[LAD] Jack MTC/MMC slaving

Stéphane Letz letz at grame.fr
Mon Nov 10 15:28:52 UTC 2008

Le 10 nov. 08 à 16:01, Alex Montgomery a écrit :

> Hello and thanks for the response :)
> I've inlined my responses below:
> JACK does not and will not support varispeed. Therefore slaving to MTC
> is going to be pretty limited. MMC also sends some speed commands that
> could not be supported by JACK.
> I understand, but I think limited MTC support is better than  
> nothing. That being said, I don't think it's necessary to change  
> JACK for this purpose, jackctlmmc's approach of being a jack  
> compliant app that can drive JACK transport by listening to MMC  
> signals seems like an elegant approach.
> > There's a small program called jackctlmmc that seems to do want I
> > want, but it's a bit limited (seems pretty hardcoded) and I think  
> that
> > such a desirable feature should be integrated into a more widely  
> used
> > program. Any thoughts?
> what does "hardcoded" mean?
> I made that comment in ignorance. I didn't see a place in the code  
> where you could choose which MIDI device/port you were reading  
> from, so I assumed that it just took the first registered one or  
> hardcoded (specified in code instead of parameters passed into the  
> program) a particular port. On closer inspection, it seems that you  
> either have to patch a midi port to the application via JACK so it  
> knows which one to read from, or it just reads from all alsa midi  
> signals, either of which is perfectly reasonable. I'm new to the  
> JACK and ALSA APIs, and I'm still waiting on a cable order to get  
> my 8-track to talk to my PC, so I can't really test to see how this  
> stuff works.
> Currently jackctlmmc supports the "play", "stop", and "rewind"  
> messages from MMC, and I'm going to play with the code to see if I  
> can add support for "locate" using jack_transport_locate() so that  
> players can have a primitive seek-to type of function. I think that  
> even this limited subset of MMC functionality is very desirable for  
> linux audio engineers. I just want to be able to sit at my 8-track  
> with my guitar and play/stop, rewind, and maybe seek around without  
> having to keep clicking a mouse on a JACK app.
> I'm surprised that jackctlmmc doesn't seem to be that widely used  
> (I can't find any packages for it in the major linux distros, and  
> it only seems to be distributed via source code) when it provides  
> such useful functionality. I'd love to see it's functionality  
> integrated into a GUI app like QJackCtl (which already has JACK  
> transport buttons for play, rewind, fast-forward) instead of being  
> a console application, but that's a subject for another email.
> Thanks for your reply,
> -- Alex

I think this kind of tools is a good candidate to be developed as an  
"in server" JACK client. With the dbus based dynamic access of  
available "in server" clients installed on a machine, it will be  
quite easy to load/unload this kind of tool when needed (Netjack2  
uses this model of "in server" clients).


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