Re: [linux-audio-dev] Utility for recording audio track with midiclock

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Subject: Re: [linux-audio-dev] Utility for recording audio track with midiclock
From: Gerrit Niestijl (g.niestijl_AT_rijndam.nl)
Date: Thu Jul 01 2004 - 17:16:44 EEST


> getting a general purpose computer to output MIDI Clock (and/or MIDI
> Time Code, just so nobody confuses the two of them) is a very hard
> problem. There are 24 MIDI Clock messages per quarter note. This means
> that for a piece in 4/4 at 120bpm, you need to output 1 MIDI byte
> every 20ms. Not so bad - its a nice even multiplier of the system
> interrupt frequency. However, just change the tempo or the time
> signature, and all of a sudden you have situations where the MIDI
> Clock byte needs to be output every 18ms or every 32ms or ever 9.7ms
> or every 56.5ms.

Yes, our music involves many (subtle) tempo changes, accelerando's
etc. Accuracy would be very important.

> until the high resolution clock timers patch is solid enough to be
> used by any system, there is no way to schedule MIDI output with this
> kind of resolution, and if you can't schedule it, then the receiver of
> your MIDI clock signal will see a lot of jitter and may refuse to lock
> to it. Even if it locks, its not clear what it will do with the jitter.

So it will not work then. That is a pity!

>>signal in a audio file, for example with ecasound. I know, this is
>>ugly! For playback i would have the program monitor the jack input port and
>>output a midi clock message when a sample value of 1 passes by.
>
> JACK does block structured audio. This won't work. If the "1" is in
> the model of a JACK process cycle, when does it actually get delivered
> to the MIDI output port?

In my naieve view this would simply be done in the process cycle, but
you suggest it is not possible?

> We'd love to provide this kind of functionality in Ardour, BTW, so if
> you're serious about doing the research and work needed to check on
> the HRT patch etc etc etc, we'd love to see it done in a way that we
> could use too.

I would like to contribute to the Ardour project. But i am neither
a skilled or an experienced programmer, so i doubt if i am capable to
do this. But if there is anything i can do i will do it. Is there any
particular documentation i should look at?

-- 
Gerrit


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