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on Using FFmpeg as a HLS streaming server (Part 1) – HLS Basics

We have set up our fist livestream that was already playable in safari. But we have seen, that the segmentation is not working as expected.

After a look in the stream.m3u8 we have a big surprise: The given target duration of 4 seconds are not used. Instead a TARGETDURATION of 10 is set and the segment length (value after #EXTINF:) is also not constant. A constant value is required for a good buffering in the player. We don’t want any interruptions.


The problem here is that the segmenter splits starts a new file only at keyframes (I-Frames). This makes sense, because if you e.g. seek in the stream the player chooses the nearest new stream*.ts file and starts playing it.

How to solve (step 1)

-g 25 sets the group picture size to 25. My video source has 25 FPS (25 pictures per second). So each second will start with a new picture group. This option forces a target size of 25 frames. After that a new group will be created (starting with a new I-Frame).

Well, now it is much closer to what we want. But there are still some glitches above and also below 4 seconds.

How to solve (step 2)

The reason for this is that FFmpeg has a scene detection. This means, that always when a new scene starts an I-Frame will be set. Since it’s FFmpeg we can easily disable this feature.

-sc_threshold 0

And the new playlist looks as we were expecting: Segments with the exact same duration.

The new command so far is:

./ffmpeg -listen 1 -i rtmp:// \
    -c:v libx264 -crf 21 -preset veryfast -g 25 -sc_threshold 0 \
    -c:a aac -b:a 128k -ac 2 \
    -f hls -hls_time 4 -hls_playlist_type event stream.m3u8