Building an advanced stream: file-based sources

The purpose of this part is to document and illustrate the creation of an advanced stream using static files.

In order to be self-contained, we will use here only pure liquidsoap scripting functionalities. However, all the parts that use pre-defined functions can be implemented using external scripts, which is the most common practice, and has proved to be very convenient in order to integrate your liquidsoap stream into the framework that you use to manage your radio.


In order to make things more clear and modular, we will separate the code in two parts:

  • radio.liq is the script that contains the definition of the main stream
  • library.liq is the script that contains the functions used to build the stream

The scripts here should be tested using the following command line:

liquidsoap /path/to/radio.liq

Thus, we do not define here daemonized script. In order to make things work smoothly, you should put the following lines at the beginning of radio.liq:


Finally, we add the following line at the beginning of radio.liq, in order to load our pre-defined functions:

%include "/path/to/library.liq"

We will use the telnet server to interact with the radio. Thus, we enable the telnet server by adding the following line in radio.liq:


An initial model

In this part, we describe the initial stream that we want. We start with a simple stream that contains songs from a static playlist, with some jingles, with 3 songs for one jingle, and output the result to an icecast server. This is described by the following graph:

Initial stream modelThis very simple stream is defined by the following content in radio.liq:

# The file source
songs = playlist("/path/to/some/files/")

# The jingle source
jingles = playlist("/path/to/some/jingles")

# We combine the sources and play 
# one single every 3 songs:
s = rotate(weights=[1,3], [jingles, songs])

# We output the stream to an icecast
# server, in ogg/vorbis format.
               host="my_server", password="hack_me_not",

For now, library.liq does not contain any code so we only do:

touch /path/to/library.liq
  • Write this script to a file, change the default directories and parameters.
  • Run it.
  • Listen to your initial stream.
  • Connect to the telnet server (telnet localhost 1234)
  • Try the telnet comment: icecast.skip
  • Check that a jingle is played every 3 songs.
  • Sometimes, when skipping, the source does not prepare a new file quickly enough, which breaks the rotation. If this is the case, add the parameter conservative=true to each playlist and try again.

Now, we extend this initial stream with some advanced features:

Notify when a song is played

Once the stream is started, we may want to be able to keep track of the songs that are played, for instance to display this information on a website. One nice way to do this is to call a function every time that a new track is passed to the output, which will inform the user of which tracks are played and when. This can be done using the on_metadata operator.

First, we define a function that is called every time a new metadata is seen in the stream. This is a function of type (metadata)->unit, i.e. a function that receives the metadata as argument and returns nothing. The metadata type is actually [(string*string)], i.e. a list of elements of the form ("label","value").

Thus, we add the following in library.liq:

# This function is called when
# a new metadata block is passed in
# the stream.
def apply_metadata(m) =
  title = m["title"]
  artist = m["artist"]
  print("Now playing: #{title} by #{artist}")

Note: the string "foo #{bla}" can also be written "foo " ^ bla and is the string `foo bar'', ifblais the string“bar”`.

Now, we apply the on_metadata operator with this function just before passing the final source to the output, so we write in radio.liq, before the output line:

s = on_metadata(apply_metadata,s)
  • Update your scripts.
  • Run the new radio.
  • Observe the lines printed on new metadata.
  • You may prefer to use the log function rather than print.


Custom scheduling

Another issue with the above stream is the fact that jingles have a strict frequency of one jingle every 3 songs. In a lot of cases, you may want more flexibility and have full-features scheduling of your songs. The best approach in this case is to externalize this operation by creating a scheduler with the language/framework of your choice and integrating it with liquidsoap using request.dynamic.list.

request.dynamic.list takes a function of type ()->[request('a)], i.e. a function with no arguments that returns an array of new requests to queue and create a source with it. Every time that liquidsoap needs to prepare a new file, it will execute the function and use its result.

Requests in liquidsoap are created with the function request.create, which takes an URIs of the form:


where protocol: is optional is arguments is the URI of a local file. For instance, is a requests using the ftp protocol, which is resolved using wget (if present in the system).

We are going to use request.dynamic.list to merge both the songs and jingles sources into one source and let our external scheduler decides when to play a jingle or a song. However, we will need later to know if we are currently playing a song or a jingle.

