Higher compression rate by using more B frames. Comments?

[SaSi]

I know the subject of using B frames is (or was) controversial. However, I have done some experiments with variations in the GOP structure using constant quality compression.

The "standard" structure for a GOP would be IBBPBBPBB..., i.e. an index frame, two B-frames, a P-frame, etc.

A B-frame occupies much less space than a P-frame (press CTRL+L in Tmpgenc to see a real time log of each frame and size). An average ratio of size would be 3:1, i.e. a P frame is as much as 3 B-frames.

Now, if we change the GOP structure to become IBBBPBBBPBBB..., i.e. 3 B-frames for every P-frame, then, less space would be required.

Increasing the ratio of B-frames to the rate of P frames, i.e. IBBBBPBBBBP..., would further increase the compression ratio (for a given quality).

The same applies for VBR compression. With the average bitrate selected, the encoder will be able to have better quality in the I, P and B frames.

I have found that for a couple of movies, doubling the B frames results in a significant decrease in file size. As I have only done 2 films and 2 tests, I can't say we have a 10% or 30% better compression (with same picture quality), but the decrease is significant.

Of course, playback on DVD players is required and to that part I have not found any problems in the playback on a yamakawa 215 standalone.

Any feedback or comments?

[DJRumpy]

As long as you stay within specs for DVD, than anything goes. Look on Kwag's site. If it involves GOP structure, he's probably already done it. Just make sure you stay within spec for your region.

You should be able to download some of his templates, and examine the gop structure to see what makes them tick. I'd be curious as to what results you get.