THX is not a sound system as many think. It is actually a certification that ensures the reproduction in either the theater or home is up to what was encoded on the media. It is licensed under the control and direction of LucasFilms Inc. In order to be classified as THX certified the following criteria must be met. I will describe the home application here since most of us do not actually own a full size theater.
A left and right signal source comes into a unit called 'Input Level and Balance Controls'. This unit adjusts the sound and normalizes the source. Next the signal is passed to the 'Dolby Prologic Surround Circuitry'. Here the signal is processed into a left, right, center and surround signal. The next stage is a set of proprietary LucasFilms circuits. The left, right and center signals go into a 'Subwoofer Electronic Crossover' circuit which splits out the subwoofer signal. The left, right and center signals are then re-equalized and pass out of the LucasFilms circuits while the subwoofer signal passes directly out of this area. The surround sound signal is processed by another LucasFilms circuit 'Timbre Matching' and then goes on to the LucasFilms 'Decorreclation' circuit which breaks the surround signal into discrete left and right surround signals. Now, all the six signals are out of the proprietary LucasFilms Curtis.
Next, the six signals are processed by the 'Output Level Control' which adjusts the balance and volume of the signals to match the original source material.
I'm working on a diagram for this that should help make things a bit easier to understand.