For these reasons, we will be using the annotate: protocol. This protocol can be used to pass additional metadata along with the metadata of the file. Here, we will pass a metadata labeled "type", with value "song" if the track is a song or "jingle" otherwise.

In the context of this simple presentation, we will write a dummy script. Thus, we create a file "/tmp/request" that contains a line of the form:


And, we add in library.liq:

# Our custom request function
def get_request() = 
  # Get the URI
  uri = list.hd(default="",get_process_lines("cat /tmp/request"))
  # Create a request

Now, we replace the lines defining songs, files and the line using the rotate operator in radio.liq with the following code:

s = request.dynamic.list(id="s",get_request)
  • Update your scripts.
  • Run the new radio.
  • Edit "/tmp/request" and change its content to:
  • Use the command s.skip through the telnet server and verify that the new song is being played. This may need more than one skip..
  • Use the commands request.on_air and request.metadata through the telnet server to verify the presence of the type metadata
  • Extra: think about how to use the previous notification code to interact with this function.


Custom metadata

We have just seen how it is possible to use the annotate: protocol to pass custom metadata to any request. Additionally, it is also possible to rewrite your stream’s metadata on the fly, using the on_metadata operator.

This operator takes a function of the type metadata->metadata, i.e. a function that takes the current metadata and returns some metadata. Thus, when map_metadata sees a new metadata in the stream, it calls this function and, by default, updates the metadata with the values returned by the function.

Here, we use this operator to customize the title metadata with the name of our radio. First, we create a file "/tmp/metadata" containing:

My Awesome Liquidsoap Radio!

Then, in library.liq, we add the following function:

# This function updates the title metadata with
# the content of "/tmp/metadata"
def update_title(m) = 
  # The title metadata
  title = m["title"]
  # Our addition
  content = list.hd(get_process_lines("cat /tmp/metadata"))
  # If title is empty
  if title == "" then
  # Otherwise
    [("title","#{title} on #{content}")]

Finally, we apply map_metadata to the source, just after the request.dynamic.list definition in radio.liq:

s = map_metadata(update_title,s)


Infallible sources

It is reasonable, for a radio, to expect that a stream is always available for broadcasting. However, problems may happen (and always do at some point). Thus, we need to offer an alternative for the case where nothing is available.

Instead of using mksafe, which streams blank audio when this happens, we use a custom sound file. For instance, this sound file may contain a sentence like ``Hello, this is radio FOO! We are currently having some technical difficulties but we’ll be back soon so stay tuned!’’.

We do that here using the say: protocol, which creates a speech synthesis of the given sentence. Otherwise, you may record a (more serious) file and pass it to the single operator…

First, we add the following in library.liq

# This function turns a fallible
# source into an infallible source
# by playing a static single when
# the original song is not available
def my_safe(s) =
  # We assume that festival is installed and
  # functional in liquidsoap
  security = single("say:Hello, this is radio FOO! \
                     We are currently having some \
                     technical difficulties but we'll \
                     be back soon so stay tuned!")

  # We return a fallback where the original
  # source has priority over the security
  # single. We set track_sensitive to false
  # to return immediately to the original source
  # when it becomes available again.

Then, we add the following line in radio.liq, just before the output line:

s = my_safe(s)

And we also remove the fallible=true from the parameters of output.icecast.

  • Update your scripts.
  • Run and test the new radio.
  • To hear the security jingle, you can empty "/tmp/request" and use the skip command in telnet or wait for the end of the current song.
  • If you put a new valid request in "/tmp/request" then the stream comes back to the normal source.


Multiple outputs

We may as well output the stream to several targets, for instance to different icecast mount points with different formats. Therefore, we define a custom output function that defines all these outputs.

