Difference between automatic VBR (CQ_VBR) and constant quality (CQ)

[intelinside]

Hi all

Just a quickie.. whats the difference between automatic VBR (CQ_VBR) and constant quality (CQ). Are there any advantages or disadvantages?

Cheers

intelinside

[intelinside]

ok i found the answer in the TMPG help section...

Constant Quality Variable Bitrate
(CQ_VBR):

This bitrate changes automatically with the stream to deliver a constant quality. It's probably better to use Constant Quality (CQ) than this mode. Gives a good quality for very long films.

Constant Quality
(CQ):

If a high image quality is a priority, this mode should be used. This gives good quality even in scene with rapid motion. As always with high resolutions, the file size can also become large. This is good for short movies of high quality.

[bernd_b]

I did many searches and read many posts.

But still the real explanation of this methods is still a secret or too complicated for a guy like me.

With both methods, a quality index is choosen and a bitrate is selected by the programm coresspondending to the choosen min_max Bitrate and the movie scenes which has to be encoded.

So why should in the end Constant Quality be better for short movies and the other way round?

[intelinside]

I don't know - my reply was cut and pasted from the help section. I guess trust them as they are much more knowledgable about this than us.

[SatStorm]

In praxis and at the current stage of TMPGenc, it is like this:

CQ_VBR: You set a minimum and a higher value and TMPGenc gonna try to encode in an average value of the % you set up.
Example: minimum 1000, maximum 4000 @ 50% gonna try to encode with an average of 2500kb/s.

CQ: You set a minimum and a higher value and TMPGenc rise the bitrate over the minimum value, up to what you have set up, when the encoder request it...

Why CQ is better CQ VBR? Because in praxis CQ doen't have to calculate the need of the bitrate in an average way like CQ_VBR, it just adds bitrate when the encoder ask for it...

[bernd_b]

Do I get it right when I understand that the encoder request more bitrate when he isn't able to encode the scene without loosing information compared to the input file?

So does this mean

CQ: The encoder increases the bitrate until:
a) the picture is encoded without loosing information of the picture given by the input file
or
b) the average bitrate of the scene being encoded reaches the maximum set by the CQ-index ?

CQ_VBR: The encoder tries to encode with the bitrate given by the Q-index.
If succeeded (scene encoded without loosing quality) then the next scene is going to be encoded.
If not, a higher bitrate is taken until the maximum given by the Q-index is reached or until all information is successfully encoded?