HLS Output

Starting with liquidsoap 1.4.0, it is possible to send your streams as HLS output.

The main operator is output.file.hls. Here’s an example using it, courtesy of srt2hls:

aac_lofi = %ffmpeg(format="mpegts",
                   codec="libfdk_aac",
                   channels=2,
                   ar=44100,
                   # Bitrate is define per channel in ffmpeg,
                   # so for 2 channels, 16k per channel gives
                   # an overall bitrate of 32k
                   b="16k",
                   afterburner=1,
                   profile="aac_he_v2")

aac_midfi = %ffmpeg(format="mpegts",
                   codec="libfdk_aac",
                   channels=2,
                   ar=44100,
                   b="48k",
                   afterburner=1,
                   profile="aac_low")

aac_hifi = %ffmpeg(format="mpegts",
                   codec="libfdk_aac",
                   channels=2,
                   ar=44100,
                   b="96k",
                   afterburner=1,
                   profile="aac_low")

streams_info = [("aac_lofi",(40000,"mp4a.40.29","ts")),
                ("aac_midfi",(110000,"mp4a.40.2","ts")),
                ("aac_hifi",(220000,"mp4a.40.2","ts"))]
                
streams = [("aac_lofi",aac_lofi), 
           ("aac_midfi", aac_midfi), 
           ("aac_hifi", aac_hifi)]
                
def segment_name(~position,~extname,stream_name) =
  timestamp = int_of_float(gettimeofday())
  duration = 2
  "#{stream_name}_#{duration}_#{timestamp}_#{position}.#{extname}"
end             

output.file.hls(playlist="live.m3u8",
                segment_duration=2.0,
                segments=5,
                segments_overhead=5,
                segment_name=segment_name,
                streams_info=streams_info,
                persist=true,
                persist_at="/path/to/state.config",
                "/path/to/hls/directory",
                streams,
                source)

Let’s see what’s important here:

  • streams describes the encoded streams. It’s a list of: (stream_name, encoder). stream_name is used to generate the corresponding media playlists. Encoders can be any encoder supported by liquidsoap. However, the HLS RFC limits the list of possible codecs to mp3 and aac. Furthermore, for the best possible compatible, it is recommended to send data encapsulated in a MPEG-TS stream. Currently, the only encoder capable of doing this in liquidsoap is %ffmpeg.
  • streams_info describes additional data used to generated the HLS playlists according to section 4.3.4.2 of the RFC. It consists of a list of the form: (stream_name, (average_bandwidth, codec_name, segments_file_extension)). This list is inffered for the following formats: %mp3, %fdkaac, %shine. When using these formats, you do not need to pass an entry for the corresponding stream in this list. However, since we’re using the %ffmpeg encoder to get MPEG-TS encapsulated data, we need to add these info for each stream.
  • persist and persist_at are used to allow liquidsoap to restart while keeping the existing segments and playlists. When shutting down, liquidsoap stores the current configuration at persist_at and uses it to restart the HLS stream when restarting.
  • segments and segments_overhead are used to keep track of the generated segments. Each media playlist will contain a number of segments defined by segments and an extra set of segments, defined by segments_overhead, is kept past the playlist size for those listeners who are still listening on outdated segments.

There are more useful options, in particular on_file_change, which can be used for instance to sync up your segments and playlists to a distant storage and hosting service such as S3.

Liquidsoap also provides a output.harbor.hls and output.harbor.hls.ssl which allow to serve HLS streams directly from liquidsoap. Their options should be the same as output.file.hls, except for harbor-specifc options port and path. It is not recommended for listener-facing setup but can be useful to sync up with a caching system such as cloudfront.