AVI file size decreases after embedding subtitles. Why?

[Sulli]

Can someone please help. Im trying to embed aubtitles to AVI files. It works successfully but when finished the the size of the AVI file is substantially smaller than it was to begin with e.g from 1.5gb to 850mb. Firstly i convert my SRT file to SSA using Divxland Media Subtitler 2.0.8. When converfted i then add/embed the subtitles to the AVI file using virtual dub 1.9.11. When complete the subtitles are embedded as they should be but the file size has shrunk. Can someone please offer a helpful suggestion.

[Baldrick]

The file size depends on the video and audio bitrate. You can adjust it in virtualdub under video->compression and click configure on the codec you are using.

[Sulli]

Thanks Baldrick this has indeed solved my problem. However i now have a firther question. After when in virtual dub i go video>compression and click configure it opens up a box with many options none of which i understand. I did some experimentation and used the slider to change the rate on the "Target Quantizer" ( whatever that is) and by changing the number ( between 1 and 31) i was able to obtain different completed file sizes. However to finish with the exact same size file with which i began with it is now trial and error. So it may take 2, 3, 4 even 5 tries before i get the exact size. The question is, is there anyway to determine before hand what number i need to place in the "target quantizer" field before i begin?

Once again thanks for your help. If there is no solution and it has to be trial and error im still much better placed than i was before you answered my intial question.

[manono]

The question is, is there anyway to determine before hand what number i need to place in the "target quantizer" field before i begin?

No, there is not. You can just run a 1-pass quant 3 encode and be done with it, knowing the end result will look reasonably decent. The final size may be larger; it may be smaller. Is there any particular reason it has to be the same size as the original AVI?

The only way to achieve a final size close to the original is to Direct Stream the audio (keep it the same - don't reencode it) and use the same bitrate as for the source video, doing a 2-pass encode. Actually, since you're sticking in subs this time, it might be better to aim for a slightly larger size.

[Sulli]

Hello Manono, my understading of these things is limited so im not sure how necessary it is to have the exact size. My intial query came about because the file reduced from 1.50g to 850mb and I thought that cant be right. I also thought that cant be good. I dont want files to be too much smaller as I imagine the quality would be compromised. Is that a correct assumption? I dont want them much larger as they will end up using more space than necessary on the drive they are stored on. If larger size meant better quality i would accept that but i dont see how one can improve the quality of the original file just by making it larger? Surely you cant improve on the original? So I just thought if the subs are usually less than 100kb the files should be pretty much the same size even with subs embedded. I guess if i had a better understanding of things id know how much tolerance i could have either way without it being a problem. Just one further thing- the virtual dub instructions say to do "direct stream copy" on the audiio so i am doing that. Thanks greatly for your help.