Bitrate vs Percentage and cropping

[W0mble]

Hi,

I have done a number of backups with AutoGK and have tried to extend my skills to the GK front end however I am confused by the bitrate concept.

My question for the wise is: How do I choose the same quality of encode in GK as I can in AutoGK using the Percentage Option?

The rest of this post is ramblings as to why I want to be able to do this, I might (very likely) be wrong in the way I am thinking.

I always use the Percentage Option in AutoGK, at the default of 75%, I have never had any problem with the quality. I care more about quality than file size, but that said I would like my file size to be as small as possible for the 75% quality I am getting and I understand this is better acheived with a two pass encode which AutoGK will not do - I always use XviD.

I always choose a fixed width of 720 in AutoGK (and AC3).

My concern with using AutoGK to do the encoding is because I have no idea what it is doing with the Auto Cropping, I would like to start using GK because then I can control the cropping. I have read in another Thread that AutoGK will never have a cropping tool so I have to work out how to work GK :)

I would prefer to be able to crop my DVDs manually so as to never remove any of the picture, I would always rather have two or four pixels of black at the top and bottom than remove even one pixel of picture.

But I do not understand how to select a bitrate that will give me an equivilant quality to what choosing 75% does in AutoGK, sure I can always choose a great bit rate so that my backup looks perfect, but the file size is substantially larger than what AutoGK produces and the quality is no better.

Any help greatly appreciated.

Thanks
W0mble

[manono]

Hi W0mble-

There are 3 variables at work here; percentage quality, resolution, and file size/bitrate. You can control any 2 of them, but not the third. So, if you want 720*xxx resolution, and you want 75% or thereabouts quality, then file size control flies out the window. Run a compress test before encoding and jiggle the file size/bitrate until the results show 75%.

I would always rather have two or four pixels of black at the top and bottom than remove even one pixel of picture.

I recommend not keeping any black, even if it means removing a few pixels of picture, but you didn't ask about that. In any event, keep an eye on the Aspect Error as you're cropping for your chosen resolution.

[W0mble]

Run a compress test before encoding and jiggle the file size/bitrate until the results show 75%.

This is the bit I am having difficulty with, how do I know what is a good bitrate for the %'age wanted? How does AutoGK go about calculating this? As I said before I don't care about file size.

Thanks.

W0mble

[manono]

Hi-

AutoGK's quality percentage sets an average quant based on this formula:

((MaxQuant-MinQuant)/100 * (100-quality)) + MinQuant

That was the old way of doing it, anyway. With the XviD RC4 included now, I suspect that 75% is equivalent to an average quant of 2.25. A GKnot 2-pass at the same percentage will be roughly of the same size and quality, if all the other settings are the same as AutoGK's. So, how do you get the 75% in GKnot? I already told you. Run a compression test at your chosen resolution, check the resulting percentage, and then adjust the file size/bitrate until you reach 75%. Then run your 2 passes for that file size.

I would like my file size to be as small as possible for the 75% quality I am getting and I understand this is better acheived with a two pass encode

You understand wrong. The only advantage to running 2 passes is that it will allow you to hit a specific file size (like to fill 1 or 2 CDs). If you don't care about the final size, then there's no point in using the 2-pass method. I'd suggest that you stick with AutoGK, or else just set up a 1-pass encode for XviD directly in VDubMod with a chosen quant of 2.25.