...

2. If HLS agent for the stream is stopped by REST query /hls/terminate, and there are active HLS subscribers, this agent will be restarted. In this case, active HLS subscribers must reconnect to the stream.

HLS ABR support

Legacy HLS ABR implementation in builds 5.2.484 - 5.2.582

Since build 5.2.484 HLS ABR playlists support was added. This feature can be enabled with the following parameter

...

curl -s http://localhost:8082/test/test.m3u8
sleep 1
curl -s http://localhost:8082/test-240p/test-240p.m3u8
sleep 1
curl -s http://localhost:8082/test-480p/test-480p.m3u8
sleep 1
curl -s http://localhost:8082/test-720p/test-720p.m3u8
sleep 1

Actual HLS ABR implementation in build 5.2.585 and newer

Since build 5.2.585 HLS ABR implementation is significally changed. As usual, HLS ABR can be used in CDN only, but Edge server captures all the transcoded streams for ABR manifest stream variants within the same mediasession to syhchronize the stream variants. This requires to configure Tanscoder nodes and Edge nodes simultaneously, and adds some limits. Let's explore it below.

Transcoder node settings

To synchronize all the variants of the same stream, encoding should be aligned on Transcoder node

transcoder_align_encoders=true

Also, FPS filter should be enabled

video_filter_enable_fps=true
video_filter_fps=25

All stream variants key frames (GOPs) should be synchronized. For example, we will send key frame every 2 seconds for 25 fps stream

video_filter_fps_gop_synchronization=50

HLS Edge node settings

HLS preloader and streams resizing should be disabled on HLS Edge node

hls_preloader_enabled=false
hls_player_width=0
hls_player_height=0

The transcoding profiles should be set as follows in cdn_profiles.yml file

profiles:
 -240p:
  audio:
    codec : mpeg4-generic
    rate : 48000
  video:
    height : 240
    bitrate : 300
    gop : 50
    codec : h264
 -480p:
  video:
    height : 480
    bitrate : 600
    gop : 50
    codec : h264
 -720p:
  video:
    height : 720
    bitrate : 1000
    gop : 50
    codec : h264

Note that audio parameters can be set for the first profile only because those parameters should be identical for all the profiles, and will be applyed according to the first profile.

Then HLS ABR should be enabled

hls_abr_enabled=true

Usage

Client should request ABR manifest as usual playlist

https://server:8445/test_0/test_0.m3u8

The playlist contains links to stream variants playlists, a client can switch between then

#EXTM3U
#EXT-X-STREAM-INF:BANDWIDTH=614400,RESOLUTION=852x480,CODECS="avc1.42e01f,mp4a.40.2"
-480p/-480p.m3u8
#EXT-X-STREAM-INF:BANDWIDTH=1024000,RESOLUTION=1278x720,CODECS="avc1.42e01f,mp4a.40.2"
-720p/-720p.m3u8
#EXT-X-STREAM-INF:BANDWIDTH=307200,RESOLUTION=426x240,CODECS="avc1.42e01f,mp4a.40.2"
-240p/-240p.m3u8

Known limits

1. HLS Edge can be used for HLS streaming only, client sessions of another kinds will not work.

2. Stream recording, snapshots, mixing, stream capturing from another server and other captured stream management functions will not work.

3. Currently, HLS ABR segments can only be stored on disk, not in memory.

HLS segments storage

Using disk

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