[linux-audio-user] some notes regarding sync (JACK, MTC) under Linux

Dave Phillips dlphilp at bright.net
Thu Dec 18 08:53:05 EST 2003


  I followed the recent thread here regarding this issue but I was too 
busy to contribute. Here are a few observations resulting from my own 
tests :

    Hydrogen works very nicely under JACK. However, when Song mode is 
set to Loop the JACK transport works only on the first play through, 
i.e., Hydrogen doesn't loop. Pattern mode looping is fine, but not Song 
mode looping. Also, I'm losing the first beat when the transport starts. 
Anyone else noticing that ?

    Ecasound also works nicely with JACK but I haven't tested all the 
transport functions yet. I just realized that ecasound should be able to 
rewind to start under JACK. Kai, am I right about that ? Where can I 
find a summary of ecasound's current JACK capability ?

    MusE and Rosegarden don't yet work with the JACK transport, but 
that's not news. However, I did sync MusE to external MTC and it worked 
very nicely. I thought I had been able to do the same thing internally 
with the ALSA virmidi module, but upon sober reflection it appears I was 
smoking bad crack. Am I correct in assuming it's not possible to send 
MTC over the virtual MIDI connection ?

    Again not news: Ardour works nicely as a JACK master but it doesn't 
seem to like being a slave (but who does ?). Also, am I correct to note 
that Ardour doesn't respond to incoming MTC or MMC ? I'll have to check 
that again...

    The latest Hydrogen release notes state that MMC/MTC are 
experimentally supported, but I don't see any switches for those 
controls. Has anyone else tried utilizing MMC or MTC with the new Hydrogen ?

    Qjackctl is very sweet (Thanks Rui!), but a rewind transport control 
might be nice to see on it.

    Finally, a non-sync issue (?) that I think has been reported before. 
Sometimes the sound gets distorted while running an app under JACK (not 
necessarily under the transport control). It cleans itself up after a 
little time, but it's a Very Bad Thing to be happening. Instruments with 
lower frequencies seem to trigger the distortion (bass drum, low toms, 
etc. in Hydrogen). Has anyone else noticed and fixed this problem ? In a 
similar vein, I tried recording Hydrogen to  two channels in Ardour, 
running Hydrogen as slave, Ardour as master. The first play-through of 
the Song in Hydrogen records fine in Ardour, but then I get a terrible 
distortion. I can supply a soundfile and screenshot that clearly 
sounds/shows what's happening. Perhaps it's related to the first problem 
above ?

  System here is as follows:

    800 MHz Duron w. 512 MB RAM
    current PlanetCCRMA RH9 w. low-latency kernel
    ALSA 1.0.0rc2
    SBLive Value

  Kudos to all the developers working on fastening their apps to the 
JACK transport model. As joq stated this is one of the most impressive 
aspects of the whole JACK schema, at least for the normal user.

Best regards,

== dp

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