Dolby Pro Logic

The format was adapted for home use in 1982 as Dolby Surround when HiFi capable consumer VCRs were introduced.

The same four-channel encoded stereo track was largely left unchanged and made available to consumers as "Dolby Surround" on home video.

It also had Dolby Noise Reduction and an adjustable delay, for improved channel separation and to prevent dialog leaking and arriving to listeners' ears first.

A Pro Logic decoder/processor "unfolds" the sound into the original 4.0 surround—left and right, center, and a single limited frequency-range (7 kHz low-pass filtered)[3] mono rear channel.

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.

[4] DPL II processes any high-quality stereo signal source into five separate full frequency channels (right front, center, left front, right rear and left rear), while also decoding five channels from stereo signals encoded in traditional four-channel Dolby Surround.

Because of the limited nature of the original Dolby Pro Logic encoding, many consumer electronics manufacturers introduced their own processing circuitry, such as the "Jazz", "Hall", and "Stadium" modes found on most common home audio receivers.

DPL II forgoes this type of processing and replaces it with simple servo (negative feedback) circuits used to derive five channels.

It identifies spatial cues in low-level, uncorrelated information, such as ambience and effects like rain or wind in the side and rear surround channels, and directs it to the front height speakers.

The term now refers to a new upmixer whose purpose is to enable Dolby Atmos receivers and speaker configurations to serve non-Atmos signals.

Dolby Surround is a complete replacement for Pro Logic that upmixes stereo and multi-channel inputs to play over Atmos configurations.