Re: [proaudio] Real-Time for audio playback?

[Mark Knecht <markknecht@xxxxxxxxx>]

On Mon, Dec 28, 2009 at 11:09 AM, Grant <emailgrant@xxxxxxxxx> wrote:
<SNIP>
>
> Thanks Mark and Emery.  I don't think my post was very well written.
>
> One of the ways to increase sound quality in digital audio playback is
> reducing jitter.  In the following thread, Dynobot brings up how he
> used the real-time kernel to reduce latency and improve sound quality:
>
> http://www.computeraudiophile.com/content/Linux-audio
>
> Shouldn't reducing latency reduce jitter and thereby improve sound
> quality?  I realize we are talking about very small changes in
> quality, but I'm interested in seeing Linux fully optimized for
> digital audio playback.
>
> - Grant

There are a lot of sources of jitter. I don't know what effect the
kernel has on overall jitter. I wouldn't have thought it to be very
large though as typically jitter is a hardware issue. Of course, if
some sound card is using the system to set its timing then the kernel
could be a big source.

The overriding clock architecture of all the Alsa drivers is that the
sound card is in control of data flow using it's crystal based clock.
Basically, the card asks the system to fill a buffer and then the card
drains that buffer into your speakers. As long as the buffer isn't
starved then sound comes out on time. If the sound card clock has
jitter then it will appear in the sound, but even so I'm not sure that
this will be a big problem for home audiophile applications. (It is
for recording studios but even that is 50% political and possibly not
audible...

What Jack sets up is a way for Jack-enabled applications to talk to
the sound card in a very deterministic way. Jack expects every app to
supply data on time and in the right format.  Jack then mixes streams
if desired and sends the data on to the sound card, again on time and
in the right format. If you are running Jack then it can report to you
when this doesn't happen correctly and you can debug what the causes
are and fix them. This leads, in my experience, to better sound
because nothing is missing, but it doesn't reduce jitter, at least in
my systems.

Jack and *most* Jack applications are real-time safe so that if you
run the real-time kernel with JAck you can get very low latencies.
This is important in recording applications, but not quite so
important in playback applications. (My opinion...)

Jack runs perfectly with a non-real-time kernel. It just doesn't run
'real time' because the kernel can stall you for long periods of time
while the sound card drains the buffer and you get clocks and pops.
That's what the real-time kernel, along with proper system
configuration for sound cards and audio apps, does for you.

Hope this helps.

Cheers,
Mark