Question about Encoding Video to DVD...Constant or Variable Bitrate ??

[VEBouto]

I have encoded a few videos to DVD (Pal 4:3 Format) and was wondering about what the difference in quality is between choosing constant or variable bitrates when encoding the video.
I take it that Variable will give the better quality, cos it does the 2 pass encoding and it says that it will take longer, but i have noticed that the finished file size of the video compared to constant bitrate videos is smaller.
So if it is smaller, how can it be better quality?

[Alex_ander]

VBR gives better quality compared to CBR of the same file size in case the average bitrate is lower than DVD specs limit. The best quality is at CBR at maximum allowed bitrate since in this case VBR could only reduce bitrate for some portions of video for reducing file size. So when you are not limited in space, CBR is better in quality and encoding speed (only one pass needed).

[jagabo]

With MPEG compression some frames don't require as much bitrate as others. For example, an all black frame can compress down to nearly nothing and still be perfectly reproduced. Since MPEG often encodes only the difference between successive frames, a series of identical frames also compresses down to nearly nothing. In a talking head shot with only the speakers head moving only the moving head needs to be updated at each frame, the still background doesn't. This requires much less bitrate than encoding the entire frame at each frame.

CBR encoding uses a fixed bitrate for every frame regardless of whether it really needs it. A VBR encoding will use less bitrate for the above sequences allowing more to be used in scenes where it's needed.

If you are putting less than an hour or so on a DVD, CBR at a high bitrate (eg 8000 kbps) will look pretty much the same as VBR at that same average bitrate. But if you are putting more than an hour on a DVD there will be a noticeable difference.

Say for example you have a two hour movie. The first half of the movie is all black (and rather boring!). The second half is all action shots with fog, smoke, lots of movement, and static noise. To fit that on a single layer DVD you would use ~4000 kbps. With CBR encoding all those black frames in the first half of the DVD would get 4000 kbps. All action in the second half would also get 4000 kbps and wouldn't look too good. With VBR encoding the first half of the movie would get very little bitrate, maybe 200 kbps. That means the second half of the movie can use 7800 kbps which will look much better.

[VEBouto]

Thanks for the reply.
I only use between 60 and 65 mins at the highest quality on my DVD-R's, so i suppose that i should go with the Constant Bitrate.

[dcsos]

if you ever need to pull the material offa the DVD you've burned, USE CBR
VBR is no good for re editing.

Also, this is the rule of thumb I would think..
USE CBR unless you cannot fit the project to the media with sufficient quality using CBR
this is when VBR is useful.