ffmpeg alsa recording multiple applications audio

Posted 01-28-2016 at 10:56 PM by number22
Updated 02-07-2016 at 03:30 AM by number22

Tags alsa, dmix, html5, loopback, recording

The trick is using dmix on loopback device:
2 dmixers; one for regualr onboard audio; the other for loopback virtual device, then duplicates streams send to onboard audio card, so you can hear what is being sent to loopback device.

Need to load snd_aloop module first;

Code:

modprobe snd-aloop index=2 id=Loopback pcm_substreams=1 enable=1

My first audio device is onboard soundcard; 2nd is HDMI card, so I put index=2 for Loopback device.

file for .asoundrc

Code:

# .asoundrc
# periods INT # when buffer_size or buffer_time is not specified
#defaults.pcm.rate_converter "speexrate_medium"

pcm.snd_onboard { type hw card 0 device 0 }
pcm.snd_loopout { type hw card 2 device 0 channels 2 }
pcm.snd_loopin { type hw card 2 device 1 channels 2 }

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

pcm.dual {
      type asym 
      playback.pcm "looptoreal"
      capture.pcm "dsnooploop "
}

pcm.looptoreal {
    type plug 
    slave.pcm {
          type multi
          slaves {
                 a { channels 2 pcm "dmix_onboard" }
                 b { channels 2 pcm "dmixloop" }
          }
          bindings {
                 0 { slave a channel 0 } 
                 1 { slave a channel 1 }
                 2 { slave b channel 0 }
                 3 { slave b channel 1 }
         } 
    }
    route_policy "default"  #default; duplicate; and copy
    #Manual setting up ttable if you have problem of out sync audio with video in flash.
      # ttable.0.0  1
      # ttable.1.1  1
      # ttable.0.2  1
      # ttable.1.3  1
#    ttable [ [ 1 0 1 0 ] [ 0 1 0 1 ] ] #same as above


}

pcm.dmix_onboard {
     type dmix
     ipc_key 234567
     ipc_key_add_uid false
     ipc_perm 0660
     slave.pcm "snd_onboard"
     slave {
           rate 48000
           channels 2
           buffer_time 0
           period_size 4096 
           buffer_size 8192 
           format S32_LE
     }
     bindings {
          0   0
          1   1
     }
}

pcm.dmixloop {
     type dmix
     ipc_key 345678 
     ipc_key_add_uid false
     ipc_perm 0660
     slave.pcm "snd_loopout"
     slave {
           rate 48000
           channels 2
           buffer_time 0
           period_size 4096 
           buffer_size 8192 
           format S32_LE
     }
     bindings {
          0  0
          1  1
     }
}

pcm.dsnooploop {
      type dsnoop
      ipc_key 456789
      ipc_key_add_uid false
      ipc_perm 0660
      slave.pcm "snd_loopin"
      slave {
             rate 48000
             channels 2
             buffer_time 0
             period_size 4096
             buffer_size 8192 
             format S32_LE
       }
       bindings {
             0  0
             1  1
       }
}

pcm.duplexloop {
        type plug
        slave.pcm {
              type asym 
              playback.pcm "dmixloop"
              capture.pcm  "dsnooploop"
        }
}

pcm.card0 { type plug pcm "dual" }
clt.snd_onboard { type hw card 0 }
clt.!default { type hw card 2 }
ctl.multiloop { type hw card 2 }
ctl.mixloop { type hw card 2 }
clt.dmixer { type hw card 0 }
ctl.snd_loopout { type hw card 2 }
ctl.snd_loopin { type hw card 2 }
ctl.dsnooploop { type hw card 2 }

ffmpeg command to capture onboard sound card's microphone input and running desktop application's audio:
turn on capture in alsamixer, default is muted.

Code:

ffmpeg -f alsa -acodec pcm_s32le -ac 2 -ar 48000 -i hw:0,2 
       -f alsa -acodec pcm_s32le -ac 2 -ar 48000 -i duplexloop
       -filter_complex amix=inputs=2
       -f x11grab -r 25 -s 1366x768 -i :0.0+0,0
       -vcodec libx264 -preset veryfast -crf 18
       -acodec libmp3lame -ar 48000  -pix_fmt yuv420p
       -y test.mkv

note:
using buffer_time = 0 or period_time = 0 is optional for buggy card; using buffer/period_size or buffer/period_time; but not both. format S32_LE works great for my card.

Changes:
for my card, I have to use period_time and period_size to fine tune:

If you have problem with flash player hogs one cpu core;
1. test sample rate 48000 back to 44100.
or and
2. using export FLASH_ALSA_DEVICE=plug:dmix_onboard

Code:

period_time=50000 #125000usecs. default 
period_size=2048

note: html5 audios problem 
period_time = 15625 #125000 / 8
period_size = 1024

Thanks for hints from

https://unix.stackexchange.com/quest...ugins-together

https://bbs.archlinux.org/viewtopic.php?id=181922

https://youtu.be/mEV2O8C272g

ffmpeg alsa recording multiple applications audio

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