Liquidsoap 0.9.2 : Customize metadata

Customize metadata using liquidsoap

Liquidsoap has several mechanism for manipulating the metadata attached to your stream. In this page we quickly detail and compare the different operators, see the language reference for full details about them.

Warning. The protocol used by Shoutcast and Icecast before version 2 does not support many fields. It mainly support one: song. So, if you need to customize the metadata displayed by these servers, you should customize only the song metadata.

Rewrite metadata

rewrite_metadata rewrites metadata using a list of (target,rules). The semantic for the replacement rules is that of the % function. Namely, (pattern % [...,(k,v),...]) changes in pattern occurences of:

A sample code using this operator can be:

# The 'display_artist' field is passed using annotate.
pattern = 
  '$(if $(display_artist),"$(display_artist)","$(artist)")'
rewrite_metadata([("artist",pattern)],source)
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Map_metadata

The map_metadata operator applies a specified function to transform each metadata chunk of a stream. It can be used to add or decorate metadata, but is also useful in more complex cases.

A simple example using it is:

# A function applied to each metadata chunk
def append_title(m) =
  # Grab the current title
  title = m["title"]

  # Return a new title metadata
  [("title","#{title} - www.station.com")]
end

# Apply map_metadata to s using append_title
s = map_metadata(append_title, s)
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The effect of map_metadata by default is to update the metadata with the returned values. Hence in the function append_title defined in the code above returns a new metadata for the label title and the other metadata remain untouched. You can change this by using the update option, and you can also remove any metadata (even empty one) using the strip option.

See the documentation on map_metadata for more details.

A more complex example is the rewrite_metadata operator, which is implemented using map_metadata as follows:

# Rewrite metadata on the fly using a list of (target,rule).
# @category Source / Track Processing
# @param l  List of (target,value) rewriting rules
# @param ~insert_missing \
#           Treat track beginnings without metadata \
#           as having empty ones.
def rewrite_metadata(l,~insert_missing=true,s)
  def map(m)
    def apply(x)
      label = fst(x)
      meta = snd(x)
      if list.mem_assoc(label,l) then
        pattern = l[label]
        (label,pattern % m)
      else
        (label,meta)
      end
    end
    m = list.map(apply,m)
    def add(m,x)
      label = fst(x)
      pattern = snd(x)
      # If m does not have (label,_), then it was
      # not processed previously, we have to
      # add it now..
      if not list.mem_assoc(label,m) then
        list.append(m,[(label,pattern % m)])
      else
        m
      end
    end
    list.fold(add,m,l)
  end
  map_metadata(map,insert_missing=insert_missing,s)
end
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Insert_medatata

This operator is used for inserting metadata using a server command. If you have an insert_metadata node named ID in your configuration, you can connect to the server (either telnet or socket) and execute commands like:

ID.insert key1="val1",key2="val2",...