Bit rates - 1 cd vs 2, help naive newbie here

[whitetigerboy]

I've been converting DivX to MPEG-2 w TMPGenc and making SVCDs (I have about 40 now) and I've been really curious about why some of the converted movies take up 2 CDs and some 1, regardless of the length of the movie or the quality of video contained. (If the resulting file is over 750mb I cut it using the MPEG tools before burning). For example, I just converted a 1-1/2 hr DivX horror movie to mpeg2 and burned it to an SVCD and it all fit on one CD and it's sound and image quality are as good as some shorter movies that took up 2 CDs. (Is it because much of it takes place at night and dark scenes compress better?)
I've never messed with any of the bitrate settings, but I gather this is the key. I'd rather have 1 movie/1 CD but not if it means sacrificing a noticeable amount of quality. Since I've never changed the settings, I'm guessing that the compression variables of the DivX file I start out with have a lot to do with it. Is there a pretty good tutorial on dealing with bitrates in TMPGenc here? Are there other settings I can adjust? I have noticed that I can convert some of these .avi files that are in a very small screen format and fit them on one SVCD and just use the 'zoom' feature of my DVD player and the quality is about that of a DAZZLE 1 captured mpeg-1 file. I'm hoping for better. Thanks

[bewley]

I don't mess with DivX, so I can not offer you any advice to improving quality, achieving best bit rates, etc... But I can offer you a possible answer as to why TMPGEnc will produce different file sizes although your bit rates never change...

When doing VBR and CQ encodes -- and padding is not selected -- TMPGEnc utilizies its built in encoder efficiency to save space. What this means is if the bit rate required to encode a GOP is less then the minimum bit rate specified, then the encoder reserves the extra bits to apply later. The more bits placed in reserve -- and especially if this reserve is never used -- the smaller the resulting MPEG2 file. So if your source has a small picture, it will usually require much less bit rate then a file with a larger resolution. Even with padding turned on, the same thing will happen if the encoder rarely surpases the average bit rate. Although you may set an average of say 2520, if the encoder rarely hits that 2520, the resulting MPEG2 will be much smaller than expected.

Note: This same principal applies to the original encoding of the source file. A DivX file encoded at 800 kbps will encode smaller than one at the same length and resolution encoded at 1200 kbps.

I hope this offers some insight.

[whitetigerboy]

Thanks, it might if I had a glossary. VBR - variable bit rate, if selected tells TMPGenc to only use as many bits as it needs per frame, per sec?
Not sure what a GOP or a CQ is. What the mysterious thing to me is, is that the movie I decoded, Jeepers Creepers, was 90 minutes, DVD quality and on my TV, without using the DVD player's zoom I got a letterbox picture that filled the screen (width-wise) 4:3. Dude Where's My Car? for example, is about the same length, same quality, same picture size and took up 2 whole CD-Rs (filled them). The only setting I ever change on TMPGenc is to select Full Screen (Keep aspect ratio). I never know how big the resulting file is going to be. - Why would anyone choose padding? I'm just curious if there's some guide as to what settings I could select to have some control over the resulting file size with all the appropriate caveats regards picture quality. In another posting, a guy set off a storm of controversy by claiming that he achieved single-cd file size by converting a movie to .asf format from .avi before running TMPGEnc on it, and no loss of quality. I think I may try it and see.

[bewley]

Sorry about that... Let's see if I can clarify a few things...

GOP = Group of Pictures. MPEG's are made up of three picture types - I, P, B. I Frames are the largest while B Frames are the smallest. I have heard it said that one can get better quality by setting these to 1 each (IBP), while people looking to fit lots of movie on a single disc will up the number of B and P frames for better compression.

CQ = Constant Quality. I think this works like a one pass VBR as it applies a percentage of quality per frame. The problem with CQ is that the file size is extremely unpredictable -- it sounds like you are using CQ on your encodes.

The "min" setting in VBR is not a true setting unless you choose padding. The encoders I have used (TMPGEnc and CCE) will ignore this setting and actually use the lowest amount possible to encode the frames. For example, if you set your VBR min to 1200 kbit (without padding) and the current frames only require 800 kbit, then the encoder only uses the 800 kbit. So why pad? Here is one reason... Some standalone DVD players do not function correctly when trying to play movies that fall below a given bit rate. Some people are forced by their equipment to use this setting.

Why did Jeepers Creepers encode better than Dude Where's My Car? For one, Jeepers Creepers doesn't have as much action (movement scenes) as does Dude. The scenes in Jeepers are darker -- which takes less bit rate to encode. Jeepers is anamorphic (2.35:1) versus normal widescreen (1.85:1) for Dude. This means there is more area to encode on Dude than Jeepers.

If you want control over the number of discs it takes for a movie, get a bit rate calculator (like FitCD) and use CBR or 2 Pass VBR mode. This will give you absolute control on the number of discs it will take to encode a movie.

This link provides some good info on TMPGEnc:

http://www.vcdhelp.com/tmpgencexplained.htm

Good luck,

[whitetigerboy]

Thanks much. This is pretty much exactly what I was looking for. I'm printing it out and keeping it as a guide.

[bewley]

Glad to be of help.

I would recommend checking out the rest of the guides on VCDHELP... There is some good information contained within them.

Good Luck,