[FFmpeg-user] Combining -muxrate and -copyts results in unexpected high bitrates
Roman Huy-Prech
rossi at diehalbstarken.at
Thu Oct 17 11:57:43 EEST 2019
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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