[FFmpeg-user] Altering mp4 container and video stream headers without transcoding

[Tom Evans]

On Wed, Dec 12, 2012 at 11:23 AM, Philip Kahle <philip.kahle at gmail.com> wrote:
> Hi all,
>
> I have a huge amount of mp4 files (video mpeg4, audio aac) with wrong
> aspect ratio and I want to fix that on container/header level without
> touching the streams (due to time and quality reasons).
>
> I already tried altering the header with mp4box and it seems to work.
> However, the source file and the resulting one slightly differ in
> bitrate according to ffprobe. The only differences in ffprobes output
> (-show_streams -show_format):
>
> Source file:
>
> Duration: 00:00:10.40, start: 0.000000, bitrate: 5809 kb/s Stream
> #0:0(und): Video: mpeg4 (Simple Profile) (mp4v / 0x7634706D), yuv420p,
> 720x576 [SAR 1:1 DAR 5:4], 5612 kb/s, 25 fps, 25 tbr, 25 tbn, 25 tbc
>
> size=7549051
>
> Result:
>
> Duration: 00:00:10.40, start: 0.000000, bitrate: 5806 kb/s Stream
> #0:0(und): Video: mpeg4 (Simple Profile) (mp4v / 0x7634706D), yuv420p,
> 720x576 [SAR 16:15 DAR 4:3], 5612 kb/s, 25 fps, 25 tbr, 25 tbn, 25 tbc
>
> size=7551734
>
> My questions are:
> How can the aspect ratio possibly impact the bitrate of the stream? Or
> does this value just change with the file size and the way ffprobe
> computes the VBR?
> Do I have to expect defects with this approach?
>
> And what I could not find in the documentation: Is there a way to fix
> these files with ffmpeg without doing any transcoding?
>
> Thanks and kind regards,
> Philip

Wild guess: the first bitrate reported (which differs) is the
estimated bitrate of the combined audio and video streams. It differs
by a tiny amount because it is an estimate.

The second bitrate reported is the bitrate of the video stream, and is
the same in both files.

Cheers

Tom