Ladspa (plugin)

From the ALSA wiki

See http://www.ladspa.org/ for an overview of LADSPA. Using LADSPA plugins for standard alsa apps has many uses. My motive to get this working is so i can use a compressor when playing movies with mplayer.

There is a "ladspa" plugin available for alsa's pcm plugin layer. See .asoundrc for and overview on how to edit this configuration file..

See http://www.alsa-project.org/alsa-doc/alsa-lib/pcm_plugins.html for an overview of all pcm plugins available. The info we are interested in is on the bottom of that page..

Here's an entry from the asoundrc.txt included in the alsa-lib source package (It searches the "/usr/lib/ladspa" directory for a .so file that has the plugin "delay_5s" stored in it. The controls section sets the delay time and the dry/wet level i think):

   pcm.ladspa {
       type ladspa
       slave.pcm "plughw:0,0";
       path "/usr/lib/ladspa";
       plugins [
           {
               label delay_5s
               input {
                   controls [ 0.8 0.3 ]
               }
           }
       ]
   }

Using aplay -Dplug:ladspa some_sound_file.wav will play this file with the requested delay. To save the "plug:" prefix we can just define another pcm device called "pladspa":

   pcm.pladspa {
       type plug
       slave.pcm "ladspa";
   }

We can use this pcm device by using aplay -Dpladspa some_sound_file.wav

The usage of the plug plugin is important because LADSPA plugins only can han handle FLOAT data. Most sound data is usually saved as some sort of Integer data..

Ok, back to my objective. Getting compressed sound out of mplayer. I can use the above pcm device for playback with mplayer:

   mplayer -ao alsa:device=pladspa some_movie.avi

All alsa apps that can be configured to use a specific pcm device should be compatible (i have to use "-abs 1" to make mplayer work with any pcm-device. Depending on your hw this might be necessary, too). So installing a compressor should just include finding the right label string for the above pcm.ladspa definition and setting the parameters appropriately..

Ok, i found the "Dyson Compressor" and here is my corresponding .asoundrc entry:

   pcm.ladspa {
       type ladspa
       slave.pcm "plughw:0,0";
       path "/usr/lib/ladspa";
       plugins [
           {
               label dysonCompress
               input {
                   #peak limit, release time, fast ratio, ratio
                   controls [1.0 0.1 0.1 0.9]
               }
           }
       ]
   }

If somebody might have a hint on how to set the parameters to effectively compress typical movie audio streams, so they loose some of their dynamic range (nice for watching movies late at night, so they don't wake up the neighbours at loud spots), please add them here :)

- kokoko3k

You could add a limiter after the compressor and tweak some parameters. The one i'll use is fastLookaheadLimiter (from swh-plugins). My .asoundrc follows:

LadComp: in -> compressor -> limiter -> out

 pcm.ladcomp {
      type plug
      slave.pcm "ladcomp_compressor";
  }
 pcm.ladcomp_compressor {
      type ladspa
      slave.pcm "ladcomp_limiter";
      path "/usr/lib/ladspa";
      plugins [
          {
              label dysonCompress
              input {
                  #peak limit, release time, fast ratio, ratio
                  controls [0 1 0.5 0.99]
              }
          }
      ]
  }
 pcm.ladcomp_limiter {
      type ladspa
      slave.pcm "plughw:0,0";
      path "/usr/lib/ladspa";
      plugins [
          {
              label fastLookaheadLimiter
              input {
               #InputGain(Db) -20 -> +20 ; Limit (db) -20 -> 0 ; Release time (s) 0.01 -> 2
               controls [ 20 0 0.8  ]
              }
          }
     ]
  }

As you see, the sound is first "smoothed" by dysonCompress slowly (release time=1) then it is passed to fastLookaheadLimiter which pumps it with 20db and limits it to 0 db. If you feel that this filter is too strong, you may want to lower the input gain replacing

 controls [ 20 0 0.8  ]

with

 controls [ 10 0 0.8  ]

And so on, then play with mplayer: mplayer MyNotSoLoudMovie.avi -ao alsa:device=ladcomp

- tapas

You can find out which plugins you have on your system by installing the ladspa-sdk (at least that is the name of the Debian package).

• listplugins -- prints out a list of all plugins found in LADSPA_PATH.

• analyseplugin -- gives you details about a plugins i.e. what controls exist

• applyplugin -- applies a plugin to a wav file to hear how the plugin sounds

- jzedlitz

I figured out that many of the swh-plugins (like buttworth-bandpass filters) are only working with recent versions of alsa-lib. With 1.0.3b I got segfaults. After a change to 1.0.5 they are working fine!

- julian

Since Alsa 1.0.11rc2 multichannel Ladspa plugins can now be used

- rob

When using ladspa plugins in .asoundrc as above, aplay says:

aplay: set_params:860 Broken configuration for this PCM: no configurations available

What is wrong?

- michael

[edit] See also

• Low-pass filter for subwoofer channel (HOWTO)