[linux-audio-dev] Re: [Linuxsampler-devel] buffers, periods, & fragments - Linux-audio-dev

18 Sep 2005

Flo, Thanks for the jack suggestion. I definetly do need to spend some 
time working with jack on my system. Also, thanks for the great low 
latency documentation on tapas - ugh!. After getting 2.6 setup with | 
Preemptible Kernel (Low-Latency Desktop), fiddeling with my hardware to 
get the soundcard irq's right, and some tweeking with chrt all I can say 
is "wow. WOW!".
I don't think there is anything you can do on a 2.4 kernel to get close 
to this level of performance and control. Sweet! I almost giggle as the 
entire system temporarily hangs while linuxsampler loads gig files at 
outrageous speed. Also, I wanted to let you know that after a bit more 
hacking I was able to get dshare working with the ice1712. It's awesome. 
I can send linuxsampler out of channels 1-4 and have fluidsynth playing 
out of channels 5 & 6, with 2 channels to spare for ecasound or 
something... all simultaneously, at very low latency, & hardly any 
processor load. I paid for the hardware... might as well take advantage 
of it. Besides, this way I can give my processor a break, under-clock 
it, & keep fan volume very low. Here is a snippet from asound.conf. -Garett

|pcm_slave.66_slave {
    pcm "hw:1,0"
    channels 8
    rate 44100
    buffer_size 256
    period_size 128
}

pcm.66ch1234_dshare {
    type dshare
    ipc_key 18273645
    slave 66_slave
    bindings.0 0
    bindings.1 1
    bindings.2 2
    gindings.3 3
}

pcm.66ch1234 {
    type plug
    slave.pcm "66ch1234_dshare"
}

pcm.66ch56_dshare {
    type dshare
    ipc_key 18273645
    slave 66_slave
    bindings.0 4
    bindings.1 5
}

pcm.66ch56
    type plug
    slave.pcm "66ch56_dshare"
}

Florian Schmidt wrote:

...
  On Thu, 15 Sep 2005 22:40:32 -0600
 Garett Shulman &lt;shulmang(a)colorado.edu&gt; wrote:

  Hello, I have been fooling around with my alsa
asound.conf in an attempt
 to take advantage of the hardware mixer in my ice1712 and have multiple
 apps output to it. 

 The hw mixer on the ice1712 is not doing what one normally refers to as
 hardware mixing with consumer grade cards (i.e. allowing several apps
 concurrent access at the same time). It mixes and routes channels from
 its single 10 in/12 out channel device.

 The way to get concurrent access with an ice1712 based card is _software
 mixing_. That is, ALSA needs to do it (or whatever sounddriver you use).

 I would urge you to take a very long look at JACK though. LinuxSampler
 has excellent jack support and most apps made for peolpe creating music
 usually have jack support, too. You'll save yourself lots of hassles
 (setting up jackd isn't so tough when you read some docs). It is very
 simple to route specific LS patches to specific output channels on your
 ice based card when using jack.

  In order to acomplish this I need linuxsampler to
be
 able to access devices I setup in asound.conf instead of just hw:x,x
 devices. This was easy enough to acomplish by nuking all of the '"hw:"
 +' code from AudioOutputDeviceAlsa.cpp. The next issue relates to the
 fact that the alsa dshare plugin requires that all of the "virtual
 devices" that I create from the ice1712 card share the same buffer_size,
 period_size, periods, & period_time settings. So, linuxsampler crashes
 when it tries to set buffersize and periods. I figured I would just find
 out what linuxsampler was trying to use for those values, setup the same
 values in the asound.conf and then comment the code that sets them in
 linuxsampler. This seems to work except that when linuxsampler connects
 to the device I notice click and pops from the device & the output from
 linuxsampler is rather distorted. I suspect that I'm just not setting
 the values quite right in asound.conf. However, I guess I also may be
 just mangleing linuxsampler beyond proper function. :) I was wondering
 if anywone has any suggestions. When linuxsampler tries to set the
 buffersize it is using FragmentSize=128 and Fragments=2. So I am
 interpreting this as period_size=128, periods=2, & buffer_size=256. The
 slave device in asound.conf looks like this:
 pcm_slave.66_slave {
 pcm "hw:0,0'
 channels 8
 rate 48000
 buffer_size 256
 period_size 128
 periods 2 #I also tried 1 here... counting from 0...
 period_time 0
 }
 Then I create some dshare devices from this and some plug devices for
 each dshare device and connect linuxsampler to one of the plug devices.
 Any suggestions would be greatly appreciated. -Garett 

 dshare still won't allow you to have multiple apps use your ice1712
 card. You'll need dmix for that. Have a look at alsa.opensrc.org if you
 want to go this route. dmix will probably kill latency though.

 But better take a look at jackit.sf.net.

 Regards,
 Flo