one pass vs two pass?

[wallace4793]

Hi

Ive just bought an awesome nslu2 (unslinged) and am currently converting all my dvds to xvid to store on the attached 500gb hdd. Having so many to do im doing them one pass at the moment as im impatient and theres a few to do. Is there a marked improvement in qulaity with 2 pass? Im using xbmc (another awesome prog)to watch em on my 42" plasma

john

[BigDid]

Hi

Ive just bought an awesome nslu2 (unslinged) and am currently converting all my dvds to xvid to store on the attached 500gb hdd. Having so many to do im doing them one pass at the moment as im impatient and theres a few to do. Is there a marked improvement in qulaity with 2 pass? Im using xbmc (another awesome prog)to watch em on my 42" plasma

john
Hi,

You are giving very little details, users here may not be familiar with your project:
Using a NSLU2 http://forum.doom9.org/showthread.php?t=124753
Using a xbox AND xbmc to pass (frameserve) your xvid encoded with GK to your 42 plasma ...

You may want to read this recent related thread: http://forum.doom9.org/showthread.php?t=124673
that can give insights on 1 vs 2 passes.

Also you may or may not have blockings, stutters, strange videos colors or lines or patches... depends on the hardware possibilities with 1 pass or 2 passes unrestricted; try by yourself and/or search the SAP hardware section if some of these problems occurs.

Did

[wallace4793]

Hi

Thanks for the reply. It get a bit indepth the second thread. To be honest i try to keep my xvids down to 2mb/s as my wireless network is rubbish sometimes. I can only dream of streaimg HD. What i meant was in general is the quality significantly better with 2 pass? the size isnt quite so important as its stored locally an my server hard drive

[aldaco12]

Is there a marked improvement in qulaity with 2 pass?
What do you think?
Remember that on 1-pass you'll create a clip with a constant quality, for instance 700 kbps. On 2-pass you'll create a video with a quality which has, for instance:
min = 300 kbps
max = 1500 kbps
average = 700 kbps.
Is having a variable quality much better then having a constant quality? The answer, of course is: it depends.
If the video clip you have keeps more or less the same rhythm in any of its parts, the answer is no.
If in the video clip you have some parts at a relatively different rhythm (like a clip, which has some 'dialogue' parts plus some 'action' parts), the answer is yes.
Another thing to consider:
- if the average bitrate will be very large, so the average bitrate is enough (like a 3*CD 2h movie), you won't notice great enhancements.
- if the average bitrate won't be so large, you'll appreciate the use of a VBR with respect to the use of a CBR.
Again, the answer is 'it depends'.

[weaver4]

Constant Quality is not constant bitrate. Using constant quality on Frames that do not change very much will not use many bits, but on action it will use a lot more bits. So once you set the quality it will maintain the quality you want throughout the movie regardless of what the content is. The problem with two pass is that a person can not make a good guess most of the time on what a good bitrate would be because; it is not just action, it is how much texture is in the movie, how busy are the backgrounds, how may dark scenes are there, etc.

In my experience I get better results us Single-Pass Constant Quality than guessing at a bitrate. Let me give you an example. Several moves like Firewall give great results at 700kbs, but the movie Casablanca requires 1500kbs. I would of never guess those bitrates, but a Constant Quality Xvid with a Quantizer of 3 did.

One last thing, I took my favorite movie and encode it at CQ3 (67% in AutoGK) and the movie size came out to 1.1G. I then set that as my filesize and did two pass. As hard as I tried I could not tell any difference in quality, not even in the fast motion scenes.

So my rule is: Use single-pass constant quality, fixed quantizer, unless you are trying to hit a specific size; then use two-pass.