MediaInfo reported bit rate query

[Bully9]

Not a very sexy title, but i'd be interested to know...

I've made an AVC video file from a recorded HDTV broadcast. In mediainfo (just the video file) it tells me the length is 22mn 38s and bit rate is 3500kbps, which is what i specified before encoding.

But then if i run the file through mkvmerge and cut off the first 2 seconds (Global->split after timecode->00:00:02.00), mediainfo reports bitrates of 3500kbps for the 2s file and 3432kbps for the 22m 36s file. This seems unlikely - indeed mathematically impossible - to me. Can anyone please explain? Thanks.

[poisondeathray]

Not a very sexy title, but i'd be interested to know...

I've made an AVC video file from a recorded HDTV broadcast. In mediainfo (just the video file) it tells me the length is 22mn 38s and bit rate is 3500kbps, which is what i specified before encoding.

But then if i run the file through mkvmerge and cut off the first 2 seconds (Global->split after timecode->00:00:02.00), mediainfo reports bitrates of 3500kbps for the 2s file and 3432kbps for the 22m 36s file. This seems unlikely - indeed mathematically impossible - to me. Can anyone please explain? Thanks.

why would it be mathematically impossible? "kbps" is an average bit rate . If that intro section had lots of complexity , motion, it might require a large I-frame, large P-frames

what is the filesize of the 2sec segment? That is what determines if it's "mathematically" possible. Avg bitrate is simply filesize/running time

filesize = avg bitrate * running time

avg bitrate = filesize / running time

(don't forget to do bits vs bytes conversion here)

Moreover - mediainfo isn't necessarily accurate for these types of things; it doesn't scan the entire file, it derives information from reading the header and inferring from other info like the filesize . Also, very small files may skew averages

use something like bitrate viewer if you want more accuracy, it will scan through the video instead of reading the header

[jagabo]

Not a very sexy title, but i'd be interested to know...

I've made an AVC video file from a recorded HDTV broadcast. In mediainfo (just the video file) it tells me the length is 22mn 38s and bit rate is 3500kbps, which is what i specified before encoding.

But then if i run the file through mkvmerge and cut off the first 2 seconds (Global->split after timecode->00:00:02.00), mediainfo reports bitrates of 3500kbps for the 2s file and 3432kbps for the 22m 36s file. This seems unlikely - indeed mathematically impossible - to me. Can anyone please explain? Thanks.

why would it be mathematically impossible?

For the two segments to average the 3500 kbps of the original, the 2s segment must have a kbps greater than 3500, to make up for the lower average of the other segment.

The real issue is that the values reported by MediaInfo aren't exact. It doesn't actually parse the video data. It reports the value in the file header or it estimates by taking the total file size, subtracting the audio size, then dividing by the run time (or some such crude method). Use MPEG File Bitrate Viewer for more accurate reports.

[Bully9]

Thanks guys, that's cleared that up for me. The data from bitrate viewer make complete sense.