[LAD] midi beat clock

Tim E. Real termtech at rogers.com
Sat Feb 20 21:22:50 UTC 2010

On February 20, 2010 02:38:19 pm Tim E. Real wrote:
> On February 20, 2010 01:19:52 pm Paul Davis wrote:
> > On Sat, Feb 20, 2010 at 11:03 AM, Ralf Mardorf
> >
> > <ralf.mardorf at alice-dsl.net> wrote:
> > > If Linux apps would use JACK transport the coders only would have to
> > > take care to handle this. Just having on app doing sync among JACK and
> > > eternal MIDI would reduce the risk of trouble. Btw. MTC should be part
> > > of such an app too ... I didn't read the whole thread, perhaps this was
> > > mentioned before.
> >
> > this isn't true.
> >
> > one of the most deliberate and conspicuous design decisions of the
> > JACK transport API is that there is no support for varispeed.
> >
> > sync requires knowing two things: where we are, and how fast we are
> > going. JACK transport has only 2 answers the the second question
> > (stopped, rolling). this isn't adequate to fully capture the notion of
> > sync.
> MTC: Maybe veering OT a bit but if we could just touch on this for a
> moment. Was going to ask about this anyway, as we had talked about it a
> bit.
> I thought "it's too bad we can't adjust Jack's sample rate on the fly"
>  thus allowing Jack to behave like a Voltage Controlled Oscillator in a
> PLL, syncing to MTC frame.
> I was thinking the VCO 'control' signal would come from a client.
> Else Jack itself would have to support MTC.
> But raises the question - which Jack back-ends allow dynamic sample rate
>  changes? And with a finite resolution small enough?
> I concluded that the best way for me to sync Jack audio with an
>  external MTC is with app-side audio time stretchers.
I forgot to add that if one is faced with external MTC (or any frame-based
 sync signal) then it should really be fed (somehow) into the sound card 
 as a sync signal, shouldn't it? Everything supports SPDIF, but MTC?
Then sound card + Jack + apps are happy, as they all run off it.

And using MTC to send sync between apps just seems odd, as they're
 using Jack already, for audio. 


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