Specifications

Metrics Tested on THX Controllers:
Reference Output Voltage
Voltage Gain
Input Impedance
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
Polarity
D.C. Offset at the Output
Frequency Response Deviation
Signal Time Delay
Noise Output Voltage
Input Level Indicator (Clipping Indicator)
Video Path Switching
Video Level
Sync Level
Bar Tilt
Pulse/Bar Ration
K-Factor (K-2T)
Frequency Response Amplitude (0.5 to 4.2
MHz)
A few words on "Reference Level"
Before we start talking about the pieces and parts, we first need to take a quick refresher on the concept of
"Reference Level", as you are going to be hearing that term quite a bit in the coming paragraphs. Simply
stated, Reference Level is a standard, known, predictable and reproducible playback volume level. When
movie sound tracks are crafted, they are done so on systems which are locked at this level. The sound artist
does not play around with a big volume knob when doing his/her work. If the sound artist wants something to
be loud, they make that sound loud within the sound track. When they want something to be soft, they make
that element soft within the sound track. Movie theaters set their playback level by the exact same rules, so
when the movie is shown, you hear EXACTLY what the sound artist heard when they were making the piece.
Loud, soft, in-between, it's all there, and no one touches the master volume knob over the course of a two-hour
movie.
To achieve THX certification, components must play at this reference level without breaking, distorting,
buzzing, rattling or any other distracting effects.
Reference level is by any definition, objective or subjective, quite loud. It basically mirrors the dynamic range
of the studio system, which in the case of all movie sound tracks, is 105 dB. Any single channel of the system is
calibrated to play 0 dB FSD (the loudest sound the sound track can contain) at 105 dB (115 dB for the LFE
channel). While that is really, REALLY loud, its important to remember that there is 105 dB of dynamic range
and the artist can put a sound at any level they want. So while a system's volume may be set to reference
level, dialogue within the sound track can, and most often is, at a normal, natural level. Reference level, with
the dynamic range available, permits a movie to have that normal, natural dialogue, and then suddenly a
spectacular, loud car chase without anyone touching the volume control. Every element in the sound track
comes out as it should.
Now, having said all that, watching a movie at reference level in a home theater is almost never done. It can
be extremely loud to begin with, but the close spaces typical of home theaters make it perceptibly even more
so. Reference level is still very important in home theater though for several reasons. Because it is the
absolute loudest a sound track should ever be played, its fairly intuitive that its a good idea to have a system
that can competently go that loud. It gives you a sort of "safe maximum" volume level, even though you may
never push it that high. Even more important though is knowing what volume you are at RELATIVE to reference
level because if we go too low, we literally lose the quietest sounds since they are pushed below the audible
threshold, surrounds lose their presence, the perceived spectral distribution of the track is altered, and
dialogue intelligibility suffers.
The THX Controller
We refer to the THX controller as both dedicated
surround sound processor/preamplifiers, and the
processing section of integrated receivers.
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