Bitrate Allocation

[apfraats]

As far as I can see a segment in DVD-RB-PRO is equal to a chapter in the original movie, correct me if i'm wrong.

The second fact is that each segment gets dynamically assigned bitrate, as far as I know.

I see this when DVD-RB-PRO is making its VAF files.

I see different average bitrages for different VAF's.

How does DVD-RB-PRO determines what the average bitrate for a particular segment (=chapter) should be ?

It infuences CCE 2.66 I use. Some segments get average of under 4 Mbps, other to about 5 mbps ????

Why this difference ? And can it not be that a segment that gets assigned a lower average bitrate has one demanding scene in it, that can't be kept up by CCE because of the lower bitrate choosen for the particular segment ?

I mean, can an action scene gets 'lost' in bitrate, because the overal segment bitrate is low, but the scene itself is very demanding ?

What's the logic of using different bitrates for each segment, if a segment indeed is a chapter ?

Thanx.

[wmansir]

Each segment is a cell. Most chapters are 1-2 cells long, but can be longer.

DVD-RB's requires the video to be broken down into cells.* DVD-RB originally gave each cell the same bitrate. Then jdobbs added an option called "dynamic bitrate assignment". It soon became the default method and after a long while it was decided that there really wasn't a need to NOT use DBA, so it's always turned on now and the option was removed.

DBA works by reducing each cell by the same % of bitrate from the original. So if you overall compression is 70% then a cell with an original 5000kbps average bitrate is reencoded at 3500kbps and 4000kbps original cell is given 2800kbps. The logic being that the original bitrate reflects the action/bitrate demand of the cell in the overall movie.

Regarding a sudden high demand scene in an overall low demand cell, the responsibility for distributing the bits within the cell falls on the encoder. Some do a better job than others. AFAIK all supported encoders are utilize some type of VBR when used with DVD-RB, so they should at least attempt to distribute the bitrate according to demand.

*I know this method greatly increases support for encoders that cannot force I-frames in the output but I don't recall if that was the sole reason for method, a consideration, or just an extra benefit from jdobb's re-authoring engine design.

[Trahald]

IIRC jdobbs reasoning was specifically for cce basic (since price was reasonable over SP ) because of its lack of support of forced i-frame (chapter) setting.