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Directly from the Dolby site:
What’s the difference between Pro Logic®, Pro Logic II, and Dolby® Digital 5.1?
Dolby Pro Logic is a matrix decoder that decodes the four channels of surround sound that have been encoded onto the stereo soundtracks of Dolby Surround program material such as VHS movies and TV shows. Dolby Surround is a matrix encoding process that in essence “folds” Left, Center, Right, and Surround channels onto stereo soundtracks. A Pro Logic decoder “unfolds” the four channels on playback (without a Pro Logic decoder, the encoded program plays in regular stereo).
Dolby Pro Logic II is an advanced matrix decoder that derives five-channel surround (Left, Center, Right, Left Surround, and Right Surround) from any stereo program material, whether or not it has been specifically Dolby Surround encoded. On encoded material such as movie soundtracks, the sound is more like Dolby Digital 5.1 (see below), while on unencoded stereo material such as music CDs the effect is a wider, more involving soundfield. Among other improvements over Pro Logic, Pro Logic II provides two full-range surround channels, as opposed to Pro Logic’s single, limited-bandwidth surround channel.
Dolby Digital 5.1 is a method of transmitting and storing 5.1-channel soundtracks via newer digital media such as DVD, digital cable, digital broadcast TV (DTV), and satellite transmissions. Unlike the Dolby Surround encode/Pro Logic decode process, which sacrifices channel separation to get surround onto any stereo soundtrack, Dolby Digital 5.1 is a discrete system that keeps the multiple channels fully separated throughout the encoding and decoding processes. In addition to having full-range Left, Center, Right, Left Surround, and Right Surround channels, Dolby Digital 5.1 soundtracks carry a sixth (“.1”) channel recorded with low-frequency effects (those bass rumbles and booms you feel as well as hear in a well-equipped cinema).
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