CQ vs VBR vs CBR

[WickedDVD]

I know what VBR and CBR mean... but how is CBR different from CQ=Constant Quality??

I'm a little lost there.

[thayne]

CQ is like 1 pass VBR. it has good quality and is faster than 2-pass vbr, but the output size is harder to predict.

[adam]

When you encode in 2-pass VBR the encoder adjusts the quantization value (Q-value for short- basically amount of information thrown away) in order to achieve the set target bitrate average. Basically, it throws out more data in some scenes than in others, and it tries to do this intelligently. CQ mode is just 1-pass VBR which allows you to set a specific Q value and allow the bitrate to fluctuate around it instead. So it will throw the same amount of data out of all scenes. (sort of...hard to explain. Its more like throwing out the same % of data relative to the complexity of that scene, point is its a constant amount.) So you can think of it as setting a specific quality level you want and telling the encoder to use just enough bitrate to achieve that level, and like thayne said the resulting size will be unknown, though you can run some shorter sample encodes to better predict the filesize.

Realistically, both modes are a different means to the same end. Ultimately the file has to be stored, and that means its either going to x number of cdrs or to a dvd-r. So in the end you are always going to be working with the same avg bitrate. So if you did the same encode using both CQ and 2-pass VBR, than in any given scene the 2-pass encode may have a higher or lower Q value as compared to the CQ encode, (lower is better) but overall the avg Q value should be lower, which equals better quality, at least as far as quantization is concerned. The CQ encoded file will have the same Q level throughout, but overall it should be higher than the avg of the 2-pass encode= overall lower quality. Now this is how it works in theory, in practice the resulting quality entirely depends on how well the encoder is programmed. Many people feel that TMPGEnc's CQ mode actually yields superior quality to its 2-pass VBR mode. If this is true than its because its 2-pass VBR algorithm is lacking. Personally, I think in later versions of TMPGEnc, both modes achieve very similar quality, but multipass VBR is just more foolproof.

As for other encoders, many such as CCE will let you set a constant Q value, which is essentially all CQ mode does. So if you wanted, you could essentially do multipass "CQ" encoding. From my personal tests, I find this way of encoding inefficient and lower quality.