What are the Different Surround Sound Formats?

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Dolby Pro Logic II replaced the earlier version of Pro Logic. It added the ability to derive five-channel surround (left, center, right, left surround, and right surround) from any stereo program, whether or not it had been encoded with Dolby Surround. Encoded material yields results more like Dolby Digital 5.1, including two discrete surround channels rather than a single mono channel and an LFE (low frequency effects) channel for the subwoofer.

Dolby Pro Logic IIx builds on Dolby Pro Logic II technology. It processes native stereo and 5.1 channel material to produce 6.1 or 7.1 output channels. This differs from Dolby Digital EX in that rather than decoding a 6.1 or 7.1 signal, it synthesizes the additional channels from non-encoded material. The extra one or two channels add rear surround to the left and right surround channels.

Dolby Digital 5.1 takes the encoding digital for DVDs, digital cable and digital television (DTV). Each of the 5.1 channels is discrete, meaning each channel is encoded, transmitted and played back without ever being matrixed as is done in Dolby Surround. This discrete handling results in improved range and channel separation and so yields greater fidelity. Dolby Digital is backwards compatible and will decode older Dolby formats.

Dolby Digital EX adds a center surround channel to DD 5.1 which can be played through one or two speakers. This differs from ProLogic IIx in that DD EX uses an encoded channel while PL IIx has to synthesize a channel from the other channels.

Newer Dolby Digital Surround EX soundtracks contain a digital flag that can automatically activate the EX decoding in a home theater receiver. For titles released prior to late 2001, you must turn on the EX decoding manually.

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