[Alsa-user] Create disfunctional volume control

[Muffinman <ne...@ko...>]

Hello all,

I want to create a disfunctional volume control. I'm running MPD for
playing music which uses hw:0 for playing back sound. However, I want to
route the volume control to a sortof 'null' device and have a
selfwritten application listen for changes in the volume level,
translate and send that data to my amplifier through a serial port
without it affecting the actual audio data stream to my USB-dac. I'm not
sure if it's all going to work, but I figure I have to begin somewhere
to find out. So I'm trying to create that null device first.

Based on the steps I've taken as seen below, can someone give me some
advice on how to approach this task?

Thanks in advance, Maarten

************************************
I created a null device in /etc/asound.conf:
pcm.blackhole {
type null # Null PCM
}

However MPD now says its an invallid CTL blackhole and apparently tries
to hook it up to hw:0 (mpd settings are copied below):
ALSA lib control.c:882:(snd_ctl_open_noupdate) Invalid CTL blackhole
Oct 23 16:27 : Failed to open mixer for 'Ayre QB-9': failed to attach to
blackhole: No such file or directory

If I want to create that CTL, the MPD wiki gives the example below. Now
I'm guessing that if I can create a (virtual) hw:1 things would be a lot
easier, but is that possible? And if so, how?
##################
# mpd volume control
pcm.mpdvol {
  type softvol
  slave.pcm "dmixer"
  control {
    name "MPD"
    card 0
  }
}
# ctrl for mpd volume
ctl.mpdvol {
  type hw
  card 0
}
# src:http://mpd.wikia.com/wiki/Alsa

#####################
######### MPD settings:
audio_output {
        type            "alsa"
        name            "Ayre QB-9"
        device          "hw:0"  # optional
        mixer_type      "hardware" #none"
        mixer_device    "blackhole" # hw:0"     # optional
#       mixer_control   "PCM"           # optional
#       mixer_index     "0"             # optional