We add the following in library.liq:

# A function that contains all the output
# we want to create with the final stream
def outputs(s) =
  # First, we partially apply output.icecast
  # with common parameters. The resulting function
  # is stored in a new definition of output.icecast,
  # but this could be my_icecast or anything.
  output.icecast = output.icecast(host="my_server", 

  # An output in ogg/vorbis to the "my_radio.ogg"
  # mountpoint:
  output.icecast(%vorbis, mount="my_radio.ogg",s)
  # An output in mp3 at 128kbits to the "my_radio"
  # mountpoint:
  output.icecast(%mp3(bitrate=128), mount="my_radio",s)

  # An output in ogg/flac to the "my_radio-flac.ogg"
  # mountpoint:
  output.icecast(%ogg(%flac), mount="my_radio-flac.ogg",s)

  # An output in AAC+ at 32 kbits to the "my_radio.aac"
  # mountpoint
  output.icecast(%fdkaac(bitrate=32), mount="my_radio.aac",s)

And we replace the output line in radio.liq by:

  • Write your own output function.
  • Run and test your new radio.

Note liquidsoap may fail with the following error:

Connection failed: 403, too many sources connected (HTTP/1.0)!

In this case, you should check the maximum number of sources that your icecast server accepts.


More advanced functions!

Now that we have a controllable initial radio, we extend our initial scripts to add advanced features. The following graph illustrates what we are going to add:

Advanced stream model* The Replay gain node normalizes all the songs using the Replay Gain technology. * The Smart crossfade node adds crossfading between songs but not jingles. * The Smooth_add node adds the possibility to insert a jingle in the middle of a song, fading out and then back in the initial stream while the jingle is being played.


The replaygain support is achieved in liquidsoap in two steps:

  • apply the amplify operator to change a source’s volume
  • pass a "replay_gain" metadata indicating to the amplify operator which value to use.

The "replay_gain" metadata can be passed manually or computed by liquidsoap. Liquidsoap comes with a script that can extract the replaygain information from ogg/vorbis, mp3 and FLAC files. This is a very convenient script but it generate a high CPU usage which can be bad for real-time streaming. In some situations, you may compute beforehand this value and pass it manually using the annotate protocol.

If you cannot compute the value beforehand, liquidsoap comes with two ways to extract the replaygain information

  • The replay_gain: protocol. All requests of the form replay_gain:URI are resolved by passing URI to the script provided by liquidsoap. This method allows to select which files should be used with replay gain. However, it will only work if URI is a local file.
  • The replay gain metadata resolver, enabled by adding a line of the form enable_replaygain_metadata () in your script. In this cases, all requests and not only local files can be processed and you cannot select which one should be used with replaygain.

The most simple solution, in our case, is to change the requests passed to request.dynamic.list to something of the form:


However, in order to illustrate a bit more the functionalities of liquidsoap we present another solution. The method we propose here consists in using map_metadata, which we have already seen to update the metadata with a "replay_gain" metadata when we see the "type" metadata with the value "song". Thus, we add the following function in library.liq:

# This function takes a metadata,
# check if it is of type "file"
# and add the replay_gain metadata in
# this case
def add_replaygain(m) = 
  # Get the type
  type = m["type"]
  # The replaygain script is located there
  script = "#{configure.bindir}/extract-replaygain"
  # The file name is contained in this value
  filename = m["filename"]

  # If type = "song", proceed:
  if type == "song" then
    info = list.hd(get_process_lines("#{script} #{filename}")) 
  # Otherwise add nothing

And, we add the following line in radio.liq after the request.dynamic.list line:

s = map_metadata(add_replaygain,s)

Finally, we add the amplify operator. We set the default amplification to 1., i.e. no amplification, and tell the operator to update this value with the content of the "replay_gain" metadata. Thus, only the tracks which have this metadata will be modified.

We add the following in radio.liq, after the line we just inserted:

s = amplify(override="replay_gain",1.,s)

Note we can also apply amplify only to songs, before the switch operator

  • Update your scripts.
  • Run and test the new radio.
  • Change the content of "/tmp/request" with something of type "file" and skip the current song.
  • Use the telnet server to make sure that the new "replay_gain" metadata has been added.
  • Also, in the logs, you should be able to see that the replay_gain information was used to override the amplification factor.
  • Can you find a content for "/tmp/request" that will enable replay gain on a file which is not of type "song"?

Note in this case, the replay_gain metadata is not added during the request resolution. Thus, it is not visible in the request.metadata. However, you should be able to find another command that displays it!


Smart crossfade

The smart_crossfade is a crossfade operator that decides the crossfading to apply depending on the volume and metadata of the old and new track.

