OPV vs 6 Pass VBR

[halpern]

What are your opinions on OPV vs. 6 pass VBR (or something like it). After doing some testing of OPV and comparing it to the original DVD I can not really see that much of a difference. Has anyone done extensive tests on this besides MaxT? I have been using the 6 pass VBR method for a few years now but the convenience of doing it in a much much shorter time with the same result is appealing. I apologize if this was discusses elsewhere. Thanks.

[TheSeeker]

Here is an extensive discussion and experiments done by Sir Didymus. I think what we ended up observing is that a three pass (2+1) is pretty much the sweet spot. Check it out for yourself.

http://forum.doom9.org/showthread.php?s=&threadid=86038&highlight=opv

[halpern]

Does anyone have the data that was in those links? It seems as though those links are dead. Thanks.

[Sir Didymus]

Hi halpern :)

The links are dead, since the data were hosted by a provider using dynamic links, allowing their access just for few days from the post date...

The content of the link isn't really necessary to take some decision about nr. of passes...

They pointed just to the source cell [an m2v file sized 192.660 KB] and to one of the encoded m2v, from Kill Bill Vol. 2, PAL, R2, movie...

The SSIM objectivation metric was used, and the thread indicated by TheSeeker is full of recommendations about taking this metric as a tool for measuring the encoding quality... It isn't...

Said this, if you think the information posted is not enough [it is evident... quite long and annoying to read the whole...], or if you need clarifications, please ask...

To say the thruth, I was planning to repeat the test on a totally different type of cell [from Balto 3 title...], but you know, it takes lot of time, and I am not sure it is really relevant...

Cheers,
SD

P.S. If you are just looking to some quick answer to the question you have posed, I am almost sure you may reduce your VBR passes to 1+1 or 1+2 without warrying too much about the quality, since you will loose nothing relevant. I was also totally surprised by the excellent quality of the OPV encodings. Looking at the numbers, it seems that in case OPV size is matching precisely the target size then OPV encodings are even better than Multipass VBR...

[MaxT]

http://www.maxt.dk/archives/2005/01/26/the-best-way-to-backup-dvd/

I posted some of my results with OPV vs 6 pass VBR. Could give you some ideas :)

[halpern]

Yes MaxT I read your website and it looks like OPV is just as good as 6 pass. I have been using the 6 pass method for 3 years now but if a tool comes along that makes the process easier I'll take a look at it. It takes me 1 hour to do a movie in OPV which is really good for the quality that comes out of it. I am not familiar with this SSIM thing. I have never heard of it before. I don't mind testing on my machine since it is fast I just need to familiarize myself with this new process since I'm used to the 6 pass VBR way.

[archaeo]

I assume you are talking about CCE regarding these issues?

I think the quality really relies on what Q-value you achieve when you run OPV. The lower the Q, the closer the encode is to the original. At higher Q values, you may want to lean towards doing multi pass. But even with multi pass, from what I have seen and read, a two pass (1+1) is more than enough for most sources.

Here's an excellent explanation that jdobbs wrote regarding OPV vs multipass:

In multiple pass Variable Bit Rate (VBR) you define an average bitrate that the encoder must meet. That bitrate is fixed based upon the maximum size of the output, number of frames, and length of the video. The encoder scans the source on the first pass (usually doing a mock encode) to see where the available bandwidth is needed. In the ensuing passes it allocates that appropriate bitrate to each section of the movie (taking from the rich and giving to the poor) so the Q (Quality) is consistent across the movie (and the bitrate meets the average selected). The resulting Q is as high as can be accomplished within the constraints of the output size.

One Pass Variable Bit Rate (OPV) encoding takes a different view. It lets you select the Q you want up front (note: lower is better). The encoder makes a single pass, and as it goes through the source it uses whatever bitrate it required to reach the Q you've selected (within the limits that are set).

The problem with OPV is that since the bitrate isn't controlled -- the output size isn't fixed, and you don't know whether it will even fit on a DVD-5 when its done.

In order to try and correct that, DVD-RB selects a representative sample of the source (usually 1%) during the PREPARE phase, encodes it, and trys to predict what Q is the best that would fit onto a DVD-5 disc (-R, +R, etc). It then uses that Q to do the OPV encode.

The advantage of multipass encode is that it is very accurate in predicting output size. It can use almost every bit available on the output disc -- which could improve quality (slightly).

The advantage of OPV is speed. It can do a complete encode a lot faster. Each pass of a multipass encode takes the same amount of time as a complete OPV encode. So OPV is twice as fast as a 2 pass encode, 3 times as fast as a 3 pass encode and on-and-on. The disadvantage is that Q prediction is a really roughly scaled and it could end up smaller or larger than what you thought (possibly even too big to fit on the output disc). DVD-RB minimizes that possibility but doesn't remove it. The bigger concern for most people is that OPV IS ONLY AVAILABLE on the expensive versions of CCE (you know, the $2000 one you've heard about), so unless you are a video professional you probably wouldn't have a legal copy.

My experience is that you can't really tell the difference between CCE OPV and CCE multipass VBR in most cases.

In QuEnc you don't select a Q in OPV but a bitrate. My experience with that encoder shows that multipass encoded is very much superior to OPV -- so I don't recommend QuEnc OPV unless you're just in a hurry.

[MaxT]

I used 25/16 setting which looks like to be more or less "standard"