Dolby Digital EX (THX EX)

The most advanced surround formats for home theater are the extended formats, Dolby Digital EX and its competition DTS ES.

Dolby Digital EX was co-developed by Lucasfilm's THX, so it's also called THX-EX. Legend has it that George Lucas himself, unsatisfied with Dolby Digital 5.1, felt there was a way to improve on this format. THX and Dolby Labs teamed up and came up with an extra surround channel they called rear surround. This extra channel is just a matrix of the left and right rear channels, meaning it's only made up of the existing rear channels sound, so it's not a true 6.1 or 7.1 process at all.

Dolby Digital EX (THX EX) format settings

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Dolby Digital (THX) EX at a glance:

It's Dolby Digital with an extra back surround matrix.
Requires 6 or 7 speakers and a subwoofer and a compatible receiver or preamp.
Extra surround channel is not discreet, not a true 6.1 or 7.1 surround format.
EX-encoded material is fully backward-compatible to Dolby Digital 5.1.
Extra surround back speakers might be a tough sell in most living rooms.

The new format was released to theaters amid the highly commercial synergistic hoopla of Lucas's own Star Wars Episode One: The Phantom Menace.

DVDs encoded in Dolby Digital EX have soundtracks optimized for the use of that extra rear channel but receivers decoding Dolby Digital 5.1 won't miss out on any sounds. In fact, properly positioned rear speakers should create the illusion of a phantom center speaker. The presence of the extra channel can only accentuate the surround channels imaging.