Secrets Q & A
- Written by Brian Florian
- Published on 30 November 2007
The THX Controller
Reference Output Voltage
Gain Control Range
Gain Tracking of the Master Gain Control
Overload Source Voltage
D.C. Bias Current at the Input
Maximum Output Voltage of Low-Level Outputs
Output Source Impedance
D.C. Offset at the Output
Frequency Response Deviation
Signal Time Delay
Noise Output Voltage
Input Level Indicator (Clipping Indicator)
Video Path Switching
Frequency Response Amplitude (0.5 to 4.2 MHz)
Group delay (0.5 to 4.2 MHz)
Harmonic Distortion and Noise
Digital Data Sampling Rate Support
Input Mode Control and User Interface Design
Input Mode Switching
Automatic Detection of Data Types
Output Modes and Post Processing
Equalization and Tone Controls
LFE Channel Level Scaling
Loudspeaker Position Time Synchronization
There are two distinct halves to the certification of a THX Controller. The first is the actual performance metrics, shown in the table at right. As you can see, it goes well beyond any review you've ever read, including our own best. What is absolutely key here is that these are not simply "minimums" a product must meet, but many are actually design considerations. For example, the line level outputs must not only meet a certain voltage capability, but their output voltage must be a certain level relative to an input signal, be it analog or digital.
Things like this are important because not only must each THX piece be excellent at what it does, but it must also work in concert with the other pieces to create a synergistic whole. By predetermining the output voltage for a reference level signal, as well as the output impedance, the amplifiers (which we'll talk about in a minute) can be designed and spec'ed in anticipation of that output, and the S/N performance of the WHOLE system is assured, without any "matching" effort required by you, the end user. THX controllers are a perfect match for THX amplifiers, end of story. You can think of this in terms of dynamic range windows: when they all line up, you get maximum performance all the way through.
The second, somewhat distinct aspect of certification of a THX Controller is the inclusion and implementation of various design features, including the THX Post-Processes. Here we are starting to get into what I was previously saying about THX addressing the realities of home theater.
All THX Controllers include one or more THX modes which are NOT surround sound decoding schemes or codecs. When engaged, the THX modes apply a set of processes or filters to the soundtrack AFTER it has been Pro Logic, Dolby Digital, or DTS decoded.
The most basic THX mode is called simply "THX Cinema". This mode includes:
Re-Eq, or ReEqualization. Ironically this is the best known, and the single most misunderstood element of a THX processor. Remember we said that Tom Holman found sound tracks sounded different in the home-style rooms? One of the reasons was that they were coming across as too bright, which is a fancy way of saying there is too much treble. Why? The equipment was excellent, high fidelity stuff. The "culprit" if you will, was the room itself. Lacking the extensive acoustical treatments of professional sound and production rooms, high frequency energy is bounced around and sustained. At elevated playback levels (at or close to reference) it gets subjectively worse and you start to squint. Here is the very first example of what THX does to address the realities of the consumer market: THX knew that no one would be talked into turning their family's living room into a sound studio. Instead, Re-Eq simply applies a rational, mild roll-off of the high frequencies. It's that simple.
Everything you have read about movie sound tracks themselves having too much high frequency energy, including what THX's own marketing department has put out there, is inaccurate. The treble is not exaggerated by the sound artist to overcome the perforated screens at the theater (the theater does its own EQ to address that). The tracks do not have too much treble because movie theaters are aligned to something called the X-Curve. We have an entire article on the history of cinema sound system curves which discusses the relevance of the X-Curve, and why it has absolutely nothing to do with why sound tracks sound too bright in a home. Please feel free to digest it when you are done here.
Another quick sidebar: For many years, my living room was my home theater. As a reviewer, I had the privilege of playing with some very nice amps, preamps, and more speakers than I care to remember, but regardless of the caliber of equipment, movies just sounded harsh without a THX controller and Re-Eq. As soon as I created a serious screening room complete with extensive acoustical treatments, I found Re-Eq necessary only at the absolute loudest output levels. Go figure. This is why our Secrets SSP Benchmark specifications require that THX controllers offer the option of turning Re-Eq off independently.
Adaptive De-Correlation / De-Correlation. The THX Home Theater program was launched in 1991 at a time when Dolby Pro Logic, the consumer equivalent of Dolby Stereo, was the only thing going in surround sound. While virtually all surround sound configurations used two surround speakers, Pro Logic decoding yields only one surround channel, meaning both speakers get 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 corresponds to reference level playback. In turning the volume down, the volume level is expressed in -dB, or how many dB below reference level you are.