TMPGEnc CBR or (2 pass) VBR ?

[nettroller]

I took an avi and converted it using CBR and then (2 pass) VBR. The CBR I used 5927 per the bitrate calculator. The VBR, I used http://members.dodo.net.au/~jimmalenko/calc.htm which gave me max 9650, ave 4200 and min 1650. The VBR conversion took longer and resulted in a smaller file (3/4 of a DVD) as opposed to the CBR conversion which filled the DVD.
Then I watched the same cuts on the copies and, to my naked eye, I couldn't see any difference on my 34" Toshiba HDTV.
Now since the VBR took twice as long to convert - I'm wondering if it's worth it in the future and wondered if anyone else had opinions on it?

[jimmalenko]

2-Pass VBR takes twice as long as CBR. CBR uses the exact same bitrate on each and every frame (note though that the bitrate is in kilobits per second, so the value you calculate actually refers to the number of bits in total used for 1 second of video), irregardless of if it truly needs it or not, whereas 2-Pass VBR analyses every frame on the first pass, and works out how best to "stagger" the bitrate across all frames such that frames that don't need much don't get much, while those that do need a lot get enough (provided your min, ave and max settings are sufficient).

So if using CBR you generally have to use a bitrate value which exceeds the required value for any given frame, unless you want macroblocking or pixelation in some areas. 2-Pass VBR can enable you to distribute the bitrate a lot better IMO, especially when you have a size constraint of, say, 1 SL DVDR.

But as you've seen by your own experiment, if filesize (and therefore bitrate, since size = running time * bitrate) is not an issue and won't affect quality, then CBR is obviously the quicker (and therefore better for this situation) method. Unfortunately the decision of when to use which method is pretty subjective, and depends on a user's own preferences, as well as the quality of your source. You can also vary frame size in some situations too - ie. your calculated bitrate is 4000kbps, but you want it done quickly. so you can choose 352 x 480, which should look perfectly fine at CBR 4000kbps, whereas 720 x 480 would most probably require 2-Pass VBR to obtain suitable results. but again, it's all up to your own tastes.

BTW, If you had have used 5927 as your average for 2-Pass VBR, you'd find the filesizes would be almost identical. 4200 is roughly 3/4 of 5927, so that's why the filesize was roughly 3/4 of the CBR encode's.

[nettroller]

Thanks for the very detailed explanation - I do understand it a bit better now. I see where the VBR can be worth it - and in my case since time isn't an issue - I'll go with that...but I will try and play with the ave that your calc gave me - bump it up a bit, more inline with the smaller bitrate calc on the site.
Thanks again

[junkmalle]

I almost always us Constant Quality mode. Pick a quality you're happy with, encode in one pass. You don't know how big the file will be beforehand though.