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Can anyone point me to that article or if in fact it doesn't exist here then point me to another web site which does explain it all.

DigitalFAQ.com has a chart. It says 10.08Mbps total combined bitrate with a maximum video bitrate of 9.8Mbps.

You can more details at DVDfaq.com.

Hi John

If your final video length is under75 minutes then use ¡¥Constant¡¦ bit rate.
Over this and you can think about ¡¥Variable¡¦.

The only reason to use variable is to reduce the file size allowing you to fit more video per disc, with the advantage of keeping quality.

We cannot directly compare these two. Variable is an Average where as Constant is the fixed Maximum rate.
So 7000 Constant is more like7500 Variable, if you get my meaning.

Difference between constant and variable

CBR - Constant Bitrate
Goal: Constant bitrate through encoding
The bitrate does not vary up or down - it remains constant.
This produces constant size but variable quality.

1-pass VBR - Single pass Variable Bitrate
Goal: Constant quality through encoding
The bitrate varies from the base video bitrate up to the defined max video bitrate.
When the complexity of the video increases, the bitrate will be increased up to the max video bitrate.
This produces constant quality but variable size (assuming the max bitrate is sufficient to reproduce the complex video sections).

2-pass VBR - Two pass Variable Bitrate
Goal: Set average bitrate with variable instantaneous bitrate
The bitrate varies up to the defined max video bitrate, with the overall average maintaining the defined base video bitrate.
When the complexity of the video increases, the bitrate will be increased up to the max video bitrate but bitrate in areas of lower complexity will be decreased in order to "free" bits for the complex sections.
This produces constant average quality and constant size (assuming the max bitrate is sufficient to reproduce the complex video sections and the defined average bitrate is not so low that bitrate peaks create poor quality in lower-bitrate sections).

Very important points:
1) CBR is fastest, 2-pass VBR is slowest (assuming same Speed/Quality settings)
2) 2-pass VBR is not necessarilly better than 1-pass VBR - especially if predictable size is not a concern.

An easier way to think about it:
CBR is like a straight cylinder. Its size is constant throughout.
1-pass VBR is like a balloon - it can increase as necessary but won't get bigger than the max you set.
2-pass VBR is like a bean bag - some parts can have more beans, other parts can have little or no beans, but overall - it's always a set number of beans.

The only comment I would make is that these are general guidelines. Many users come away after reading this that CBR is inferior to VBR. Certainly CBR set at 5000 to fit 2 hours of video on a disc would be inferior to VBR setting of 5000 average and 8000 max. There will be scenes where 5000 is more than enough and CBR will look just as good as the VBR footage at the same segment, but there are going to be scenes where more bitrate is needed, and the 5000 setting will fall short and look inferior to the VBR file at that same point.

Putting it another way, if one has only an hour of video that they are going to put on a disc, then they aren't concerned about file size. They can set the CBR at 8000, and it should be pretty darn close visually to what any VBR setting with a max of 8000 would give, if not better. The VBR encoded file shouldn't really have any segments encoded higher than the CBR file, and may potentially drop to a lower bitrate than what you would like just by the way it analyzed the file. I have had some graphics quicktime files created in India Pro that look better with CBR than they do with VBR because the bitrate dropped fairly low during the encode when it shouldn't. Since you can't put a minimum on how low it can go in Procoder, that can be a problem with some encodes. Impression Pro from Pinnacle struggles with playback of very low bitrates, and for that reason files that I encode from graphics programs (primarily as intros or motion menus for dvd's) I use CBR.

Time and place for each setting made.

section VBR vs CBR: All modern video codecs are variable (in bitrate) in the sense that not every frame uses the same number of bits as every other frame.---- Even codecs labeled "CBR" can vary data rate quite a bit throughout the file.

section Buffered v whole file data rate control: For example, for a five-second buffer, the data rate for each five seconds must be the same. This isn't a question of multiple, discreet five-second blocks, but that any arbitrary five seconds plucked from the file must be at or under the target data rate. This is also called sliding window and CBR.

In order not to infringe the copyright of the author, I shall not copy the whole page here. Just to summarize, the cbr mode is not an absolute constant for every frame. It is a constant over a buffered period along the timeline of the file. The buffer period slides along the timeline of the file and is equivalent to the concept of moving average. But at any one single point, it is not a constant.

Thanks everyone for your input.
I was aware of much of what you have all advised.
I have THREE separate movies totaling 75 minutes. I normally render at 8000 CBR LPCM but obviously cannot for 75 minutes. I tried 7500 CBR but still wouldn't fit so decided to try 2 pass VBR at max 8000 and still it wouldn't fit so then used DOLBY (audio compression) and it fits.

I remember using TmpGenc years ago when rendering to SVCD and also remember that when using VBR you could set not only max bit rate but also minimum (and maybe even average). I wondered what VS used but it seems that doesn't matter -

it seems that VS will NEVER use a bit rate too low to compromise quality and that in fact if every frame is 'complex' and requires 8000 (when using max 8000) then 8000 will always be used, that minimum is dynamic and only ever be what the least 'complex' frame requires - is this correct?

If Ken (Berry) is still reading replies to this post I now pose this question -

Personally, I would go for the higher video bitrate if you are displaying it on a large HDTV

I have also read that some DVD players "have trouble" with DVDs which use a bit rate greater than 8000kbps. Should I then not exceed 8000 either with CBR and VBR (maximum).

Is there somewhere I can read to explain exactly what happens to DVD playback as the bit rate is reduced? Is the main or only difference a reduction in resolution/sharpness?

If you read my first post in this thread, you will see that I too said that many stand-alone players have difficulty with bitrates over 8000.

Next, you question about whether one should choose a higher or lower bitrate was in the context of your own postulate that we were talking about 8000 kbps or 7300 kbps. I suggested the higher within the context of my earlier comment about not going above 8000 kbps.

Thanks Ken
This post has now been around for a few replies and together with two other posts I had going I guess I forgot all details of previous replies.

Hi John

Its a bit like comparing it to still images using Jpeg compression.
You have three default settings, low, medium and high.
The scale runs from 1 to 10, but has a further top setting of 12.
10 is usually the ¡¥high¡¦ setting but can be overridden by selecting 12.(max)
Not many users would push this to 12, I doubt if you will see any difference in quality between 10 and 12, the file size will be a little higher.

So 8000 kbps is like 10 in the jpeg scale. Giving full quality DVD.
4000 kbps being low, can be compared to giving good quality VHS.

Creating a file size of 4.3 Gb is the best you can do, but remember that you have to create a menu, adding to the final size.
Aiming at 4 Gb as your video will allow 0.3 Gb free. Ample for the menu structure.