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How do I create a perfect MP4 to HLS encode?

name69

name69

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How do I create a perfect MP4 to HLS encode?​



Hi everyone,

I have a VOD MP4 file with:

  1. Scenario A: 23.976 FPS (CFR Source)
  2. Scenario B: 23.976 FPS (VFR Source)
  3. Scenario C: 24.0 FPS (CFR Source)
I want to convert it to HLS with 6-second segments, single bitrate, fully compatible with Apple/iOS devices.
 
14 comments
Byse said:
Huh? What encoding has to do with smooth playing? Encoding has effect only on what you see in the screen (pixelation). Your streaming server is responsible for smooth playback.
Click to expand...
I mean,

  • Video freeze / black frame at segment start
  • iOS / Apple devices stutter
  • Seeking inaccurate
 
name69 said:
I mean,

  • Video freeze / black frame at segment start
  • iOS / Apple devices stutter
  • Seeking inaccurate
Click to expand...

The black frame at segment start comes from lack of IDR per segment, for that you need to ensure that each segment starts from keyframe. For 6 second segments you go with

-hls_time 6
-hls_flags independent_segments (This enforces keyframes for each segment).
-force_key_frames "expr:gte(t,n_forced*6)" (This ensures key frames every 6 seconds independently from fps since you have each video with diff fps).
-sc_threshold 0 (This eliminated GOP floating)

Stutter at IOS usually happens if your video is having floating VFR so if you want your videos always working on IOS/Mac you should go with stable CFR eg 25/30. This will fix the seeking as well.
 
Byse said:
The black frame at segment start comes from lack of IDR per segment, for that you need to ensure that each segment starts from keyframe. For 6 second segments you go with

-hls_time 6
-hls_flags independent_segments (This enforces keyframes for each segment).
-force_key_frames "expr:gte(t,n_forced*6)" (This ensures key frames every 6 seconds independently from fps since you have each video with diff fps).
-sc_threshold 0 (This eliminated GOP floating)

Stutter at IOS usually happens if your video is having floating VFR so if you want your videos always working on IOS/Mac you should go with stable CFR eg 25/30. This will fix the seeking as well.
Click to expand...

Is it true?

Dangerous Keyframe Method: -force_key_frames:v expr:'gte(t,n_forced*2.000)' is risky for 23.976fps.

  • The Math: 2.000 seconds at 23.976fps = 47.95 frames.
  • FFmpeg has to guess whether to put the keyframe at frame 47 or 48. If it drifts, your HLS segments will snap, causing playback glitches.
  • The Fix: Use strict frame counting (-g 48).
 
I have provided you example and explaination, my post is not "ready to go" commands since each video is different and settings should be adjusted per video, so kindly refer to ffmpeg docs and learn what are you trying to do/achieve, you have exact reason of your issues provided in my previous post.
 
name69 said:
Is it true?

Dangerous Keyframe Method: -force_key_frames:v expr:'gte(t,n_forced*2.000)' is risky for 23.976fps.

  • The Math: 2.000 seconds at 23.976fps = 47.95 frames.
  • FFmpeg has to guess whether to put the keyframe at frame 47 or 48. If it drifts, your HLS segments will snap, causing playback glitches.
  • The Fix: Use strict frame counting (-g 48).
Click to expand...
check mail bro
 
For reliable HLS encoding that works across all devices, here's what I've found works best:
Use ffmpeg with these key flags:
- -c:v libx264 -profile:v high -level 4.1 for maximum compatibility
- -c:a aac -b:a 128k for audio
- -hls_time 6 -hls_flags independent_segments for proper segment boundaries
- -g 144 (for 24fps) to force keyframes every 6 seconds at the GOP level instead of using force_key_frames
The -g approach is more reliable than force_key_frames because it works at the codec level. For 23.976fps, use -g 144 (6 × 24 = 144). This avoids the frame drift issue that Charliee mentioned.
Also make sure to add -hls_segment_type mpegts if you need backward compatibility with older iOS devices. For newer devices, fMP4 segments work fine and are more efficient.
 
Byse said:
The black frame at segment start comes from lack of IDR per segment, for that you need to ensure that each segment starts from keyframe. For 6 second segments you go with

-hls_time 6
-hls_flags independent_segments (This enforces keyframes for each segment).
-force_key_frames "expr:gte(t,n_forced*6)" (This ensures key frames every 6 seconds independently from fps since you have each video with diff fps).
-sc_threshold 0 (This eliminated GOP floating)

Stutter at IOS usually happens if your video is having floating VFR so if you want your videos always working on IOS/Mac you should go with stable CFR eg 25/30. This will fix the seeking as well.
Click to expand...
How can i do stable CFR eg 25/30

my most of video FPS is 23.98 or 29.97
 
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