Liquidsoap dev : 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.

The annotate protocol

The metadata are read from files, so the most simple way is to properly tag the files. However, if it not possible to modify the files for some reason, the annotate protocol can be used in playlists to insert and modify some metadata. For instance, in the playlist

annotate:title="Title 1",artist="Artist 1":music1.mp3
annotate:title="Title 2",artist="Artist 2":music2.mp3

the title metadata for file music1.mp3 will be overridden and changed to “Title 1” (and similarly for the artist).

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:

is found in the list, "b" otherwise.

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:

# 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 metadata

Using the telnet server

This operator is used for inserting metadata using a server command. If you have an server.insert_metadata node named ID in your configuration, as in

server.insert_metadata(id="ID", source)
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you can connect to the server (either telnet or socket) and execute commands like

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

Sometimes it is desirable to change the metadata dynamically when an event occurs. In this case, the function insert_metadata (not to be confused with server.insert_metadata above) can be used: when applied to a source it returns a pair constituted of a function to update metadata and the source with inserted metadata.

For instance, suppose that you want to insert metadata on the stream using the OSC protocol. When a pair of strings “title” “The new title” is recieved on /metadata, we want to change the title of the stream accordingly. This can be achieved as follows.

# Our main music source
s = playlist("...")
s = mksafe(s)

# Create a function to insert metadata
ms = insert_metadata(s)
# The function to insert metadata
imeta = fst(ms)
# The source with inserted metadata
s = snd(ms)

# Handler for OSC events (gets pairs of strings)
def on_meta(m) =
  # Extract the label
  label = fst(m)
  # Extract the value
  value = snd(m)
  # A debug message
  print("Insert metadata #{label} = #{value}")
  # Insert the metadata
  imeta([(label,value)])
end

# Call the above handler when we have a pair of strings on /metadata
osc.on_string_pair("/metadata",on_meta)

# Output on icecast
output.icecast(%mp3,mount="test.mp3",s)
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We can then change the title of the stream by sending OSC messages, for instance

oscsend localhost 7777 "/metadata" ss "title" "The new title"