It is defined using a generic smart_cross operator, that takes a function of type (float, float, metadata, metadata, source, source) -> source, i.e. a function that take the volume level (in decibels) of, respectively, the old and new tracks, the metadata of, resp. the old and new tracks and, finally, the old and new tracks, and returns the new source with the required transition.

We give here a simple custom implementation of our crossfade. What we do is:

  • Crossfade tracks if none of the old and new track are jingle;
  • Sequentialize the tracks otherwise.

We identify the type of each track by reading the "type" metadata we have added when creating the request.dynamic.list source.

A typical smart_crossfade operator is defined in utils.liq but you may do much more things with a little bit of imagination.

Here, we add the following in library.liq:

# Our custom crossfade that 
# only crossfade between tracks
def my_crossfade(s) = 
  # Our transition function
  def f(_,_, old_m, new_m, old, new) = 
    # If none of old and new have "type" metadata
    # with value "jingles", we crossfade the source:
    if old_m["type"] != "jingle" and new_m["type"] != "jingle" then
 # Now, we apply smart_cross with this function:

Finally, we add the following line in radio.liq, just after the amplify operator:

s = my_crossfade(s)
  • Update your scripts.
  • Run and test the new radio.
  • Modify the custom crossfading to fade out the old song if it is not a jingle, and fade in the new song if it is not a jingle.



Finally, we add another nice feature: a jingle that is played on top of the current stream. We use the smooth_add operator, which is also defined in utils.liq. This operator takes a normal source and a special jingle source. Every time that a new track is available in the special source, it fades out the volume of the normal source, plays the track from the special source on top of the current track of the normal source, and then fades back in the volume of the normal source when the track is finished.

Typically, you use for the special source a request.queue where you push a new jingle every time you want to use this feature.

We modify radio.liq and add the following line just before my_safe:

# A special source
special = request.queue(id="special")
# Smooth_add the special source
s = smooth_add(normal=s,special=special)
  • Update your script.
  • Run and test the new radio.
  • Use the telnet server to push the request say:My new radio rocks! in the special source.
  • Listen!


What about DJs?

We present now another important part of an advanced stream: the addition of a live stream in order to allow DJs to broadcast their shows.

We are going to add the following features:

  • A live input that is played immediately when it is available
  • Use different harbor ports, to replace mount points for shoutcast source clients
  • A transition jingle that is played when switching between live and files
  • Skip the file currently being played when switching to a live source so that the file-based source starts with a fresh song when the live stops
  • Define different authentications for the DJs and make sure that a DJ can only broadcast when its show is scheduled

Live inputs

The live inputs in liquidsoap are of two types:

  • Network-based, mostly input.http and input.harbor.
  • Hardware-based, with operators like input.alsa, input.jack, etc.

We focus here on the first type, and more precisely on input.harbor. When using this operator in your script, the running instance will be able to receive data coming from icecast and shoutcast source clients. Then, your DJs can broadcast a live stream using their favorite software. Liquidsoap supports most of the usual data formats, when enabled as encoder:

  • MP3
  • Any supported ogg stream
  • Aac and Aac+

You may also communicate data between two liquidsoap instance, one using output.icecast to send data and the other one input.harbor to receive it. In this case, you wan also use the WAVE or FLAC format to send lossless data.

We add a live source in radio.liq, anywhere before the outputs:

live = input.harbor("live")

Note "live" is the name of the mountpoint that will be associated to this source. The default parameters for the port, user and password are contained in the following settings:


We want the live source to be played as soon as it becomes available. Thus, we use a fallback to combine it with the file-based source, and add the following code after my_safe in radio.liq:

s = fallback(track_sensitive=false, [live,s])

Note the track_sensitive=false parameter tells liquidsoap to switch immediately to live when it becomes available instead of waiting for the end of the track currently played by s.

  • Update your script
  • Run the new radio
  • Try to connect to the harbor mountpoint. You may use a separate liquidsoap script and output.icecast
  • Why is the output still infallible ?


Enabling shoutcast clients

By default, shoutcast source clients are not supported. You can enable them by adding the following settings:


Note ICY is the technical name of the original shoutcast source protocol.

