[alsa-devel] plug:dmix slows playback

[Steve Langstaff]

Hi All.

I've got an installation of ALSA (1.0.19 on an embedded device) where using
plug and dmix as my output device slows down playback - voices and tones are
lower in pitch and the playback takes longer than when not using plug:dmix,
e.g.

/usr/share/sounds/alsa # time aplay ignition.wav
Playing WAVE 'ignition.wav' : Unsigned 8 bit, Rate 8000 Hz, Mono
real    0m 3.08s
user    0m 0.04s
sys     0m 0.07s

/usr/share/sounds/alsa # time aplay -D plug:dmix ignition.wav
Playing WAVE 'ignition.wav' : Unsigned 8 bit, Rate 8000 Hz, Mono
real    0m 6.00s
user    0m 0.09s
sys     0m 0.08s

See how the 'real' time for playing the file goes from 3.08s to 6.00s

I have also noticed that the audio is played out of both (L+R) channels when
not using -D plug:dmix, but is only played out of the right channel when
using -D plug:dmix.

I am an ALSA beginner - any ideas where I should start looking?

Using the -v option on aplay we get more output...

/usr/share/sounds/alsa # aplay -v ignition.wav
Playing WAVE 'ignition.wav' : Unsigned 8 bit, Rate 8000 Hz, Mono
Plug PCM: Route conversion PCM (sformat=S16_LE)
  Transformation table:
    0 <- 0
    1 <- 0
Its setup is:
  stream       : PLAYBACK
  access       : RW_INTERLEAVED
  format       : U8
  subformat    : STD
  channels     : 1
  rate         : 8000
  exact rate   : 8000 (8000/1)
  msbits       : 8
  buffer_size  : 4000
  period_size  : 1000
  period_time  : 125000
  tstamp_mode  : NONE
  period_step  : 1
  avail_min    : 1000
  period_event : 0
  start_threshold  : 4000
  stop_threshold   : 4000
  silence_threshold: 0
  silence_size : 0
  boundary     : 2097152000
Slave: Hardware PCM card 0 'LM4549' device 0 subdevice 0
Its setup is:
  stream       : PLAYBACK
  access       : MMAP_INTERLEAVED
  format       : S16_LE
  subformat    : STD
  channels     : 2
  rate         : 8000
  exact rate   : 8000 (8000/1)
  msbits       : 16
  buffer_size  : 4000
  period_size  : 1000
  period_time  : 125000
  tstamp_mode  : NONE
  period_step  : 1
  avail_min    : 1000
  period_event : 0
  start_threshold  : 4000
  stop_threshold   : 4000
  silence_threshold: 0
  silence_size : 0
  boundary     : 2097152000
  appl_ptr     : 0
  hw_ptr       : 0

/usr/share/sounds/alsa # aplay -v -D plug:dmix ignition.wav
Playing WAVE 'ignition.wav' : Unsigned 8 bit, Rate 8000 Hz, Mono
Plug PCM: Rate conversion PCM (48000, sformat=U8)
Its setup is:
  stream       : PLAYBACK
  access       : RW_INTERLEAVED
  format       : U8
  subformat    : STD
  channels     : 1
  rate         : 8000
  exact rate   : 8000 (8000/1)
  msbits       : 8
  buffer_size  : 2720
  period_size  : 170
  period_time  : 21250
  tstamp_mode  : NONE
  period_step  : 1
  avail_min    : 170
  period_event : 0
  start_threshold  : 2720
  stop_threshold   : 2720
  silence_threshold: 0
  silence_size : 0
  boundary     : 356515840
Slave: Route conversion PCM (sformat=S32_LE)
  Transformation table:
    0 <- 0
    1 <- 0
Its setup is:
  stream       : PLAYBACK
  access       : MMAP_INTERLEAVED
  format       : U8
  subformat    : STD
  channels     : 1
  rate         : 48000
  exact rate   : 48000 (48000/1)
  msbits       : 8
  buffer_size  : 16320
  period_size  : 1020
  period_time  : 21250
  tstamp_mode  : NONE
  period_step  : 1
  avail_min    : 1020
  period_event : 0
  start_threshold  : 16320
  stop_threshold   : 16320
  silence_threshold: 0
  silence_size : 0
  boundary     : 2139095040
Slave: Direct Stream Mixing PCM
Its setup is:
  stream       : PLAYBACK
  access       : MMAP_INTERLEAVED
  format       : S32_LE
  subformat    : STD
  channels     : 2
  rate         : 48000
  exact rate   : 48000 (48000/1)
  msbits       : 32
  buffer_size  : 16320
  period_size  : 1020
  period_time  : 21250
  tstamp_mode  : NONE
  period_step  : 1
  avail_min    : 1020
  period_event : 0
  start_threshold  : 16320
  stop_threshold   : 16320
  silence_threshold: 0
  silence_size : 0
  boundary     : 2139095040
Hardware PCM card 0 'LM4549' device 0 subdevice 0
Its setup is:
  stream       : PLAYBACK
  access       : MMAP_INTERLEAVED
  format       : S32_LE
  subformat    : STD
  channels     : 2
  rate         : 48000
  exact rate   : 48000 (48000/1)
  msbits       : 32
  buffer_size  : 16320
  period_size  : 1020
  period_time  : 21250
  tstamp_mode  : ENABLE
  period_step  : 1
  avail_min    : 1020
  period_event : 0
  start_threshold  : 1
  stop_threshold   : 2139095040
  silence_threshold: 0
  silence_size : 2139095040
  boundary     : 2139095040
  appl_ptr     : 0
  hw_ptr       : 0