HLS streaming explained

How HLS works, step by step

1. The encoder cuts the stream into segments. Each segment is a small file holding a few seconds of video and audio, commonly in MPEG-TS or fragmented MP4 format.
2. A media playlist lists the segments. This .m3u8 file names the most recent segments and how long each one lasts.
3. The player downloads the playlist, then the segments, one after another, and plays them back to back.
4. For live TV, the playlist keeps changing. New segments are added and old ones drop off, so the player reloads the playlist every few seconds.

Adaptive bitrate: several qualities in one stream

HLS can offer the same channel at several qualities. A master playlist lists each version, for example 1080p at 6 Mbps, 720p at 3 Mbps and 480p at 1.5 Mbps, and the player chooses one based on the speed it measures. If your connection slows, it switches to a lower quality instead of stopping. Adaptive bitrate streaming explains how players decide.

Many IPTV channels are offered at a single quality, so this only helps where the provider publishes several.

What it looks like inside

A master playlist:

#EXTM3U
#EXT-X-STREAM-INF:BANDWIDTH=6000000,RESOLUTION=1920x1080
1080p/index.m3u8
#EXT-X-STREAM-INF:BANDWIDTH=3000000,RESOLUTION=1280x720
720p/index.m3u8

A live media playlist:

#EXTM3U
#EXT-X-VERSION:3
#EXT-X-TARGETDURATION:6
#EXT-X-MEDIA-SEQUENCE:48213
#EXTINF:6.000,
segment48213.ts
#EXTINF:6.000,
segment48214.ts

This .m3u8 is a different job from the M3U file that lists your channels; see M3U vs M3U8.

Strengths and weaknesses for IPTV

Using HLS in your app

If your app or playlist offers a choice, HLS usually appears as m3u8 or “HLS”. Try it when MPEG-TS channels drop out on a patchy connection, and switch back if channel changes feel slow. HLS vs MPEG-TS walks through the decision.