Because of the limited nature of the original DPL, 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. In addition to five full range playback channels, Pro Logic II introduced a Music mode which includes optimized channel delays, and adds user controls to - for example - adjust apparent front sound stage width.
Grrr... Trade secrets.Anyway, I got my hands on "Principles Of" worksheets straight from Dolby. The original Pro Logic specification calls for applying Dolby Type-B noise reduction to the surround channel. Is this really necessary with PCM?