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Interview with Lennart Poettering (LinuxFR.org)Interview with Lennart Poettering (LinuxFR.org)Posted Jul 6, 2011 22:32 UTC (Wed) by drag (subscriber, #31333)In reply to: Interview with Lennart Poettering (LinuxFR.org) by jrigg Parent article: Interview with Lennart Poettering (LinuxFR.org)
>JACK will only handle one input device and one output device at a time.
No, I don't think that is true at all. I haven't used Jack in a while, but it seems with it's routing capabilities and qjackctl (to enable easy access to those capabilities) it should have no problem at all handling multiple devices.
In a system with using multiple devices at the same time it may become problematic to try to get low latency and multiple simultanous output working with correct timing. Probably need some mechanism to keep the clocks on your sound cards in sync and such things.
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Interview with Lennart Poettering (LinuxFR.org) Posted Jul 6, 2011 22:45 UTC (Wed) by tialaramex (subscriber, #21167) [Link]
Yes, pro-hardware is capable of using an external clock. So you pick one reference clock and all your gear keeps time with that.
Most consumer hardware does not offer this capability, you will need to investigate nasty hacks if you want to try to strap three cheap PCI soundcards together as an alternative to buying one six channel pro card.
JACK Posted Jul 6, 2011 22:53 UTC (Wed) by tialaramex (subscriber, #21167) [Link]
Oh, I forgot to address the actual point you made. No, JACK only works with one device.
If you have several physical devices which are genuinely sync'd you can just use ALSA to weave them together. If they aren't synchronised bad stuff may happen.
The reason for working with one device is that JACK's graph execution goes like this:
1. Interrupt from hardware indicates that, say, 128 frames are ready
If JACK was willing to work with more than one device, which interrupt does it wait for? What does it do with parts of the graph which require data from more than one input device? Or send data to more than one output device?
Lots of complexity, for a corner case no professional audio people care about (they either have only one device, or their devices can be clock-locked and woven into a single virtual device) and no consumers have the gear to test.
Interview with Lennart Poettering (LinuxFR.org) Posted Jul 7, 2011 19:32 UTC (Thu) by jrigg (subscriber, #30848) [Link]
I was referring specifically to combining several interfaces of the same type into a single virtual device, eg. using three 8-channel interfaces as a single 24-channel device, or a couple of 64-channel MADI interfaces as a single 128-channel device.
JACK has no trouble working with multiple devices as long as you only want to use one at a time. It can use different devices for capture and playback, but it can't combine several hardware devices into a single virtual device as I described above. That's what the pcm_multi plugin in ALSA is for (and as you mentioned, hardware clock sync is required).
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