post on MIDI interfaces), so I
can't see the problem here...
Of course, you'll still have to wait until a full frame has been
decoded to get a result, but you'll also find out exactly where in
the stream the SMPTE frame was. Still very high quality info for
decent sync code, especially for audio apps.
//David
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