I opened https://trac.ffmpeg.org/ticket/8297
Sorry for not posting the complete output here, I thought that was
something for the bugtracker (I did it there =) )
Cheers, Roman
On 16.10.19 16:23, Moritz Barsnick wrote:
On Wed, Oct 16, 2019 at 14:10:58 +0200, Roman Huy-Prech wrote:
and the least amount of arguments needed to reproduce:
https://bitdash-a.akamaihd.net/content/sintel/hls/1500kbit/seq-38.ts
Incorrect URL, this is the correct one:
https://bitdash-a.akamaihd.net/content/sintel/hls/video/1500kbit/seq-38.ts
Original Chunk:
Duration: 00:00:02.00, start: 76.083333, bitrate: 919 kb/s
Using -copyts I can copy over the timings. Good. But I want 300k.
ffmpeg -i seq-38.ts -vf scale=320:240 -f mpegts -muxdelay 0 -copyts
-vcodec libx264 -crf 19 -y seq-38-copyts.ts && ffprobe seq-38-copyts.ts
Duration: 00:00:02.00, start: 76.083333, bitrate: 245 kb/s
You should always provide us with the command and its complete, uncut
console output. In this case, thanks to the sample (almost), we can
reproduce, but it's nicer to see *your* actual results, not our own.
Now I'm combining the two, I have the correct start time again, but wtf?
11696 kb/s?
ffmpeg -i seq-38.ts -vf scale=320:240 -f mpegts -muxdelay 0 -copyts
-vcodec libx264 -crf 19 -y -muxrate 300k seq-38-copyts.ts && ffprobe
seq-38-copyts.ts
Duration: 00:00:02.00, start: 76.083333, bitrate: 11696 kb/s
Apparently, ffmpeg is taking the additional 78 seconds offset from the
copyts into consideration:
frame= 48 fps= 15 q=-1.0 Lsize= 2856kB time=00:01:17.95 bitrate=
300.1kbits/s speed=24.3x
video:54kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing
overhead: 5149.501953%
(This is what we ask for the complete output for - we can point out
where iut shows you the issue!)
Note that ffmpeg thinks it encoded 1:17.95 (not 0:02.00) at
300.1kbits/s. (See all that muxing overhead!)
Looking at my resulting file size, that's even quite precisely correct:
(no. of bytes in filesize, times bits in a byte, divided by assumed
length in seconds, is the bit rate:)
$ calc "2924340 / 77.95 * 8"
~300124.69531751122514432328
Except that the resulting video isn't actually 77.95 seconds long. ;-)
Therefore, the resulting overall bit rate is wrong.
So I believe copyts is confusing the muxrate calculation, pretty much
like the guess in your second e-mail. I consider it a bug.
(BTW, there do exist other external tools to pad an MPEG-TS to a CBR.
ffmpeg's MPEG-TS does have some deficits, but this isn't supposed to be
one of them.)
Cheers,
Moritz
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