Separate Amplifiers and the Amplifiers in Receivers
|Metrics Tested on THX Amplifiers:
Reference Output Voltage
Output Source Impedance
Overload Restoring Time
Stability with Capacitive Load
Harmonic Distortion and Noise
Noise Output Voltage
D.C. Offset at the Output
Acoustic Noise Level
Load Impedance Range
Voltage Output Capability
Current Output Capability
Transient Output Capability
Transient Overload Recovery Time
Total Harmonic Distortion
SMPTE IM Distortion
IHF IM Distortion
DIM 30 Distortion
It's too easy for people, including us here at Secrets, to
say "get as much power as you can", "you should have at least 100
channel", and so on. While having more power than you need, even lots
more, is not a bad thing and in fact is much preferred by speaker
manufacturers (in that more speakers are damaged from underpowered amps
driven to clipping, than ones which were fed too much power), the reality is
that power, REAL power, is expensive. The question is, how much is
needed to drive a
reasonably efficient speaker in a reasonably sized room to reference level
and still have adequate headroom left over?
THX has done something which, to my knowledge, no one else
in the industry has done, at least on the scale which they have done it:
They have logged the dynamic content of virtually every piece of finished
sound track they can get their hands on (we're talking hundreds of
sound tracks here) and from that developed a practical "dynamic" requirement for multi-channel
All of the THX amplifier tests use bursts in various combinations at
various frequencies, at various lengths of time, repeated cycles and
combinations of speaker loads. This enables THX to uniquely qualify an amplifier,
particularly a receiver, as capable of playing
sound tracks to reference level (given of course a certain speaker
efficiency and room size limit).
Make no mistake: THX Ultra power amplifiers tend to be very
powerful. We've never seen one with less than 100 watts/channel continuous
RMS, full band,
all channels driven, BUT because of the "practical sum" THX has defined for
themselves, even seemingly modest receivers under $1,000 can get THX Select2
certification and provide a satisfying experience for a lot of people who
can't afford a stack of THX Ultra power amps (we'll talk about Select- vs. Ultra-Certification a little later).
But it's not all about raw power. As we mentioned when
talking about the THX Controller, the input and output levels and impedances
of everything "THX" are within a certain tolerance so that all pieces "talk" to
each other synergistically. One can see from the metrics list above
that everything down to the hum of the power supply transformer must be in
check to be THX certified. Yikes!
Speakers and Subwoofers
|Metrics Tested on THX
Axial Frequency Response Analysis
Low Frequency Cut Off
Stray Magnetic Flux
Maximum Output Level
Acoustic Noise Level
Back once again to the realties of the consumer market, and
once again knowing that a living room will remain a living room, much of
what makes THX speakers unique and special has to do with addressing this fundamental
Starting with the more pedestrian facets of speaker
performance, it almost goes without saying that THX speakers must have no
compromise in terms of neutral frequency response, power handling, output
capability, and the often overlooked dynamic, or transient, response.
Any good speaker should be able to make such claims. THX speakers are
designed as such and can, without hesitation, be driven (by a THX amplifier)
to reference level. However, THX speakers go
beyond this by designing in certain features and characteristics which put them
in a position, as a system, to excel at faithfully reproducing the program in that
"living room home theater" we keep talking about.
|Myth: "THX speakers only
sound good for one person in the room"
Reality: The opposite is true: while THX speakers
must have a narrow listening window in the vertical to attenuate
reflections from the floor and ceiling, they must simultaneously provide
a VERY wide horizontal listening window, ensuring good sound for
everyone in the room.
Main/Front Channel Speakers
THX Speakers have to meet very specific design goals in
terms of their radiation pattern: In the horizontal, they must have a
very, VERY wide listening window so that everyone across the couch hears
good sound. At the same time they must have a limited, or narrow,
listening window in the vertical because reflections off the floor and
ceiling can smear and distort the sound in the time domain.
back to THX realizing people will not likely acoustically treat their
ceilings so that conventional speakers can be used, recently (as of
these requirements have changed in terms of emphasis (less on vertical
roll-off, more on off-axis linearity), but we'll talk about that when we
cover Ultra2 a little later.
The other major design characteristic of a THX speaker is
that it is a dedicated satellite speaker which REQUIRES the support of a
subwoofer. The THX speaker system therefore is categorically a sub/sat
Full-range speakers are nice. I love full-range
speakers, but they have no place in a multi-channel sound system if we want
to have any chance of realizing a flat, uniform reproduction. Again,
this goes back to acoustics: it is very difficult to get a similar low end
response from five speakers spread out through a room, or even just three
across the front of it, even if all the speakers are identical, because
their different physical positions in the room are going to result in
different acoustical loading (i.e., the bass response will not be
the same from speaker to speaker). By
summing all the bass in the sound track and sending it to a subwoofer, or set
of subwoofers (all getting the same signal), the system's reproduction of
bass from each channel will be uniform.
