file size/bitrate confusion, advice sought.

[super8rescue]

I have a silent (no audio track, just video) video file which is split in to three parts.

mp4 container, total file size 1.93gb. each clip is 30fps @ 14735kbps

I put the three parts in to DaVinci Resolve to join them together.

I set the timeline frame rate at 30fps and then used 'change clip speed' to 15fps

I then exported the video in mp4 limiting the data rate to 9000Kb/s with an empty audio track at 117kbps

Am I right in thinking that the fact DV Resolve added a 'silent' blank audio track has taken the exported video up 2.36gb even tho' I have squashed the video data rate?

Or is it because changing the speed to 15fps has doubled the length of the video?

[davexnet]

Has changing the speed to 15 fps doubled the runtime of the video?
Even if it did, filesize = bitrate x runtime and fps is not part of the formula

Look at the result in mediainfo in text view to get the details

[Cornucopia]

If in Davinci Resolve you chose "maintain timing", the duration/running time is the same. If you chose "stretch to fit", it would (in this case) double the run time.

Doing the math:
Assuming you meant that the total of all original clips was 1.93 GiB (aka 16,190,013.44kb), and the overall average biitrate was 14735kbps, plugging that into the formula...

16,190,013.44 / 14735 = 1,098.7453980319 sec (aka 18.3124233005 min.)

Assuming your processing did modify the duration to 36.624846601 min. (aka 2,197.49079606 sec), and your video AVERAGE bitrate (not MAX - are you sure about this?) was 9000kbps and your audio bitrate was set to 117kbps (a very non-standard rate for audio!), you would do the math this way:

2,197.49079606 sec * 9117 kbps = 20,034,523.587679 kb (aka 2.3883013234 GiB)

This corresponds very closely with what you experienced, and given rounding, seems to match.

Note: 2,197.49079606 * 117 = 31.3850614183 MiB (aka 0.030649474 GiB), so it is clear the audio barely contributes to the overall.


Scott