VirtualDub + Xvid: problem with file size in 2 pass encoding.

[marcinkk]

Hello,

I'm encoding movies from my standalone DVD recorder (some kind of VOB format, MPEG-2) to Xvid. I'm using VirtualDub (1.9.11) with fccHandler's MPEG-2 plugin (4.1). In VirtulaDub: deinterlace filter, next resize with cropping and Xvid encodnig (1.3.2 version). 1st pass to get statistics data and then 2nd pass with desired bitrate calculated with internal calculator. It worked perfectly (I think so...) 2 months ego for any desired filesize but last time I have a problem. I had 3 movies about 1.5 hours long each. I've set 192kbps for audio and the rest (about 1800 kbps) for video. It should give 2CD size, means 1400MB. But the size of the rendered file was about 2000MB The same for all 3 files. I've deinstalled and reinstalled Xvid, instaled previous (1.2) version, deinstalled/reinstalled all codecs (it resolved the one old playback problem ) and still the same. I've tried many times with 2CD and 1/3DVD size. And finally I've tried 1CD size (the setting used previously for most of the files) ... and the rendering was made as expected - the resulting file size was 700MB. All the time defalt settings was used: profile Xvid Home, general purpose quality setting. It looks like that when I'm setting video bitrate about 1000kbps everything works. If bitrate about 1800kbps was set than the resulting bitrate is about 2700kbps. PS. I've tried DivX and the result was equal to expected...

And some questions:
1. Is the described problem cause by Xvid or one of the rest of the programs used (VirtualDub + plugins)? Because the DivX worked properly then I think the Xvid is the problem.
2. What should I change (try to change...) in the profile or quality settings to get Xvid working as expected? I found a lot of options there and I don't have any idea which of them are useful in the described problem. I can make some tries but I hope somebody wil give me some useful hints

I'm new on forum. I found using google some similar problems but with too small not too big files.

Marcin

[jagabo]

If your source video has too little or too much detail, or if you are requesting too much or too little bitrate, Xvid can miss the bitrate target. Some things you can check: Near the bottom of the main Xvid configuration dialog, on the Quality Preset line, press the More button, then go to the Quantization tab. Make sure all the min values are set to 1, the max values to 31. If your files are coming out too large make sure that B-VOPs are enabled and set to 1 or 2. Adaptive quantization helps achieve lower bitrates but may also degrade the picture quality a bit.

But do you really need 1400 MB files? Are you planning on storing your files on CDs? Why not save time and encode in Target Quantizer (constant quality) mode? It requires only on pass and always delivers the quality you ask for. And the quality is pretty much the same as in 2-pass encoding. That is, if you were to encode at say, Target Quantizer = 3, and a video turned out at 1500 kbps, then did a 2-pass encoding at 1500 kbps (and assuming it actually delivers that bitrate) the two videos will be nearly identical.

In short, in 2-pass mode you know what the file size will be but you don't know the quality. In Target Quantizer mode you know what the quality will be but you don't know the file size.

The only real down side to Target Quantizer mode is that the bitrate limit for the selected profile is ignored. Ie, if a shot requires more bitrate than the current profile supports it will get it. The bitrate peak may cause stuttering on the intended player. 2-pass mode always limits bitrate peaks according to the profile.

[marcinkk]

Sorry for a long time without answer. I just need to find some time for the test.

"If your source video has too little or too much detail, or if you are requesting too much or too little bitrate, Xvid can miss the bitrate target." Yes, it is possible that my source has too little detail and it makes the problem, but ... I've changed B-VOPs from 2 to 1, like suggested: "If your files are coming out too large make sure that B-VOPs are enabled and set to 1 or 2.", and my encoded file has the requested size

Thanks jagabo for help Only one little change, but very helpful for me

About constant quality and file size requirements: Yes, I agree that I can just set the quantizer and it should be enough but I have some reasons for 2-pass encoding. First: yes, I often store files on CD/DVD so I need the them in the requested size. 1400 MB is not always my aim but the unexpected file size makes storing a little complicated Second: As I know 2-pass encoding gives "the best quality for the desired filesize"? Am I right? Second pass gives the information about bitrate and it is enough form me to decide: 1CD, 2CD, 1/3 DVD or something else.

[jagabo]

As I know 2-pass encoding gives "the best quality for the desired filesize"? Am I right?

Yes. But CQ encoding gives the same quality at the same size*. It's just very difficult to get a particular size with CQ encoding.

* At very low bitrates 2-pass VBR gives slightly better image quality.