Automatic VBR and bitrate calculation (TMPGENC)

[dvdnub]

Hi, all. I was wondering how automatic VBR works in regard to bitrate. In TMPGENC, automatic VBR setting include: Quality control (0 - 100), Maximum Bitrate, Minimum Bitrate, and padding. From what I understand so far, maximum setting is the highest bitrate the movie will ever have and minimum is the lowest bitrate; I'm assuming that the encoder will use higher bitrate when necessary, especially during a lot of motion, and it will use lowest setting as a default. The padding setting will make sure that the movie doesn't fall below the minimum bitrate. The quality setting is something I don't understand.

The reason I'm asking is because I need to know how accurate the bitrate calculator is going to be when it gives me the result-- if I use automatic VBR. Constant bitrate, I'm assuming, is always going to be accurate because the bitrate, thus the size, won't vary and the final .mpg will not be over the predicted size.

Basically, I want to know if I could just calculate the bitrate, take the result and enter it in automatic VBR maximum setting, set minimum to something low, like 1000, enable padding, set the quality to something like 75, and assume that the final mpg will have quality of the calculated bitrate and still fit on 80min cd.

Hope you guys understand what I'm trying to say here because I think I just lost myself. Thanks in advance.

[FOO]

The way to make sure the average bitrate is on target
is to do a 2 pass vbr. Otherwis the encoder has to be able
to predict the future.

[Vejita-sama]

The bitrate calculators only work for multipass VBR or CBR encodes. TMPEnc's CQ_VBR is just that (constant quaility) so you CAN NOT predict filesize if you use it.

Lots of people here have a good 'rule of thumb' to predict filesize, but it's jus that. If you have a video with lots of high motions scences you'll get a different file size then if there are mostly low motion scneces for the same runtime. So if you want to encode a 795MB VCD/CVD/SVCD or a 4,650MB DVD, stick with multipass VBR or CBR.

[ZippyP.]

CQ: 1-pass, unpredictable filesize.

2-pass: Slow (up to twice as long), predictable filesize.

I encode overnight so speed is not a factor using 2-pass. Under environmental settings - CPU, you can enable the caching of the first pass in order to speed up the process.

[FOO]

Seems like a few people are doing it too big on purpose
and using Shrink to fix screw-ups . Might make sense.