Constant vs variable

[GrandMasterPixel]

What difference would it make if I was to use a constant bit rate over variable?

What are the pros and cons of each method?

[Trevor Andrew]

Hi

This subject has been discussed in length many times, a search on the forum should find them. I’m assuming you are using standard DVD video.

Variable was developed to allow us to add more video to a disc whilst keeping quality.
Basically reducing the bit rate to produce smaller file sizes allows for more minutes per disc.

If your video is under 60 minutes then the video will fit to disc using the highest rates.
So Constant would be used.
I use constant for all video using 7000kbps and above, this allows me about 75 minutes per disc. It really depends on the length of your video.

Just bought my first Duel Layer discs, should allow me increase the bit rate.
8000 would give me 120 minutes per disc. (not used them as yet) for which I would defiantly use Constant.

Its difficult to compare Constant to Variable as the first is a maximum rate and the other an average.

[skier-hughes]

Simply constant is not a constant anyway...........
More precisely

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.

[GrandMasterPixel]

Cool, thanks for the information. its been helpful.

From that information, I gathered and VBR is superior over CBR in terms of image quality?

I want the highest quality possible for a movie that was recorded in VBR 9600, so I am going to save it encode it back into VBR 9600.

Thanks!

[skier-hughes]

Cool, thanks for the information. its been helpful.

From that information, I gathered and VBR is superior over CBR in terms of image quality? From that info it will tell you it depends on lots of things, 6000 is 6000, is 6000, but fitting 2 hours on one dvd would be better in vbr than a lower cbr

I want the highest quality possible for a movie that was recorded in VBR 9600, so I am going to save it encode it back into VBR 9600. So long as you dvd will play a 9600 encoded dvd, most new ones will, lots of old cheap ones may, lots won't. Bear in mind that mpeg is a lossy format, so each change and save will reduce quality. Whether this is distinguisable to you or not is up to you to decide. Using smart rendering where only the sections which have been changed will mean you retain most quality.
Thanks!

[sjj1805]

What difference would it make if I was to use a constant bit rate over variable?

What are the pros and cons of each method?

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