Dmix plugin on raspberry pi & WM8731 DAC

[Jakob Hansen <jakob.hansen@xxxxxxxxxxxx>]

Hello,

I am running raspbian with alsalib version 1.0.25 and driver version k3.8.10+. The kernel is compiled according to the instructions here: http://blog.koalo.de/2013/05/i2s-support-for-raspberry-pi.html. Since the wm8731 does not support hardware mixing, I am using the dmix plugin, but I have a few questions - thanks in advance for any help!

First, I have noticed a distinct degradation in audio quality when using the Master Playback volume control in amixer. Is this an artifact of truncation being done in the hardware, and is it possible to use dmix or any other means to increase the bit depth before the signal reaches the dac? Please excuse any misuse of terminology, this is all pretty new to me.

Second, sometimes setting certain parameters in asound.conf can yield unexpected results, for example in the following pcm declaration taken almost directly from the alsa documentation (this is just one block, not the entire file. ) 
 

pcm.dmixer  {
   type dmix
   ipc_key 1024
   slave {
      pcm "hw:1,0"
      format S16_LE
      period_time 0
      period_size 1024
      buffer_size 8192
      rate 44100
   }

The rate is declared to be 44100, but when I issue 'cat /proc/asound/card1/pcm0p/sub0/hw_params', the rate appears to be 48k. This is after reloading the config file, just for the record. The odd thing is that no matter what the input file format is, it plays back at the correct speed (or pitch level) whether the rate is declared to be 44.1k or 48k. But, if I declare the rate to be 96k, then hw_params shows 96k and the file plays back at double speed, which I suppose makes some kind of sense. Can anyone shed some light on this behavior? Similarly changing the format declaration apparently has no effect whatsoever. Is there some other alsa-specific means of doing rate/format conversions, and is that the key to retaining the sound qualtiy with digital volume attenuation?

Thanks very much for reading.

Jakob

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