Can someone help me understand the CQ mode in TMPGEnc?

[agc1976]

I've read that CQ produces better results (in terms of image quality) than 2 pass VBR. I want to start using this method but I just don't understand what the "quality" setting does.

Can someone point me in the right direction or give me an example of what the "quality" setting does? Thanks, Alfred

[Yirkin]

I would like more info on this setting too. I have been using CQ at 100 percent and I get great results with small file sizes. TMPGEnc has very vague explanations in their documentation.

Howard

[snowmoon]

It's easy to comprehend if you have even played with .jpg images.

When you export or edit .jpg images you can usually choose a quality factor... higher quality = larger file, lower quality = smaller file. You can see quite dramaticly with .jpg files what happens when you overcompress, the same is true of .mpg files ( bith use a similar iDCT to compress data ).

The higer you set the CQ setting the less data is thrown away ( assuming you have bits to spare ). Generally setting it to 100 is useless unless you are encoding DVD since that will generally just force it to use 100% of the max bitrate 98% of the time. You should set the CQ to whatever looks good to you on your TV.

[agc1976]

Let me see if I understood.

Let's say I encode a file using CQ 3000/4000 with 75% Quality.

Does this mean that the "average" bitrate will be somewhere around 3750?

[snowmoon]

no.... quality does not scale like that

Encoding one video clip at 0-8000 CQ %75 might result in a 3200kbps average... Encoding that same clip 0-4000 %75 would probably result in a 3050kbps average and encoding it at 0-2000 %75 might result in a 1980kbps average.

CQ tries to keeps the quality of the output the same and then tries to cram it into the bitrate you specify.

[Reaper88]

So if you use CQ for dvd output instead of VBR you are saying i will get better ruesuts?

[snowmoon]

Mabye... depends on the source. VBR and CQ are both good options. My personal understanding is that CQ would give a more consitant quality throuout vs VBR where some spots will look better and some will look worse, but it's all opinions as to which is best.

[snowmoon]

I have been using the CQ mode equivilent in CCE and have really enjoyed the output vs VBR.

[Cecilio]

well the best part about Constant Quality is that you get really good quality, with only one pass. and since snowmoon had the best idea of this whole thing, ill use his reply as an example. everything he said was correct and well said, but it also depends on what reselution your using. if your using 352*480(vcd) then you could just do this 0-1800 and set it to 95 or 90 you will get very very good results. ok now if your making a 352*480(cvd) you could use 0-3000 and also use 90 as your quality output. do the same for 480*480(svcd) and for 720*480(dvd) it depends if you want to put the whole movie in the dvd-r or 2 movies. lets say it last 1 hour and 30 minutes. just do this 0-5000 and set the quality to 90 this will get you perfect results. there is no need to go higher than 90 as 90 and 100 gets you kind of the same looking results, plus if you use 100 you use alot more bitrate than 90. This was well said.

[SatStorm]

I have done plenty of tests on both VBR and CQ VBR.
Well, it is always about the source...

If the source is avi, or mpeg 4 or a DVB channel, then I believe it is better to use 2 Pass VBR
But if the source is a DVD movie or a CBR DVB channel, then CQ VBR with some tweeking (Kwag is a practical expert on this) can be prety good. IMHO not better 2 Pass VBR, but it is very relative 'cause the what is better for me might be not better for you.

Why this happens? After a good chat with Adam almost a month ago, I realise that the X factor of the success is simply the .... Intra Frames!

You see, a DVD has intra Frames when a scene change happens. Those intra frames can be used as Guides by the CQ VBR mode. Okey, when you frameserve the encoder might not be able to determine this in theory, but in praxis there is something there, I still investigating what exactly...
This is what the "Detect Scene Change" is all about in TMPGenc.

So, in short Terms, CQ_VBR is a good alternative of 2 Pass VBR if your source has plenty Intra frames. Like DVD Video movies and I frame mpeg.

For Avi sources like mjpeg and Huffyuv, the encoder needs to determine where to set the intra frames in the mpeg file. Well, TMPGenc doesn't do a good job at this, but I don't know about last weeks new release. Other encoders, like CCE do a much better job.

How those Intra frames do the difference? Macroblocks! The correct insert of Intra frames on scene changes, can reduce a lot the macroblocks. That's why CCE is better TMPGenc (among the other factors of speed and the YUV native support).

And the no1 problem with us, are the macroblocks!