Re: [linux-audio-dev] MIDI sync issues; mmc, mtc, ...

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Subject: Re: [linux-audio-dev] MIDI sync issues; mmc, mtc, ...
From: Steve Harris (S.W.Harris_AT_ecs.soton.ac.uk)
Date: Tue Dec 19 2000 - 14:36:09 EET


On Tue, Dec 19, 2000 at 05:50:06AM +0100, pdq808_AT_t-online.de wrote:
> On Mon, 18 Dec 2000, Kai Vehmanen wrote:
>
> > I recorded multiple tracks this way, and to my suprise,
> > they stayed nicely in sync.
> >
> > Now is this the expected behaviour, or is it more common that the speed
> > of various hw devices (and of course, soundcards) vary from time to time,
> > sooner or later resulting in lost sync? Uhm, maybe a little dumb question,
> > but this is something that's hard to verify yourself. Just because my two
> > devices x and y stay in sync doesn't say much really... :(
>
> Kai, I think you already know the answer: you may be lucky now and then
> but in general two devices will not stay in sync. In a five minute track,
> if the x-tals are accurate to 10 ppm, the drift will be 3 ms, i.e. too
> large. I have a soundcard with two independent chips and even they won't
> stay in sync even though they see the same temperature and voltage drift.

Eh? I think MMC re-syncs once per second (or is it beat?), which means
that your maximum drift (at 10ppm) would be about 10 microseconds. Not that
I beleive 10ppm is feasible over MIDI, but in general MMC is fine as
long as you maintain tempo maps between the two devices.

I've done 15 min tracks synced only by MMC and its been fine. Even MIDI
Clock is fine for things that don't have to be accurate to milliseconds,
and that's really crummy.

- Steve


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