Confusion on file size vs. constant bitrate

vbimport

#1

I have a DVD that I’m trying to split into chapters, and am confused on how this works vs. what I’m thinking. I’m trying to split the DVD into separate files for each chapter, and wish to have a constant bitrate between chapters but it doesn’t seem to work this way when I select fixed bitrate. I was expecting that each chapter would be ripped at the bitrate, and would receive files of varying sizes based upon that bitrate for each chapter. What I seem to be receiving is similar to selecting fixed file size, but just based upon bitrate rather than size. Is this the way it’s supposed to work?

Here’s what I’m looking to accomplish, and if you have a way to do it other than individual chapter rips I would love to hear about it.

The DVD has 17 chapters, some of these chapters are 10 minutes long, full video clips, but some of them are only 2 minutes long, kind of like an interlude between clips. I would like to record these at a constant bitrate that would keep the same 2300 kbps between chapters, meaning the 10 minute chapters would be of a larger file size than the 2 minute chapters, but with the same bit depth recording. I don’t find this to be the case when selecting fixed bitrate…

I ripped a single chapter that is 3 minutes long with a fixed bitrate of 2300 kbps @ avi/h.264/mp3 which resulted in a file size of 62,808 KB. I then ripped two chapters, the first being the same chapter as above with another that is close to 3 minutes long as well. Split for each chapter, and fixed bitrate of 2300 kbps was selected. I would expect both files to be near the same 62,808 KB, but no, file 1 was 48,431 KB and file 2 was 48,368…what am I missing here? Shouldn’t both files be around 62,808 if I’m ripping two chapters that are close to the same size, with a fixed bitrate?


#2

No thoughts?


#3

What is the particular virtue of keeping the identical bitrate? Try setting the video bitrate slider for the same number of bits/pixel rather than a particular target bitrate for each file. Examine the output files with VLC or MediaInfo and see what the video stream bitrate really is. When you say “chapters” do you really mean Titles?


#4

[QUOTE=signals;2159634]What is the particular virtue of keeping the identical bitrate? Try setting the video bitrate slider for the same number of bits/pixel rather than a particular target bitrate for each file. Examine the output files with VLC or MediaInfo and see what the video stream bitrate really is. When you say “chapters” do you really mean Titles?[/QUOTE]

Here’s another example of what’s happening. When I rip the entire DVD with the split by chapter selected, and select a bitrate of 2300 kbps, all the files end up nearly identical sizes. The problem with this is that a chapter that is 10 minutes long is, just for example, 48 MB and it looks horrid and another chapter that is only 30 seconds long is also 48 MB and it look just like the original. That’s not the way I would expect constant bitrate to work. I would expect the file size to be bigger for the 10 minute long chapter than the 30 second chapter but have the same video quality.

(this is using DVD to Mobile option / generic settings)

I’ll check it out with VLC and post back when I’m done.


#5

I think I understand better now. I will try it on my machine and see what I get. Sounds like you have selected fixed file size.


#6

Ok, here we go…

Chapter 5 of this DVD is 1:05 long.
Chapter 6 of this DVD is 5:41 long.

Here are the rip settings in the DVD to Mobile / Generic options…

The file sizes ended up with Chapter 5 being 58,547 KB and Chapter 6 being 58,823 KB.

When playing the ripped files in VLC, Chapter 5’s bitrate is listed as…

…and Chapter 6…

Certainly doesn’t appear correct to me!


#7

For 2-pass this may be correct. I tried it with the same codec (H.264) with 1-pass and it worked perfectly. Exactly the same bitrate for each of 3 chapters and the file size tracked the run time within a few percent. I never use 2-pass with H.264 and didn’t realize that’s what you were doing. When I have time I will try it again with your settings, but I think it will come out just as your conversion did.


#8

Interesting, I just tried the same to chapters with single pass settings and it seemed to work fine as well, looks like a bug when selecting constant bitrate and double pass encoding.

I also encoded a single file (chapter 6) by itself with double pass encoding with the same settings, and the filesize is about 30MB smaller than the above method.

Something’s not right here, I’ll post a bug to DVDFab.