BD Rebuilder, why a second pass?

[starscomeout]

For most movies I am encoding them at a custom setting of 20 GBs. I want the best picture quality as possible without using too much hard drive space. I am already up to 5TBs of external hard drives with close to 800 movies and growing (most are standard DVDs, which are uncompressed). I am using a front projector system with a 92-inch screen. I am always after the eye candy so I don't want to sacrifice picture quality to the extent that it is noticeable to my critical eye. Do I need to do a second pass, and what is the second pass done for? Will it make a difference in the picture quality?

When BD_Rebuilder resizes the video, does it also compress the audio, which would seem like it would provide a lesser quality of audio in such case?

-Don

[nurbs]

The first pass is for analysis (basically encoding with faster settings) and the second pass for the actual encode. The analysis is needed to ensure an optimal bitrate distribution which isn't possible with one pass alone, except if you do quality based encoding and don't care about the resulting filesize.
Example: You want to encode a video at x kbps. The video is relatively easy to encode except for the last 10 minutes which is one big difficult to encode action scene. With one pass the encoder wouldn't know that. With two passes the encoder will lower the bitrate somewhat for the rest of the movie so that it can but some more at the end and encode that scene with good quality. That way the quality is kept constant over the whole movie while if you had used only one pass the end would have looked worse than the rest.