The other benefit of a sub/sat system, known by experts such
as Ken Kreisel long before THX and home theater, is dynamics. By
asking one amp and speaker to cover the upper audible range (the main
speaker) and a completely different one the bottom, both do a better job
than either could if it was trying to do the whole shebang.
Remember we said bass management was integral to the THX
controller, long before a time when it was common in consumer equipment?
Now we're getting to the heart of that. These days, all processors and
receivers offer bass management, but what slopes do they use? What
crossover frequencies are offered? Will it all work with your
speakers? If you have a THX Controller with THX speaker, you don't
have to worry about this. Your stuff will work together famously because it was designed
as such from the ground up.
For you Techies:
The THX crossover consists of an 80 Hz, 4th order Linkwitz/Riley
filter alignment, and it was not chosen lightly or without serious
consideration. As the crossover frequency increases, it becomes
harder to blend the subwoofer with a satellite, and the subwoofer becomes
more difficult to audibly "hide". At the same time, as the crossover frequency
increases, distortion from the satellite decreases, the total dynamic range
increases, and loading the room for the flattest response becomes much
easier. The logic of the 80 Hz crossover point is that it's high enough to
ease demands on the speakers and amplifier, but low enough to make the whole
setup work without a tremendous headache. The 4th order (24dB/octave
roll-off, 6dB/octave/pole) Linkwitz/Riley alignment not only offers a steep
slope, but one that immediately transitions to that slope, maximizing the
benefits of that slope near the crossover point. The high-pass side
minimizes excursion and power requirements, while the low-pass minimizes
more localizable content at higher frequencies.
To correctly achieve this, THX satellite speakers are sealed
systems with an 80 Hz –3 dB low frequency cutoff (preferably with a Qtc of
0.71). The electronic high-pass filter applied to them is an 80 Hz,
2nd order (12dB/octave) Butterworth alignment. The
speaker and the filter sum to a 4th order Linkwitz/Riley roll-off which matches the electronic
filter applied to the subwoofer, and an excellent crossover is achieved.
surround speakers were used because of Dolby Pro Logic's mono surround
channel and are obsolete with Dolby Digital 5.1."
Reality: Di-poles are used to emulate the
multi-speaker arrays of movie theaters. It is Adaptive
De-Correlation (see THX Controller section above) which addresses
mono surround channel issues.
Surround Channel Speakers
The surround channels in movie theaters are reproduced by
arrays of speakers, anywhere from four to sixteen speakers on either side and the same again across
the back. What is the best way to reproduce that surround sound effect
at home? The answer would be to use twelve or more surround speakers of course! THX
knew that no one would even entertain such a suggestion so for their
surround speakers they mandate a dipole design.
A dipole, or dipolar speaker is one which fires sound from
two opposing sides, or poles, where each pole is out of phase with the
other (meaning that while one side is moving outward, the other is moving
inward). In a home theater, they are generally placed to the side of the
main listening area with the poles firing to the front and back of the room,
never directly at the listener. The result is that the sound bounces
off the walls of the room, successfully emulating the speaker arrays of the
|Movie theaters and large dubbing stages
use speaker arrays for uniform, enveloping surround sound.
||Dipoles in a home theater do an admiral
job of emulating that sound field using just two properly placed
THX dipoles must meet a strict requirement of flat
total power output, which means the sum of the sound coming spherically from all
the speaker must be smooth, as opposed to the sound coming from just one of the
poles. This is not easy for a designer to do, and there are plenty of
examples of poorly designed dipoles (none of which are THX-Certified).
In general no one will
deny that dipoles do an admirable job of emulating the multi-speaker
arrays of movie theaters, but many a journalist has gone on
record as saying they are not suitable for multi-channel music and that
monopole (a.k.a. conventional) speakers must be used.
Surround speakers are ok for movies but not music"
Reality: THX Surround speakers, with their flat
total power response, provide excellent, pleasing results for BOTH
movies and music.
Research done by Tom Holman in 1986 (involving both technically savvy audio
engineers as well as laypeople) revealed that while some sound engineers
preferred mono-poles in certain situations only for their ability to "expose"
defects such as pops and dropouts, when it comes to actually listening, all
persons showed a clear preference for a
diffuse ambient surround sound field when the test involved properly designed dipoles level
matched to the monopoles they were being compared to. The "bad-rap"
dipoles get is often due to evaluations clouded by the use of improperly
designed dipoles and a failure to level-match them with monopole
There are many ways of getting diffuse sound,
including strategically positioned and angled monopoles, but dipoles are simply the most
practical solution for a consumer (once again, THX is dealing with the
realities of the market).
Ultimately, the argument of monopole vs. dipole surrounds
is one of inevitable compromise, with THX and others selecting the dipoles
as the preferable of the two. These days, THX certifies mono, di and bipole surrounds, recognizing that each has its appropriate use. To achieve
that elusive balance of envelopment with some directionality of special
effects or game sounds in wildly different rooms, it's necessary to
choose the design that works best.
THX still puts dipoles first as great problem solvers in many typical rooms,
but they acknowledge that other designs have their uses.
to Go to Part IV.