Re: [Alsa-user] period size and hardware limits

[Matthieu R. <mat...@be...>]

On Thu, 2006-06-01 at 08:52 +0200, Clemens Ladisch wrote:
> Matthieu Redouin wrote:
> > I am trying to have short latency on my computer. I am using the
> > hda-intel driver and I am wondering about the output latency :
> > - with a buffer size of 64 and period of 2, I get around 1.5ms of output
> > latency
> > - with a buffer size of 64 and period of 4, I get around 4.2ms of output
> > latency ! I don't figure out this big difference.
> 
> What are the units you're using here?  Is your period size in bytes, in
> frames, or the number of period per buffer?  And what are the sample
> format and rate?
sorry, I am using frames for the buffer size and for the period, it is
the number of period per buffer.
The sample format is 32bit LE and the rate is 48000Hz (and 2 channels).

> > Setting period size to 4 allows me to set buffer size to 16 and have an
> > output latency of 1.2ms, whereas period size of 2 and buffer size of 16
> > gives me too many xruns (even with patched kernel).
> > So I am wondering if buff/period size of 64/2 was my hardware limits and
> > 16/4 settings only a trick handled by the driver ?
> 

> Have a look into /proc/asound/card0/pcm0p/sub0/hw_params to see what the
> parameters used by the driver are.
thanks !!!
It helped a lot.
So in fact, my problem is not really about buffer size, but xruns :
- when I start Jack with -p16 (period size in frames) -n4 (number of
period), I get no xruns
- when I start with -p32 -n2, I have a lot !
why ? shouldn't it be the same thing ? And my kernel is already patched.

thanks for the tips,
matt