Constant bit rate vs Variable bit rate?

[ibzomie]

In the answer to another question I recently posted, I discovered that conversions go much faster if I use constant bit rate instead of variable bit rate. I was told that using constant bit rate would result in poorer image quality, but I find that it's not enough poorer to really matter to me.

Is there any other trade-off in using constant bit rate instead of variable?
Will it make any difference on different DVD players?

[mats.hogberg]

No, besides the quality vs file size advantage to VBR, no negative side effects with CBR.

/Mats

[FulciLives]

CONSTANT BITRATE (CBR) uses the same bitrate from start to finish. It actually varies a lil bit (goes slightly up and down) but pretty much follows a straight line if you will.

VARIABLE BITRATE (VBR) will lower or increase the bitrate AS NEEDED.

Let me explain.

Scenes with little to no motion need less bitrate than those scenes that have a lot of movement.

So let us say you encode at 6000kbps CBR ... the encoder simply encodes at 6000kbps from start to finish. At times this will be more than enough bitrate (perhaps "overkill" on scenese with little to no movement) but at times this may not be enough of a bitrate (perhaps on those scenes with a lot of movement).

With VBR you pick an AVERAGE which in my example is 6000kbps but you also pick a MINIMUM (say 2000kbps) and a MAXIMUM (say 8000kbps) and the encoder will attempt to lower the bitrate when it is not needed (little to no motioin) but will also try to increase the bitrate when it is needed (scenes with a lot of movement). The result is you can get better quality because the bitrate can raise over the AVERAGE when it is needed. Despites all the bitrate ups and downs the file will over all have an AVERAGE bitrate of 6000kbps (as per my example).

For VBR to work really effectivly it has to be done in a multi-pass method (usually just 2 passes but some encoders allow more than 2 passes ... CCE for instance can do up to 9 passes I believe though truth be told 5 passes is usually the most you would ever need to do).

There is real-time VBR that works in one pass but it is not as good as a multi-pass VBR.

Obviously a multi-pass VBR will take at least double the time of a CBR encode if not more/

The trick here is if the CBR bitrate is "high enough" then it often can look very good and a VBR is not always needed. But for CBR to look good you need a "high enough" bitrate which limits the amount of time you put on a DVD disc.

It's hard to exactly pinpoint what is a "high enough" CBR bitrate as it depends on the material you are encoding but once you get up to a bitrate of 7500kbps or higher you might as well just do a CBR. A CBR bitrate of less than 7500kbps (like 6000kbps) can still look really good but again it depends on the source (how "clean" it is and how much motion there is etc.) and yes even the resolution you are encoding to can make a difference.

Hope this helps.

- John "FulciLives" Coleman