An audio system which delivers sound from behind, as well as in front of, the listener. This can be artificially generated by 'virtual surround' systems which process a stereo signal to produce the effect of sounds from the rear, even with just two speakers. More commonly, though, it refers to systems such as Dolby Pro Logic, Dolby Digital and DTS which were designed to deliver specific channels of surround sound from speakers placed slightly behind and to the sides of the listener. These typically also use a centre front channel and, in some newer versions, an additional centre rear channel. In the professional cinema context, surround sound is often called stereo. Contrast with mono and stereo.