Additionally, the shoutcast source protocol does not support the notion of mountpoint: all the sources try to connect to the same "/" mountpoint. However, you can emulate this in liquidsoap by using different harbor sources on different port.

For instance, if we replace the definition of live in radio.liq with the following:

live1 = input.harbor(port=9000,"/")
live2 = input.harbor(port=7000,"/")

And the fallback line with:

s = fallback(track_sensitive=false, [live1,live2,s]) 

Then your a DJ should be able to send data using the port 9000 and another one using the port 7000, and the one connecting on port 9000 may be played in priority if the two are connected at the same time.

  • Update your script
  • Run the new radio
  • Connect one source client to port 7000
  • Connect another source client to port 9000
  • Verify that the client on port 9000 is broadcasting


A nice transition!

Now that our radio support live shows, we deal with another issue: when switching to the live show, the current song is cut at the point where it is and the audio content switches over to the live data without any transition, which is not very nice for the listeners. Further, when switching back to the file-based source at the end of the live, the source resumes in the middle of the song that was last played..

In this part, we define a transition for switching from file to live, which fades the current song out and superposes a jingle before starting the live show. We use the transition parameter of the fallback operators.

This parameter contains functions of the type: source * source -> source, i.e. functions that take two sources as arguments, the old and new source, and returned a source that is the result of the desired transition. Finally, when defined as:

fallback(transition=[f,g], [s1, s2])

f is called when switching to s1 (and g when switching to s2).

We also use source.skip, which skips the file currently being played, in order to play a fresh file when switching back to the file-based source.

First, we add the following code in library.liq:

# Define a transition that fades out the
# old source, adds a single, and then 
# plays the new source
def to_live(jingle,old,new) = 
  # Fade out old source
  old =
  # Superpose the jingle
  s = add([jingle,old])
  # Compose this in sequence with
  # the new source

# A transition when switching back to files:
def to_file(old,new) =
  # We skip the file
  # currently in new
  # in order to being with
  # a fresh file

Note source.skip may cause troubles if the file source does not prepare a new track quickly enough. In this case, you may add conservative=true to the parameters of the request.dynamic.list source.

Then, we add the following code in radio.liq, where we defined the fallback between the two live sources and the file-based source:

# The transition to live1
jingle1 = single("say:And now, we present the awesome show number one!!")
to_live1 = to_live(jingle1)

# Transition to live2
jingle2 = single("say:Welcome guys, this is show two on My Awesome Radio!")
to_live2 = to_live(jingle2)

# Combine lives and files:
s = fallback(track_sensitive=false,
             transitions=[to_live1, to_live2, to_file],
             [live1, live2, s]) 
  • Update your script
  • Run the new radio
  • Connect to each harbor and listen to the result


Custom logins

Another powerful feature of input.harbor is the possibility to define a custom authentication. For instance, imagine that DJ Alice may connect to the live1 source only between 20h and 21h, which is the time of her shows, with the password "rabbit", while DJ Bob may connect to the live2 source between 18h and 20h with password "foo".

This can be implemented using the auth parameter of input.harbor. This parameter is a function of type: string * string -> bool, i.e. a function that takes a pair (user,password) and returns true if the connection should be granted.

You may use this, for instance, with an external script and integrate harbor and DJ authentication into the framework of your choice. Here we illustrate this functionality with a custom functions. Thus, we add the following in library.liq:

# Our custom authentication
# Note: the ICY protocol 
# does not have any username and for 
# icecast, it is "source" most of the time
# thus, we discard it
def harbor_auth(port,_,password) = 
  # Alice connects on port 9000 between 20h and 21h
  # with password "rabbit"
  (port == 9000 and 20h-21h and password == "rabbit")
  # Bob connection on port 7000 between 18h and 20h
  # with password "foo"
  (port == 7000 and 18h-20h and password == "foo")

And we use it by replacing the live1 and live2 definitions by:

# Authentication for live 1:
auth1 = harbor_auth(9000)
live1 = input.harbor(port=9000,auth=auth1,"/")

# Authentication for live 2:
auth2 = harbor_auth(7000)
live2 = input.harbor(port=7000,auth=auth2,"/")
  • Write your custom login function
  • Run and test your new radio

No solution here :-)