Video Bitrate after remuxing with MKVtoolnix

[PeterProg]

Hi

Newbie question here. I've just been editing a large 1080p music video file (around 5.5Gb, mp4 format) to both improve the sound and to reduce the length.

First, I extracted the audio and added some EQ and Compression to boost the sound then used MKVtoolnix to replace the existing audio file with the enhanced one, remuxing with the enhanced audio file in place. I now had the file in an mkv container instead of mp4 - no problem with that - audio much improved and the video bitrate remained unchanged, as I expected.

I then trimmed just over thirty minutes of filler at the start of the file and around ten minutes at the end, reducing the overall length from around two hours fifty minutes to two hours and ten minutes. Again, I used MKVtoolnix for the editing, going to the output tab and and then under Splitting and Split mode, used 'split by parts based on timecodes' and chose the relevant split points and remuxed.

The results look fine, but I notice when I check the (now shorter) output file properties that the reported video bitrate has increased, from 4108 kbps to 4905 kbps. This surprises me, as I expected the video bitrate to be unchanged given that it is a per seconds measure, not an aggregate. Am I misunderstanding kbps?

Thanks

[jagabo]

The bitrate report may or may not be correct. But note that with variable bitrate videos the bitrate is not constant over the entire video. If the part you trimmed away was largely static it may have been encoded at a lower bitrate than the average, meaning the average for the remaining portion is now higher. As a simple example say you have four frames with bitrates of:

1000 5000 5000 1000

The average bitrate is 3000 (12000/4). If you cut away the first and last frames you have

5000 5000

The average of the remaining two frames is now 5000.

[butterw]

Video bitrate isn't typically constant.

Check the bitrate of the part you cut out, it's likely significantly lower.

[PeterProg]

Thanks guys, that was exactly the issue. I was thinking in terms of a constant bitrate rather than an average.

The bits I cut out were largely static and I can see that the bitrate on those parts is considerably lower than the average on the original larger file. So that explains the higher average on the ne, shorter file.

Thanks for your help!