tmpgenc svcd bitrate setting questions

[Wills]

Hi all

I have some SVCDs that were authored at the wrong aspect ratio - using this forum I know how to get Vdub (mpeg version) to frameserve the original to TMPGEnc with a resize filter, thus correcting the aspect ratio of the movie. I've done this several times.

However, I have an issue that I really can't figure out.
The TMPGEnc wizard is great when you are converting avis to svcds, since the wizard does all the work for you.

I've figured out how to access most of the areas of the wizard manually through the Settings menu - like the source range, which is great, but:

How can I access the part of the wizard that automatically works out the bitrate needed for the image to fit onto a 700 meg CD ?

I thought I'd try something - I figured if I use the wizard on the original svcd file, the wizard would tell me what bitrate would be needed to make an 800 meg file (perfect for a 700 meg cd) I could then cancel the wizard, and do it all again manually with the resize filter from vdub.

However, if you select an mpg in the wizard, TMPGEnc crashes. It doesn't expect to be given an svcd file, so it doesn't work.

So how can the size wizard be accessed without using the wizard?

And on a related note, the bitrate settings in TMGEnc - what are their relevant merits? The automatic bitrate, for example - does that work out the bitrate on the fly - if it does, what size svcd file is it trying to make? Where can I specify that? And is it worth using?

I know you can get the average bitrate of an svcd from the file info page within vdub, but this doesn't seem to be a reliable figure - one film i have, vdub claims that the average bitrate is 1315k, but that can't be true because a) the image quality is way too good for that in the movie, and b)don't svcds have a minimum bitrate compatablity figure of 1500 something?

Thanks for any info anyone can impart.

Versions:
TMPGEnc is 2.59.47.155
VDub is MPEG2 version build 1.5.9 build 18143

cheers
will

[eas]

I think the bitrate calculator is only available though the wizard. There are, however, various bitrate calculators that do something similar. Check the tools section of this site.

The thing about bitrate is that its really simple math.

       (audio bitrate + video bitrate) * duration in seconds 
     ---------------------------------------------------------------  = filesize
      1024 bits/kilobit * 1024 kilobits/megabit * 8 bits/byte

resolution, etc, doesn't figure into it

As for the avg bitrate of your SVCD, its totally possible that its 1315kbps. According to the comparison on this site, ~1000kbps is the minimum, and since it sounds like these are homemade SVCDs, its also quite possible that they aren't perfectly to spec. Many players will play them fine anyway.

Image quality is a relationship between source image quality, bitrate, degree of motion in the source clip and resolution of the encoded image, so a low motion clip might look great at low bitrates. Also, if the original wasn't at a proper aspect ratio, it sounds like it might also be at a lower than standard resolution.

If the clip was encoded at a variable bit rate, the actual bitrate might be much higher in high-motion sceens (and lower in low motion sceens), preserving image quality and maximizing storage efficiency.

As for predicting file sizes with the various rate control modes, there are only two ways to get a relatively precise estimate: Constant Bitrate, or 2- pass VBR. The others make it possible to predict file size, but only within the range of the minimums and maximums you set.

Delving deeper into the relative merits is beyond both my time and my current level of understanding. I've found some good info on this site, I suggest you look at it.