What does the second pass actually do?

[B|izz]

I now use xvid over divx as I get like 40fps more using xvid, but what does the second pass do?
The first pass makes the movie so I have the movie and sound file in the folder ready to be muxed at the end of the first pass. Is the second pass really needed?

[Enchanter]

What you have been doing seems to be a 1-pass configuration. The 2-pass configuration works differently. Below is a simple explanation on how a 2-pass encoding algorith works.

In a 2-pass setup, the 1st pass analyses the video source and stores the compressibility graph in a .STATS file. The output of the first pass is an invalid (a.k.a. not playable) .AVI file. The 2nd pass uses the previously created .STATS file (in order to know which parts of the video will benefit from more bits and which part can make do by having some bits taken from them -> to be re-distributed to the 'more demanding' parts). This is a very efficient method of bits assignment and hence outputs much better quality than a single-pass encoding typically can.

Is the second pass really worth it? Yes.

Regards.