[alsa-devel] First 50 milliseconds of audio clipping with dmix

[Tyler Conant]

I'm trying to use dmix on an OMAP processor. I'm experiencing the first 50 ms of the sample getting clipped. When I aplay directly to the hw:0 I don't get the clipping. The difference I see between the alsa configuration between dmix and hw:0 is the start_threshold, stop_threshold and silence_size. When it's direct to hw:0 the start and stop threshold are equal to the buffer size. For dmix the start threshold is 1 and the stop threshold is 1445199872. Is this dmix which is setting the start and stop or is this aplay. I'm only guessing this is the reason why the first part is clipping. The only bug which seem similar is http://bugs.gentoo.org/show_bug.cgi?id=218641 and it's already been closed out. I'm using alsa lib 1.0.23


with hw:0

Audio_Prompts # aplay -v -D hw:0 Click03.wav 
Playing WAVE 'Click03.wav' : Signed 16 bit Little Endian, Rate 44100 Hz, Mono
Hardware PCM card 0 'TWL4030' device 0 subdevice 0
Its setup is:
  stream       : PLAYBACK
  access       : RW_INTERLEAVED
  format       : S16_LE
  subformat    : STD
  channels     : 1
  rate         : 44100
  exact rate   : 44100 (44100/1)
  msbits       : 16
  buffer_size  : 22052
  period_size  : 5513
  period_time  : 125011
  tstamp_mode  : NONE
  period_step  : 1
  avail_min    : 5513
  period_event : 0
  start_threshold  : 22052
  stop_threshold   : 22052
  silence_threshold: 0
  silence_size : 0
  boundary     : 1445199872
  appl_ptr     : 0
  hw_ptr       : 0
/opt/axonapp/Audio_Prompts # 


With dmix:

/Audio_Prompts # aplay -v Click03.wav 
Playing WAVE 'Click03.wav' : Signed 16 bit Little Endian, Rate 44100 Hz, Mono
Plug PCM: Direct Stream Mixing PCM
Its setup is:
  stream       : PLAYBACK
  access       : RW_INTERLEAVED
  format       : S16_LE
  subformat    : STD
  channels     : 1
  rate         : 44100
  exact rate   : 44100 (44100/1)
  msbits       : 16
  buffer_size  : 22052
  period_size  : 5513
  period_time  : 125011
  tstamp_mode  : NONE
  period_step  : 1
  avail_min    : 5513
  period_event : 0
  start_threshold  : 22052
  stop_threshold   : 22052
  silence_threshold: 0
  silence_size : 0
  boundary     : 1445199872
Hardware PCM card 0 'TWL4030' device 0 subdevice 0
Its setup is:
  stream       : PLAYBACK
  access       : MMAP_INTERLEAVED
  format       : S16_LE
  subformat    : STD
  channels     : 1
  rate         : 44100
  exact rate   : 44100 (44100/1)
  msbits       : 16
  buffer_size  : 22052
  period_size  : 5513
  period_time  : 125011
  tstamp_mode  : ENABLE
  period_step  : 1
  avail_min    : 5513
  period_event : 0
  start_threshold  : 1
  stop_threshold   : 1445199872
  silence_threshold: 0
  silence_size : 1445199872
  boundary     : 1445199872
  appl_ptr     : 0
  hw_ptr       : 0
/opt/axonapp/Audio_Prompts # 


asound.conf:
pcm.dmixer {
  type dmix
  ipc_key 2048
  ipc_key_add_uid true
  slave {

      channels 1
      pcm hw:0
      rate 44100
      period_time 125011
      period_size 5513
      buffer_size 22052
  }
}

pcm.asymed {
  type asym
  playback.pcm "dmixer"
  capture.pcm "hw:0,0"
}

pcm.!default {
  type plug
  slave.pcm "asymed"
}