Sanjay's a great resource here, and there are others here as well!

I only recently became aware how bass management works on surrounds in DPL II. Over on AVS at http://www.avsforum.com/avs-vb/showthread.php?s=&threadid=297444&perpage=20&pagenumber=4 Dolby Lab's Roger Dressler wrote
Quote:

Electrically, Game mode starts off as being identical to Movie mode in terms of logic steering. The only difference is in the bass management. When the surround speakers are set to “small,” Game mode includes the surround bass in the composite signal feeding the subwoofer. This bass has traditionally been ignored by bass managers ever since bass management became part of surround systems back in Pro Logic days. The basis for this goes back even further, to movie mixing and playback practices passed on Dolby Stereo which used the progenitor of Pro Logic since 1976.

With video games, players “need all the menacing cues they can get when an enemy comes up from behind,” and this ensures they will hear them, particularly when played on smaller sat/sub systems.

__________________
Roger Dressler
Dolby Laboratories

(emphasis added)

So I asked him the following:
Quote:

Q: I thought that when surround speakers were set to small, even in music or movie mode in PLII, that the bass for the surrounds was directed to the subwoofer. Are you saying that no, when not in game mode, when the surround speakers are set to small, the surround bass is ignored by the bass manager?

and he responded

It was ignored. This has been the practice since bass management was first instituted for Pro Logic originally by THX and carried forward by Dolby. When PLII was designed we revisited the question to see if the stereo surrounds would give a different result. It did not. Adding the bass from the surrounds to the fronts heading to the sub merely induced unevenness in the bass quality/level. Hard steered mixes like games are less susceptible to this, and game content is more forgiving of the finer nuances of sound reproduction, in my opinion.

Added to this the fact that all Dolby Surround ENcoders have a bass filter in the Surround input, which means that it isn't actually possible to encode surround bass on a movie soundtrack from a hard rear effect. So what is being ignored is not deliberate bass mixed into the soundtrack, as that isn't really there, but bass that leaks from the front channels because it is phasey in L/R.

__________________
Roger Dressler
Dolby Laboratories