VBR (2 pass, average bitrate) Encoding for DVD-9

[jcm0320]

Two questions:

1.) I know that VBR encoding is a good way to fit more video onto a dvd, but is the quality expected to be better than CBR? I've never been quite sure because TMPGenc Express claims that VBR "allows high picture quality", but never says anything about how it stacks up against CBR. I would imagine that since the video is being "analyzed" (my own word for it) in an extra pass that it would produce a better picture quality.

2.) If the quality is expected to be better with VBR encoding... and if I'm using a dvd-9 which can hold my entire video at the max bitrate... would I just set the Minimum, Average, and Maximum bitrate to 8000 since I have plenty of space? I don't believe there would be any reason for me to have a lower bitrate.

So question #2 depends on the answer to question #1. If CBR encoding at 8000 Kb/s produces the same quality (or better quality) than VBR with maxed out settings than I suppose I could choose either option. Although it would seems strange to use VBR encoding without varying the bitrate settings. That leads me to believe that encoding with CBR and using more than one pass would be the best option... but you can't do that with TMPGenc Express.

Hope that didn't sound too confusing.

Thanks!

[mats.hogberg]

If you're adding "within a limited file size" it's true. A VBR encode with 2000 min, 6000 average, 8000 max, will (I'm sure someone will argue about this but...) look better than a 6000 CBR, but have the same file size.
However, if you can fit the whole video at max allowed bitrate, using CBR will be as good. The effect of VBR doesn't come into play until you give it a range (min/max) to play with.

/Mats

[redwudz]

1. I think it would depend on the bitrate used. A high bitrate and CBR and VBR would probably be about the same quality. For lower bitrates, VBR would likely have the advantage.

2. If you had the minimum, average and maximum set to the same bitrate, then you wouldn't really have VBR anymore.

Just my understanding, but if you needed to encode at 4000Kbps, then VBR might be the better choice. At 8000Kbps or higher, they would likely give you the same quality.

[jman98]

Keep in mind that in the early days of DVD, the first ones were only single layer discs. I think I still have a very old "flipper" that my dad gave me that has half of the movie "Stargate" on one side and the other have of the movie on the other side of the DVD. Both sides are single layer DVD-5 discs. VBR was an important tool to try to maximize space so you could fit more video in the space by using the bit rate more effectively - high motion scenes get more bit rate and low motion scenes get less, for example.
People absolutely hated the idea of flipper DVDs with half of a movie on each side and the studios quickly moved to dual layer discs. Having to flip laserdiscs was a big part of the reason why that format failed (cost was another) and VBR allowed studios to encode at lower bit rates and jam an entire movie onto one side of a DVD-5 disc so the disc didn't have to be flipped. Flippers would have killed the DVD in its infancy and the studios were quick enough to realize that this was a terrible idea.

If your video will completely fit on a DVD-9 disc at CBR, VBR will gain you nothing. VBR is really a way to efficiently use lower bit rates. It allows the encoder to make an intelligent decision on when to use a lower bit rate and thus save space and when to use a higher bit for higher quality but use more space. I recently made a DVD for a friend where I used a minimum bit rate of 3200, and average rate of 4200 and a maximum rate of 5200. I would expect the result to be better than simply encoding at 4200 CBR because the encoder will figure out intelligently when to use more than 4200 and when to use less. In theory, the times it uses more than 4200 will result in better quality and the times it uses less than 4200 will save space, so you're maximizing quality when it makes sense to do and minimizing space when it makes sense to do that. VBR is not magically better than CBR though, it just uses the bits more intelligently. As redwudz states, at high enough bit rates like 8000 Kbps or more, you won't really notice a difference. I've done some testing at bit rates over 8000 and I don't use VBR at that bit rate because I can't tell any quality difference.