Use two 4.7k resistors connected from the left and right channels to a
common point to generate a new Mono signal. This can be used for the center
channel since the center information and LFE is mixed in this channels by
the player software. Sub can connect on this signal too if a lowpass
filter is present. Otherwise use a simple RC lowpass to generate the Sub -
Channel from this new Mono source. You can connect your 4.0 out from
Soundcard to 5.1 input on a Surround Amp ;)
Don't beat me, it looks like some new 5.1 sound cards with 4 channel YM 744
soundchip are doing it this way (buffered with OPV)
