There's more or less no way for JACK to do this. Its entire design is "be woken by hardware to process N samples; process N samples; go to sleep". It is a synchronous low latency design that does not offer scope for processing different channels or different outputs at different times.

On Sat, Dec 3, 2016 at 7:20 PM, David Klann <dxklann@gmail.com> wrote:
Greetings,

Long-time Linux user, and relatively new JACK user here. I have built
some audio workstations for the community radio station where I
volunteer (WDRT, Viroqua, WI, US). I recently switched one of the
workstations to use JACK, along with PulseAudio. We use Audacity to edit
audio and we have noticed that the "left" and "right" channels are out
of sync with each other. We have witnessed the "skew" to be as few as
two samples (which is unnoticeable to the ear) to as many as a couple
hundred samples (which sounds a lot like a phase error).

We performed a lot of troubleshooting, including swapping PCI audio
cards (ESI Juli@, Digigram VX222), disabling JACK, running Audacity on
the same hardware booted from a USB stick and a completely different
Debian environment. I am not *completely* confident, but the likely
culprit seems to be JACK.

We have recorded audio using jack-record and experienced the same
left-right channel skew.

Can anyone help point to JACK or OS configuration parameters that we
might look at to get our left and right channels in sync?

Details:

OS: Debian Jessie (8.6), up to date (kernel: 3.16.0-4-amd64 #1 SMP
Debian 3.16.36-1+deb8u2 (2016-10-19) x86_64 GNU/Linux)
JACK (jackd2): 1.9.10+20140719git3eb0ae6a~df
PulseAudio: 5.0-13
ALSA: 1.0.27+1
CPU: Intel(R) Core(TM) i5-2400 CPU @ 3.10GHz
RAM: 8GB
Audio Cards: ESI Juli@ PCI, Digigram VX222HR PCI

Thanks for your consideration!

  ~David Klann


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