Re: [fluid-dev] Re: What is the best way start fluidsynth with zero/low latency? (Louis B.)

[Pedro Lopez-Cabanillas]

On Monday, May 25, 2009, address@hidden wrote:
> Quoting Pedro Lopez-Cabanillas <address@hidden>:
> > On Saturday, May 23, 2009, Louis B. wrote:
> >> Thanks for the info. I have just update the wiki with information on
> >> how to run fluid on a netbook. see
> >> http://fluidsynth.resonance.org/trac/wiki/LowLatency. Please correct
> >> it if it is wrong or you want to add anything else.
> >
> > "Also halving the sample rate with the flag '-r22050' helps a lot."
> >
> > I doubt the above advice would be beneficial for latency. On the
> > contrary, running FluidSynth with a native sound card sample rate will
> > give the better results, because reducing the sample rate requires ALSA
> > to perform software interpolation (by the plughw: layer) to transform the
> > buffers into the native frequency before sending it to the hardware. This
> > requires larger buffers and consumes CPU cycles. On the other hand, using
> > the native frequency allows you to use the hw: interface directly.
> >
> > By the way, which netbook are we talking about? Which Linux distro?
> >
> > Regards,
> > Pedro
>
> If the system is running out of CPU and the sound card actually works
> with 22050 natively, then it could help reduce CPU usage, thus
> preventing CPU starvation.  Another idea is to decrease
> synth.polyphony in that case though.

Louis said that he is using an eeepc 901. According to my sources this device 
has an integrated Intel HDA sound card:
http://array.org/ubuntu/status.html?model=eeepc-901

The HDA cards allow only a very restricted set of native sample rates, 
typically only above 44.1 KHz, so I found very improbable that sr=22050 is 
natively supported. This can be verified compiling and running the attached 
program. Here are some results obtained on my Asus laptop:

$ ./alsa-rate
supported sample rates for hw:0,0 : min=44100, max=192000
$ ./alsa-rate dmix
supported sample rates for dmix : min=48000, max=48000
$ ./alsa-rate plughw
supported sample rates for plughw : min=4000, max=4294967295
$ ./alsa-rate default
supported sample rates for default : min=4000, max=4294967295

Note that "dmix", "plughw" and "default" are not native, they are software 
conversion layers.

Regards,
Pedro

alsa-rate.c
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