ConvertXtoDVD question re VOB files

I’ve been using ConvertXtoDVD for a bit, and generally it’s working excellent!

I was sort of wonder how it treated VOB source files. I started with a VOB file that was created by my DVR. After running it through CXtD, the resulting VOB file was quite a bit smaller (even though I selected best quality).

Now, I’m not sure if this means that there’s a quality loss or if CXtD can optimize the “guts” of a VOB file without loss of quality.

Next, I though, what if I ran the output from CXtD back through itself a few times (generations). Here are the resulting file sizes after a few passes of the output back through the program again.

2006-11-05 14:14 846,606,336 SF0.VOB
2006-11-05 15:29 741,677,056 SF1.VOB
2006-11-06 18:21 740,636,672 SF2.VOB
2006-11-07 15:43 740,528,128 SF3.VOB
2006-11-07 16:58 740,700,160 SF4.VOB

Note that SF0.VOB is the output from my DVR. SF1.VOB is the first file created after using CXtD on SF0.VOB.

Once I got SF1.VOB, I captured the CXtD log for the following recodes:
SF1 to SF2
SF2 to SF3
SF3 to SF4
The resulting VOB files get progressively smaller from SF0 to SF3. Then on the last recode (SF3 to SF4), the resulting VOB file gets bigger.

So the big question is, are all the CXtD-generated VOB files (SF1 to SF4) of equal quality, or is there going to be some degeneration on each pass? It might a cool feature if there was an option to use a VOB file AS-IS without any “processing”. The few times I’ve used Nero Recode (for the fancy menus), if it sees a VOB file, it runs it REALLY fast as a direct copy. That can be handy when I want to add a few VOB files to an image along with say one or two AVI files - the VOB files can be considered “copy” only.

Oh, I’ve attached the ConvertXtoDVD log, in case it helps.
Thanks,
Bit.

Hi Bitbasher,

I will bring this thread to the attention of the main programmer of ConvertXtoDVD.

As for the feature request ‘option to use a VOB file AS-IS’ without any “processing” ", this is not possible and will probably not be a direction the developement of ConvertXtoDVD will go (although I do understand how practical it could be).

I did find this though written by the main programmer, but I dont think it answers all of your questions

"ConvertX encoder is designed to achieve the best quality, given the maximum possible bitrate. But this best quality can sometimes be achieved without reaching this maximum bitrate, that why the result is even a little bit smaller than the theoric 0.32 Gigs.

You want to have a full DVD, use more than 5 minutes of video, and you’ll see as it’s getting longer on the screen, it’ll fill up the DVD. If you have less than 1 hour of video, the DVD will never be 100% full."

So the big question is, are all the CXtD-generated VOB files (SF1 to SF4) of equal quality, or is there going to be some degeneration on each pass?

There will always be some degradation during each conversion. No program can improve video quality, it can only try to retain as much as possible while compressing the output.

Output file size isn’t a good indicator of quality. Programs that use constant bitrate compression will produce bigger files than those using variable bitrate under most circumstances. But some variable bitrate programs are more efficient than others. A program may include more data than needed for some scenes and less than needed for others. Another program may decide to use different amounts for those same scenes. So there can be considerable variation in file size, and, even though overall quality might be the same, one program might reproduce some scenes better and some worse.

ConvertX does an amazing job of retaining overall quality and still producing small output files using a single pass variable bitrate compression scheme. Programs that use several passes to analyze content before beginning compression have the potential to do better, but not all of them do because their scheme may not be as good.

The recycling of output as you did doesn’t produce the same results as multi-pass encoding. There should be some degradtion with each recycle, but the size differences between your SF1 and SF4 files are so small, I doubt that you can see any quality difference. The size differences may be little more than something like roundoff errors.