Specifications

formats, De-Correlation became Adaptive
De-Correlation, intelligently applying itself
when discreet surrounds contain mono
content.
the exact same signal. This can cause the sound to be
"in your head" instead of pleasantly surrounding you as
it should, or if seated off center, can cause the
surround sound to collapse to the speaker nearest you.
THX's De-Correlation very subtlety alters the phase and
time relationship of the two rear speakers so that they
don't sound absolutely identical, maintaining the spacious nature the surrounds are suppose to have.
When Dolby Digital hit the consumer market, De-Correlation was NOT made obsolete by the 5.1 format's
two discrete surround channels. Many, many surround sound effects in sound tracks are still input
equally to both surround channels so that it fills the surround space, and as such, the system can suffer
the same pitfalls as Pro Logic's mono surround channel.
De-Correlation was updated to Adaptive De-Correlation, which, as the name suggests, adapts to the
incoming signal. Content which is identical in both channels undergoes De-Correlation, the rest of the
sounds intentionally steered to one side of the other remain as such.
Timbre Matching. A given sound in front of us sounds different if sourced behind us. That's because of
the shape of our outer ear and is part of how we can tell where a sound is coming from (the other is the
relative amplitude in our two ears). In home theater though, sounds which are panned from the front of
the room to the back, or vice versa, can lose continuity because of this auditory reality, since halfway
from rear to front the panned sound is actually coming at us from BOTH in front and behind. Timbre
Matching applies a generic HRTF (Head Related Transfer Function) to reshape the surround sound
speakers' sound so that it is a closer match to what we hear from the front channels, closing the "seam"
between the front of the room and the rear.
Also, part of the THX Cinema mode, and indeed any THX mode (we'll talk about some of the others
later), is that it "overrides" any superfluous alterations of the program. For example, if bass or treble
controls have been engaged, or an alteration made to the level of a speaker (outside of the master
calibration), any such "impurities" are zeroed out whenever the THX button is pressed, assuring you that
the movie is in fact being presented as it should.
In 1999, a new THX mode was added to the repertoire: THX Surround EX. It includes all the
aforementioned processes, and simply puts them on top of Dolby Digital Surround EX decoding, so its not
really a separate THX mode, but we mention it here because THX actually co-developed Surround EX.
Dolby Digital Surround EX, as it is called in movie theaters, was jointly developed by Dolby and THX. It applies
a Dolby matrix decoder to the two surround channels, and decodes a third channel between them (the
center-surround or rear channel as it is sometimes called) yielding a "6.1" configuration (though the 6th channel
is not actually discrete). Soundtracks encoded in this format became available on DVD, and the first consumer
systems to decode them were THX units with THX Surround EX (the EX decoding later became available on
non-THX equipment under the name of Dolby Digital EX).
One major design consideration of the THX Controller is the bass management system. While some form
of bass management is now common in consumer electronics components, THX designed into their
program a very specific bass management system right from the start, back in 1991 when the phrase
"bass management" was not even well known. We're going to talk more about it in the speakers section
of this article, but suffice it to say for now that the THX Controller has to implement bass management
in a very specific way, using a specific set of slopes, to accommodate and make the most of the speaker
system. The THX Controller may also provide a bass peak limiter, a feature which allows you to set a
maximum signal level sent to the sub. Again, knowing the realities of home theater and that not every
situation would permit a subwoofer to be placed in the best spot with maximum loading, THX knew that
even a THX sub could be overdriven. The Bass Peak Limiter allows you to "protect" a subwoofer in such
situations, taking away the fear you may have of pushing your volume a little closer to reference level.
While at one time required by THX, this feature is now an option implemented at the manufacturers
discretion because, by and large, powered subwoofers have their own limiter/protection.
Another key element that THX requires for receivers and SSPs is the reference setting for volume control.
When a system is calibrated according to the manufacturer’s instructions, "0.0 dB" on the volume control