A four-channel perceptual coding scheme developed by Dolby Labs where an LCRS audio signal is converted into two channels of analog audio, then recovered to yield discrete left, center, right, and mono surround channels. This is a hardware version of their surround decoder originally developed for the Dolby Motion Picture 4:2:4 matrix surround-sound system, developed for the Star Wars picture in 1977. If a subwoofer is used, it is generally fed by lowpass-filtering a mix of the three front channels at the receiver.