[FFmpeg-user] Truncated file with ffmpeg but resulting output is x2 bigger!?

[Philip Rhoades]

Moritz,


On Fri Oct 16 13:51:11 EEST 2020, FFmpeg user discussions wrote:
> On Fri, Oct 16, 2020 at 20:26:16 +1100, FFmpeg user discussions wrote:
>> People,
>> 
>> I truncated a 14s file with:
>> 
>>   ffmpeg -i 20201016_055828.mp4 -t 00:00:09 -async 1 cut.mp4
>> 
>> and although about 35% of the frames were removed, the resulting mp4
>> was double the size - why is this?  I did a before and after mediainfo
>> comparison - see here:
>> 
>>   http://pricom.com.au/Diff.html
>> 
>> and the only things that jump out at me are:
>> 
>>   Format settings                          : CABAC / 4 Ref Frames
>>   Format settings, Reference frames        : 4 frame
>> 
>> ie changed from 1 to 4 . . but is that the explanation?
>> 
>> I am using:
>> 
>>   ffmpeg-4.2.4-1.fc31.x86_64
>> 
>> Thanks,
>> 
>> Phil.
> 
> You are reencoding the streams inside the file. ffmpeg's default
> encoding parameters apparently result in streams with more bandwidth.
> You can fine-tune the encoding parameters, of course.
> 
> Re-encoding is not a good idea, as it always loses quality. If you just
> want to "clip", be sure to use the "copy" codec:
> 
> $ ffmpeg -i 20201016_055828.mp4 -t 00:00:09 -c copy cut.mp4
> 
> (Note that cutting without re-encoding may or may not be exact. Feel
> free to test first though.)


Ah! . . of course . .

Thanks!

Phil.
-- 
Philip Rhoades

PO Box 896
Cowra  NSW  2794
Australia
E-mail:  phil at pricom.com.au