[LAU] Updates

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To: Linux Audio Developers <linux-audio-dev@...>, Linux Audio Users <linux-audio-user@...>
Subject: [LAU] Updates
Date: Wednesday, March 21, 2012 - 3:45 pm

Updates available now at


* Now includes some dirty hacks (implemented via the debug
options parameter so they don't clutter the API) to support
the -L option of


* Fixed the bug that made it fail with Jack period sizes
>= 1024 (filter coefficients going out of range).

* Added some hacks, activiated by the -L option, to make
it work with ALSA's loop devices.

The latter requires some clarification.

The two apps provided by zita-ajbridge were never intented to be
used with the ALSA loop devices. They were developed to provide
additional and full quality capture or playback channels to Jack,
using a real soundcard.

Yet most people trying the first release used them with the
loop devices, to provide a Jack interface to some misbehaving
apps such as flash player or skype. Using the loop device is
required since those apps refuse to work with ALSA's Jack
plugin. It's a dirty hack using an unwanted resampling step,
but it seems there is no working alternative.

The ALSA loop devices presents themselves as hw: devices, but
they don't behave like a real one:

* Timing of period wakeups is quantised to a much too high
value, and produces lots of jitter, worse than even the
most crappy USB interface. It also makes using the device
with small period sizes impossible, as wakeups occur when
it's almost too late. The next cycle will xrun in that case.

* The loop devices have 32 channels. They allow to be opened
with a different channel count at both ends, but then make
a mess of it. For example mplayer's two channels will be
spread out over the available 32, making it run at 16 times
normal speed.

To run zita-a2j or zita-j2a with the ALSA loop device:

* Use the -L option. This forces two things: the sample format
will be 16-bit, and the device is opened in 2-channel mode.
This makes it usable to apps at the other end which can't deal
with floats or more than 2 channels.
Note that just using -c 2 (which is also the default) still
opens the ALSA device with all its channels, as must be done
on a real hw: device, so that won't work.

* Use at least -p 256 -n 3, or -p 512 or higher (on the ALSA
device - Jack's parameters don't matter).



A world of exhaustive, reliable metadata would be an utopia.
It's also a pipe-dream, founded on self-delusion, nerd hubris
and hysterically inflated market opportunities. (Cory Doctorow)

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