thumb|120px|dolby pro logic logo dolby pro logic is a surround sound processing technology developed by dolby laboratories, designed to decode soundtracks encoded with dolby surround. dolby stereo was originally developed by dolby in 1976 for analog cinema sound systems. the format was adapted for home use in 1982 as dolby surround when hifi capable consumer vcrs were introduced; it was then replaced by the newer and improved pro-logic system in 1987. however, the term "dolby surround" is still used to describe the encoding technology or matrix-encoded soundtrack, whereas pro logic refers to the decoding technology and processor. it is the domestic equivalent of the theatrical dolby stereo technology used in movie cinemas in the 1970s and '80s. the two technologies are mostly identical but a change in marketing was needed so as not to confuse cinema stereo which is at least four channels of audio with home stereo which is only two. thus dolby pro logic is the consumer version of theatrical dolby stereo. ==dolby pro logic== dolby surround/pro logic is based on basic matrix technology. when a dolby surround soundtrack is created, four channels of sound are matrix-encoded into an ordinary stereo (two channel) sound track. the centre channel is encoded by placing it equally in the left and right channels; the rear channel is encoded using phase shift techniques, typically an out of phase stereo mixdown. a pro logic decoder/processor "unfolds" the sound into the original 4.0 surround&mdash;left and right, center, and a single limited frequency-range (7&nbsp;khz low-pass filtered) mono rear channel&mdash;while systems lacking the decoder play back the audio as standard stereo. a pro-logic decoder also uses 'steering logic', which drives amplifiers, to raise or lower the output volume of each channel based on the current dominant sound direction. for example, while a mono signal is played, the strong correlation to the center channel triggers the output volume of the left, right and surround channels to be lowered. this increases the channel separation achievable, to around 30 decibels between channels. by careful tuning of the response of the amplifiers, the total amount of signal energy remains constant and is unaffected by the operation of the channel steering. additionally the response time of the system to changes in sound direction is important as too fast a response results in a twitchy feel, whi
Dolby_Laboratories Film_sound_production Surround_sound