Re: [Qemu-devel] [PATCH] hw/usb/dev-audio.c: make USB audio card sound perfect

[Gerd Hoffmann <kraxel@redhat.com>]

  Hi,

[ context for spice folks: patch was added to qemu increasing usb-audio 
  default buffer size ]

> > What bothers me is that you have no qualms about making latency on
> > everyone's system worse.
> 
> How do you know it makes sound on other people's systems worse? If you have
> actually done any testing, I would like to see the results. 

It's real.  With that change we *do* actually trade latency for better
sound quality.

You probably wouldn't notice with pure music playback.

Higher chance is with video playback.  lip sync issues might show up,
although you probably still have to watch carefully to actually notice.

Anything sending audio both ways and expecting it to run with low
latency (VoIP phone, music jam as mentioned by stefan) is affected even
more.

And we *do* actually just paper over the root cause.  Problem is the
real root cause can is very hard to track down.  It can be pretty much
anywhere in qemu, and even outside qemu.

One known issue actually is in spice-server (added spice-devel because
of that).  It does audio processing in the qemu iothread (instead of a
separate thread like it is done for the display channel).  If you turn
off audio compression in spice sound quality suddenly becomes better. I
think this this happens because the latency spikes caused by audio
compression go away.  Happens with intel-hda and windows guests (which
use very small audio buffers).  Not (yet) investigated in detail though.

cheers,
  Gerd