HC VBR/CBR Setting: What Does It Do?

[TomBrooklyn]

Under settings for the HC Encoder, what does the VBR/CBR Setting do?

Mine is set at 20. I guess that's the default. Under what conditions might someone want to change it and what value might they change it to?

[jdobbs]

It isn't really a "VBR/CBR" setting -- it is a bias setting. The output stays VBR no matter how it is set, but the characteristic of the output changes so there is less "swing" in terms of bitrate allocation. It makes it "more like VBR" or "more like CBR".

Raising the setting can improve encodes where more bitrate applied to low-demand areas is needed. For example, it might help when you are seeing blockiness in dark or fade-in scenes. Remember, though, that the bits have to come from somewhere -- so it is pulled from other areas of the video.

[Movie_Maker]

I ran into this thread while searching on the subject of VBR Bias. (you might have seen my recent post in the mpeg-2 encoding forum).

Remember, though, that the bits have to come from somewhere -- so it is pulled from other areas of the video.

I didn't quite understand this part. If a higher VBR bias is used wouldn't it just increase the file size to make room for increased overall bitrate ?
Are you saying it will give less bitrate to areas now where it would have given more bitrate had the bias been less ? Can you please explain a little more if that is true ?

[SpazzHH]

What is being said is that you still have the same bits in the segment. What the Bias does is tell the encoder where to put the bits at. With a low bias setting, there are bigger swings in the bitrate, with low action scenes receiving very little, and high action scenes receiving the lions share(More Variable). With a high bias setting, all scenes receive more or less the same bitrate, with little change from scene to scene(More Constant).

Hope this explanation helps.

Spazz

[jdobbs]

I didn't quite understand this part. If a higher VBR bias is used wouldn't it just increase the file size to make room for increased overall bitrate ? But in VBR you are telling the encoder what average bitrate is required in your encode -- and the output size of the encode is expected to be what you have specified per your bitrate. So any time you take bits, they have to come from somewhere.

This is especially true when encoding for DVD. The disc is a fixed size and you have to control the output size of all your segments in order to ensure it all fits when complete.