Suitable source programs encoded four channels of sound on the ordinary stereo sound track by phase shift, called Dolby Surround or Lt/Rt. The Pro Logic system split the sound between four channels: left and right, center, and a limited mono rear channel which was split again between two speakers.
Dolby Pro Logic has been superseded by digital 5.1 (such as Dolby Digital), 6.1 and 7.1 channel systems. It is considered to have comparatively poor channel separation and suffer from the bandwidth limits of the rear channel.