Two Pass encode in Constant Bit Setting??

[Jeff Schroeder]

See your other post…

2-Xeon X5680 @ 3.33, EVGA SR-2 Mobo, 48GB DDR3, GTX 580 3072MB, 16TB Attached Storage, Win7, Vegas 11 x64

[John Rofrano]

[Dan Myers] “But the two pass setting is only accessible in the VBR mode. Do I set all three (Min/avg/max) to 8,000,000 in VBR?”

There is no 2 Pass option in CBR because it doesn’t need it. No, you don’t want to use VBR and set all three bit-rates the same.

Perhaps an explanation of CBR vs VBR would help:

CBR uses a constant bit rate for every frame of video. All frames are treated equal. This is good to use for maximum quality when your video can fit on the media at a fairly high bit rate.

VBR varies the bit-rate depending on the demands of each frame. Frames with a lot of information will use more bits and frames with less information will use less bits. The encoder will try and achieve the average bit-rate that you set for the entire video and not use more than the maximum bit-rate or less than the minimum bit-rate on any given frame.

The problem with VBR is that it has no idea how many bits future frames will need so it’s usually conservative in its estimates of how many bits to use and still not exceed the average for the whole file. It may not make the best use of all bits but it’s fast.

2-Pass is an option for VBR where the first pass collects information on how much detail each frame needs and then the second pass allocates the bits based on the information collected in the first pass. This solves the problem of not knowing what’s coming next because you now know what’s coming next and how many bits you will need to represent future frames and still not exceed the average bit-rate. 2-Pass takes twice as long to encoder because it processes the video twice but it squeezes every last drop of quality for a given average bit-rate.

So 2-Pass is only needed when using VBR and has no meaning for CBR. Hope that helps,

~jr

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