[FFmpeg-user] why are my streams not aligned?

[Dennis Mungai]

On Mon, 18 Nov 2019, 18:38 gordon, <wgordonw1 at gmail.com> wrote:

> Okay, thank you. I will give that a go.
>
> -- aside --
>
> I thought that the "-force_key_frames expr:gte(t,n_forced*2)" was placing
> the key frames for segment boundaries and still allowing the encoder to
> place other keyframes where they wanted?  I am trying to use h264, h265,
> and vp9 so I figured the keyframes not at boundaries would need to be
> different for efficiency.
>
> How do I determine the best (or good) value for "-g" given a particular
> source file and target duration for segments?
>
> On Mon, Nov 18, 2019 at 10:19 AM Dennis Mungai <dmngaie at gmail.com> wrote:
>
> > On Mon, 18 Nov 2019 at 17:31, gordon <wgordonw1 at gmail.com> wrote:
> >
> > > Why are the segments for my stream not aligned?  The first command
> > outputs
> > > video segment.  The second command outputs an audio segment with the
> > > previous video output copied in.
> > >
> > > Ffmpeg commands are pasted below with the m3u8 outputs.  The same
> results
> > > were observed on ffmpeg 4.2.1 and snapshot
> > >
> > > /usr/local/bin/ffmpeg -y -loglevel error -f rawvideo -vcodec rawvideo
> -s
> > > 160x120 -pix_fmt rgb24 -r 12.00 -i - -an -vcodec libx264 -preset medium
> > -f
> > > hls -an -hls_flags single_file -hls_time 6 -hls_list_size 0
> > > -hls_segment_type mpegts -hls_segment_filename
> > >
> > >
> >
> /tmp/973b7b02b6e6486f90d3064527a69701/output/21b79578a1d04533932e32d28f46eb94.ts
> > > -hls_playlist_type vod -master_pl_name
> > > 21b79578a1d04533932e32d28f46eb94-master.m3u8 -force_key_frames
> > > 'expr:gte(t,n_forced*2)' -profile:v baseline -level 3.0 -pix_fmt
> yuv420p
> > > -vsync cfr -b 145k -pix_fmt yuv420p
> > >
> > >
> >
> /tmp/973b7b02b6e6486f90d3064527a69701/output/21b79578a1d04533932e32d28f46eb94-variant.m3u8
> > >
> > > #EXTM3U
> > > #EXT-X-VERSION:4
> > > #EXT-X-TARGETDURATION:6
> > > #EXT-X-MEDIA-SEQUENCE:0
> > > #EXT-X-PLAYLIST-TYPE:VOD
> > > #EXTINF:6.000000,
> > > #EXT-X-BYTERANGE:116748 at 0
> > > 21b79578a1d04533932e32d28f46eb94.ts
> > > #EXTINF:6.000000,
> > > #EXT-X-BYTERANGE:96256 at 116748
> > > 21b79578a1d04533932e32d28f46eb94.ts
> > > #EXTINF:6.000000,
> > > #EXT-X-BYTERANGE:143256 at 213004
> > > 21b79578a1d04533932e32d28f46eb94.ts
> > > #EXTINF:6.000000,
> > > #EXT-X-BYTERANGE:97760 at 356260
> > > 21b79578a1d04533932e32d28f46eb94.ts
> > > #EXTINF:5.750000,
> > > #EXT-X-BYTERANGE:75764 at 454020
> > > 21b79578a1d04533932e32d28f46eb94.ts
> > > #EXT-X-ENDLIST
> > >
> > >
> > > /usr/local/bin/ffmpeg -y -loglevel error -f s16le -acodec pcm_s16le -ar
> > > 44100 -ac 2 -i - -i
> > >
> > >
> >
> /tmp/973b7b02b6e6486f90d3064527a69701/output/21b79578a1d04533932e32d28f46eb94.ts
> > > -vcodec copy -acodec libfdk_aac -ar 44100 -strict -2 -profile:a
> aac_he_v2
> > > -vbr 1 -f hls -hls_flags single_file -hls_time 6 -hls_list_size 0
> > > -hls_segment_type mpegts -hls_segment_filename
> > >
> > >
> >
> /tmp/973b7b02b6e6486f90d3064527a69701/output/e3658a88388c4d4da25681956bccb00f.ts
> > > -hls_playlist_type vod -master_pl_name
> > > e3658a88388c4d4da25681956bccb00f-master.m3u8 -force_key_frames
> > > 'expr:gte(t,n_forced*2)' -pix_fmt yuv420p -vsync cfr
> > >
> > >
> >
> /tmp/973b7b02b6e6486f90d3064527a69701/output/e3658a88388c4d4da25681956bccb00f-variant.m3u8
> > >
> > > #EXTM3U
> > > #EXT-X-VERSION:4
> > > #EXT-X-TARGETDURATION:6
> > > #EXT-X-MEDIA-SEQUENCE:0
> > > #EXT-X-PLAYLIST-TYPE:VOD
> > > #EXTINF:6.000000,
> > > #EXT-X-BYTERANGE:146452 at 0
> > > e3658a88388c4d4da25681956bccb00f.ts
> > > #EXTINF:6.000000,
> > > #EXT-X-BYTERANGE:125020 at 146452
> > > e3658a88388c4d4da25681956bccb00f.ts
> > > #EXTINF:5.916667,
> > > #EXT-X-BYTERANGE:167320 at 271472
> > > e3658a88388c4d4da25681956bccb00f.ts
> > > #EXTINF:6.083333,
> > > #EXT-X-BYTERANGE:133856 at 438792
> > > e3658a88388c4d4da25681956bccb00f.ts
> > > #EXTINF:5.750000,
> > > #EXT-X-BYTERANGE:103776 at 572648
> > > e3658a88388c4d4da25681956bccb00f.ts
> > > #EXT-X-ENDLIST
> >
> >
> >
> > To ensure alignment, you must use a closed GOP (set via -flags +cgop) and
> > specify a GOP size via the -g option passed to your video encoder.
>
>
>


A keyframe distance of 2 is ideal. Simply set -g to 2x your content's frame
> rate. If your fps is 30, then -g should be 60. As an example. Secondly, set
> your segment length to a multiple of 2. Segment lengths of 6 seconds are
> ideal (and it's what Apple recommends in their HLS authoring guidelines).
> However, if low latency for a trade off in encoding quality is desired,
> then set segment length to 2.