Dolby™ Pro-Logic

The most widely used Home Entertainment process. Produces a surrounding sound field with Dolby Surround or Dolby Stereo encoded software. This includes practically all major films from the late seventies and onwards available on VHS videotape, LaserDisc, DVD or from stereo TV.
It has 4 perceivable channels of sound all derived from a stereo sound track, (Left Front, Centre, Front Right and Rear Surround)
This is achieved by redirecting out of phase information (Normally deliberately encoded in to the stereo tracks) to the rear speakers.
Information going to the centre channel (Pro-Logic) processes and mixes all information that is lacking in any stereo content

Dolby Pro Logic 2 is the updated version of Pro-Logic that gives stereo surround speaker channels .