Getting small file size using GKnot?

[Gondo]

I've encoded lots of movies using GKnot. I open the .d3v projext and enter an .ac3 file then calculate the bitrate using 2 or 3 CDs to get over 1500kbps. Well so far every DivX has turned exactly 1400 or 2100MB accordingly.

lately I've had a few movies that I'll encode using 3 CDs. THe final file size should be around 1800MB or so with room for a 400MB .ac3 file. But the file is ending up around 1100MB.

Why is the file size so small? Are these movies capable of being super compressed or what? It's funny how GKnot caculates based on 3 CDs and it ends up around 2 CDs

Can anyone clear this one up for me?

[khp]

http://forum.digital-digest.com/showthread.php?s=&threadid=13504

[Gondo]

I even tried reducing the bitrate from 2100 to 1500kbps and the final file size is the same? It's wierd how some movies do this while others work perfectly fine.

[khp]

No it's perfectly logical. Above some bitrate the codec simply can't use up all the bitrate it's allowed.

Exacty how high this point is, is different for each movie, because some movies compress better than others, because they contain less motion or video noise.

[audi]

did you find out the problem

gondo, i was just wondering if you were able to out why it was giving you such small file sizes, i am having the same problem .

[Gondo]

havn't figured it out yet

I have tried everything form 1500kbps and above and ti gives the same file size. There seems to be a limit and after you go above that bitrate it creates the same file size.

I guess I am stuck reducing 3 CD rips to 2 CDs. This problem has occured a lot recently on new releases. I havn't had this problem in the past.

[khp]

Re: havn't figured it out yet

Originally posted by Gondo
This problem has occured a lot recently on new releases. I havn't had this problem in the past.

If you recently started using divx5 with B-frames enabled, then this is probably the cause. At maximum quality B-frames reduce the filesize by about 20-25% on average.

[Gondo]

Reached maximum quality?

It seems as htough, after reading thorugh many posts, that I have reached the maximum quality possible.

How is this possible?

I mean its not like some songs can be made 160kbps MP3 while others cannot go past 56kbps because that is their amximum attainable quality. Just what is the reason here?

Also wondering exactly what bi-directional encoding and GMC does in DivX 5.02. Which of the 2 is realted to B-Frames, or is B-Frames?

[khp]

Re: Reached maximum quality?

Originally posted by Gondo
I mean its not like some songs can be made 160kbps MP3 while others cannot go past 56kbps because that is their amximum attainable quality. Just what is the reason here?

The reason a CBR MP3 stream can stay at a fixed bitrate, even if it's encoding total silence, is that it's willing to waste bits on absolutly nothing. With a modern vbr MP3 encoder, like lame, this won't happen, it will go to a very very low bitrate, if you encode a completly silent track. Divx5 does not allow it self to waste bits like that.

Originally posted by Gondo

Also wondering exactly what bi-directional encoding and GMC does in DivX 5.02. Which of the 2 is realted to B-Frames, or is B-Frames?

B-frame are created when you use Bi-directional encoding. B-frames are encoded using parts of both the frame just before it and the frame just after it. This means that it can be encoded more efficiently. And because no frames depend on a B-frame, B-frames can be encoded at a lower quality level than other frames without much loss of quality.

GMC, or Global Motion Compensation, uses motion vectors that acts on the entire frame, as opposed to normal motion vectors which only act on a single macroblock. This works quite well when the entire frame moves un a uniform direction, like in a panning picture or scrolling text (endcredits). As I said B-frames increase compressability by about 20-25%, while GMC improves compression by maybe 1-3%.

[Gondo]

Thanks for the help

Thanks for the help. It reallly cleared things up for me. Still hard to believe that increasing the bitrate does nothing to improve the quality/filesize. I mean when there is total action on screen you'd think it would use the extra bitrate and therefore filesize.

[khp]

In DivX (or any other DCT based video codec) quality is controlled by quantizers. Higher quantizers mean lower quality. The lowest quantizer allowed is 2, when all the frames use quantizer 2, the quality can't be increased any more.

[Vaughn]

I have been struggling with low file sizes for some time now. Most of my rips are intended to be 2CD's however they are usually 1100 to 1200 MB. I use Gknot .26 and Divx5 and i have included Khp's script of the analyse.log file from my latest rip. The quality is not bad, however it is blocky on the darker scenes, especially the black & white ones.

I display these on a Nvidia video card so that may be part of the issue hard to say. At any rate any suggestions on how to increase the file size a little? Should i be disabling b-frames somehow?

[khp]

You have reached maximum quality.

The only way to remove the blocking effects would be to increase the post processing durring playback.

[khp]

Originally posted by Vaughn
Should i be disabling b-frames somehow?

Disabling B-frames is a very drastic measure imho. Compared to a maximum quality encode with B-frames enable, you will need to increase the file size by 15%-20% untill you start getting better quality without B-frames.

[Vaughn]

Hmm... thx for your reply. How may i increase increase the post processing during playback in a player like PowerDVD or WinDVD?