Originally Posted by Shrub
I thought it used a custom one for the Dolby surround sound
Not at all. Dolby surround (Prologic) is added by simply inverting one of the sound channels. On the SPC and the Playstation chips, you set stereo volume on each voice using two values, one for the left channel, one for the right. The values can be positive or negative, with a negative value meaning that the data is played inverted (flipped upside down). Setting the volume of a voice with one channel negative and the other positive causes a prologic decoder to send the voice to the rear channel.
As a side note, around about that time Dolby were not really bothered as long as they got their logo in the game. I know a couple of games only had normal stereo panning and still qualified to get a Dolby logo on them just because they had a 5 second intro with a stereo sample that had Dolby encoded in it.