CBR one pass vs CBR two pass

[Nigel O’neill]

If you are dealing with content over say, 100~140 minutes, 2 pass VBR rendering will yield better results than a single pass VBR or CBR.

My system specs: Intel i7 970, 12GB RAM, ASUS P6T, Vegas Pro 10e (x32/x64), Windows 7 x64 Ultimate, Vegas Production Assistant 1.0, VASST Ultimate S Pro 4.1, Neat Video Pro 2.6

[Dave Haynie]

Typically, you want CBR for a streaming medium, like online video, satellite television, etc. and VBR for finite-storage-based media like DVDs and BDs.

There’s no sense to “2-pass CBR”… didn’t even think it was possible. The whole point of two-pass is optimizing VBR. In VBR, you basically have a “bit-budget”… you can spend bits here or there, and there are various strategies.

One example of is CQ… sort of a constant quality, but it actually implies that the same amount of data will be tossed filtered after the DCT. This makes the video very consistent, but the resulting size is hard to control — you only get the average bitrate you select when you have average video (eg, nothing too dynamic).

The 2-pass basically lets the encoder “cheat”… it can scan the entire video, qualify the whole thing in terms of dynamics, then apply just the right range of encoding to optimize that range of movement while hitting your bitrate target.

CBR, of course, always hits your bitrate target… and unless you’re encoding at maximum possible bitrate, it won’t look at good as VBR on most video. Sure, on a perfectly static talking head, there’s not likely much difference, but for real world video, you have sections on a DVD that look just dandy at 3.5Mb/s, and others than would really like to go past 9Mb/s.

In professional encoding, there’s a mastering engineer with realtime encoding tools who can “eyeball” these things. And even play other tricks. For example, one of the worst since in MPEG encoding is leaving visible macroblocks. That’s when the compression is so high, frame to frame, that they blocks basically don’t line up anymore, and your eye sees that mis-match as block lines. You tend to see this in fact motion, where the whole motion prediction thing kind of breaks down. If you apply a global low-pass filter to those sections of video, there will naturally be less high frequency data to toss out of the DCT, but no visible macrocells, since the filtering was global.

But I digress. If the Main Concept encoder actually does anything with the 2-pass flag and CBR, I’m at a loss to say just what. One thing to check: a CBR encoding at the same bitrate is always going to be the came length (that whole “constant” thing). Try with and without, see if you get the same file size, or even identically encoded files.

-Dave

[Borja Pe]

Wow, amazing and fast reply! Thank you so much! Ok, thats great. Size is an issue since its an upload and it takes time but I doubt the time I spend rendered will compensate the upload speed gain!

Thank you very much! I will try both and see, when I first tried both it seemed a bit smoother the two pass but that would probably be just me!