Introduction

You can use external programs in liquidsoap to decode audio files. The program must be able to output WAV data to its standard output (stdout).

Basic operators

External decoders are registered using the add_decoder operator. It is invoked the following way: add_decoder(format,decoder), where:

  • format is the name of the decoded format. It is only informative.
  • decoder is a function that takes as parameter the name of the file to decode and returns the command to start to decode the file.

You may also register new metadata resolvers using the add_metadata_resolver operator. It is invoked the following way: add_metadata_resolver(format,resolver), where:

  • format is the name of the resolved format. It is only informative.
  • resolver is a function f that returns a list of metadata of the form: (label, value). It is invoked the following way: f(format=name,file), where:
    • format contains the name of the format, as returned by the decoder that accepted to decode the file. f may return immediately if this is not an expected value.
    • file is the name of the file to decode.

Wrappers

On top of the basic operators, wrappers have been written for some common decoders. This includes the flac and faad decoders, by default, and mplayer, if enabled. All the operators are defined in utils.liq.

The FLAC decoder

The flac decoder uses the flac command line. It is enabled if the binary can be found in the current $PATH.

Its code is the following: 

if test_process("which flac") then
  def flac_p(file)=
    "flac -d -c #{quote(file)} 2>/dev/null"
  end
  add_decoder("FLAC",flac_p)
else
  log(level=3,"flac binary not found: \
        flac decoder disabled.")
end
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Additionaly, a metadata resolver is registered when the metaflac command can be found in the $PATH: 

if test_process("which metaflac") then
  def flac_meta(~format,file)
    if format != "FLAC" then
      []
    else
      ret = get_process_lines("metaflac \
        --export-tags-to=- \
        #{quote(file)} 2>/dev/null")
      ret = list.map(
               string.split(separator="="),ret)
      # Could be made better..
      def f(l',l)=
        if list.length(l) >= 2 then
          list.append([(list.hd(l),
                        list.nth(l,1))],l')
        else
          if list.length(l) >= 1 then
            list.append([(list.hd(l),"")],l')
          else
            l'
          end
        end
      end
      list.fold(f,[],ret)
    end
  end
  add_metadata_resolver("FLAC",flac_meta)
else
  log(level=3,"metaflac binary not found: \
        flac metadata resolver disabled.")
end
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The faad decoder

The faad decoder uses the faad program, if found in the $PATH. It can decode AAC and AAC+ audio files.

Its code is the following: 

if test_process("which faad") then
  def faad_p(file)=
    "faad -o /dev/stdout #{quote(file)} \
          2>/dev/null"
  end
  add_decoder("FAAD",faad_p)
  def faad_meta(~format,file)
    if format != "FAAD" then
      []
    else
      ret = get_process_lines("faad -i \
                   #{quote(file)} 2>&1")
      def get_meta(l,s)=
        ret = string.extract(
              pattern="^(\w+):\s(.+)$",s)
        if list.length(ret) > 0 then
          list.append([(ret["1"],ret["2"])],l)
        else
          l
        end
      end
      list.fold(get_meta,[],ret)
    end
  end
  add_metadata_resolver("FAAD",faad_meta)
else
  log(level=3,"faad binary not found: \
       faad decoder disabled.")
end
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Mplayer decoder

This decoder makes use of the mplayer program, if it is enabled, and the program can be found in the $PATH. It allows liquidsoap to decode any file for which mplayer is able to decode an audio track. This will include video files, if some are found in your playlists, for instance.

It is enabled by adding the following line in your script: 

enable_mplayer ()

Its code is the following: 

# Enable mplayer decoder (needs mplayer binary in path)
# @category Liquidsoap
def enable_mplayer()=
  def mplayer_p(file)=
    "mplayer -really-quiet \
        -ao pcm:file=/dev/stdout \
        -vc null -vo null #{quote(file)} \
        2>/dev/null"
  end
  if test_process("which mplayer") then
    add_decoder("MPLAYER",mplayer_p)
  else
    log(label="utils.liq",level=2,
          "couldn't enable mplayer \
             decoder: no binary found")
  end
end
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