[pulseaudio-discuss] Questions, questions and questions!

Pierre Ossman ossman at cendio.se
Mon Dec 4 00:09:55 PST 2006

Max Kempter wrote:
> 2. Optimise the Debian kernel for PA? 
> I wrote:  Is it a good a idea to use Audio kernel with RealTime Preemption, if I want a low latency system?
> Pierre wrote: Pulse does use any of the realtime syscalls, so I don't think it will get any performance gain.
> I write: Is there an other possibility to optimize the kernel for pulse audio or should I optimise it for AlSA?

If you need to have those 3 ms, then you really need to crank up the
kernel HZ. Normally you have a process switch time of 10 ms, which will
be insufficient for your needs.

> 3. A Problem with Alsa-Source!
> When I try to load my source, I can not adjust the sample rate with:
> shell/
>>>> load-module module-alsa-source rate=8000 source_name=mic
> alsa-util.c: device doesn't support 8000 Hz, changed to 48000 Hz.
> /shell
> But if I try to record with arecord, there is no problem to set the sample rate to 8000Hz?

I would guess that you somehow get a plug (ALSA's name for sample
rate/format conversion) with arecord. Your sound card probably only
supports 48000, so you have to do the conversion somewhere. Might as
well let pulse do it.

> 4. Could you explain the context between latency of alsa-modules to the fragments and fragment_size value? Is it like to set the buffer-size (eg with arecord)? 
> Tell me more than, if you decrease fragments and fragment_size you decrease latency, please.  

Sound card buffering usually consists of a number of buffers (usually
called fragments or chunks) and you transfer one of these buffers at a
time. So a program that just specifies "buffer size", usually hacks that
buffer up into fragments on its own.

If you want low latency you usually want very small fragment, as you in
many cases can only transfer an entire fragment at a time. And as usual,
low latency requires a small total buffer size.

> 5. Is there an advantage to use add-autoload-sink/source instead load-module sink/source? When I load the sinks/sources with autoload I don`t have access to the sinks/sources (eg with play-file).

Hm... you should. File a bug otherwise with complete information about
your setup.

Pierre Ossman            OpenSource-based Thin Client Technology
System Developer         Telephone: +46-13-21 46 00
Cendio AB                Web: http://www.cendio.com

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