Dolby Prologic is an analogue procedure for producing surround sound on four channels from a stereo signal. It is still widely used for television, on DVDs and in video games. Most equipment using it can produce a signal for a bass speaker, but it is not separate on the original signal.
Dolby Prologic 2: A development of the above which can produce a multi-channel environment on five loudspeakers from a previously non-encoded signal, like the signal from a CD or a radio. On a Dolby Prologic encoded cinema sound track, there is better distribution of background effects through a re-created stereo signal limited in the treble instead of a single monophonic channel.