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

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Subject: Re: [linux-audio-dev] MIDI sync issues; mmc, mtc, ...
From: Kai Vehmanen (kaiv_AT_wakkanet.fi)
Date: Tue Dec 19 2000 - 02:05:50 EET


On Tue, 19 Dec 2000 pdq808_AT_t-online.de wrote:

> 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.

Yup, just what I was afraid of. On the other hand, this situation is not
the same as with two separate sound cards. With sound cards you have clock
chips that run at slightly different speed. So as a result, the cards will
go out of sync (from software point of view) even if their speed stays
constant during processing.

In my case, I'm trying to start two complitely separate devices at exactly
the same time, and hope that they run at a constant speed. But, I admit,
there's lots of things that can go wrong.

It's just that this whole sync business is really quite a jungle from app
developer's point of view. As most audio apps usually use the soundcard as
the sync-master, it's very difficult to actually sync to any external
signal. Also, it's very difficult to test anything unless you have a full
blown studio near your development machine, and even if you can do
testing, it's hard to say whether your code will work with other hardware.

And selecting among the various sync mechanisms is another difficult
issue. Plain midi clock has little meaning for a digital audio
recorder/mixer (concept of tempo unknown). Mtc and smpte are very good for
doing precise location searches, but are they good enough for actually
syncing audio apps to hardware devices or to other apps...?

It seems that Ardour has the best set of supported sync methods available
in Linux audio apps. Thread concerning Ardour's mtc implementation (check
the LAD archive; 5-6/2000) is a very interesting read. But it also shows
how difficult these sync issues are for a user-space app to handle. Paul,
have you received any reports how Ardour's mtc-sync-out has worked with
other hardware than your Mackie?

-- 
 . http://www.eca.cx ... [ audio software for linux ] /\ . 
 . http://www.eca.cx/sculpscape [ my armchair-tunes mp3/ra/wav ]


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