History

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• History

- THX, named after THX 1138 from George Lucas'
first feature film
- 1980, Lucas set out to develop and implement a
set of quality control criteria for theatres
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THX are a design and quality control system for
the playback environment in a theatre First the
aspiring theatre must meet certain acoustic
criteria   low background noise, (no loud air
conditioners or projectors) good isolation (so
the lucky patron doesn't get to hear the gunfight
in the theatre next door) specific reverberant
characteristics based on the volume of each
individual theatre the theatre can't have any
nasty slap echoes or reflections.  When an
individual theatre meets these standards, it
means that..  the dialogue will be more
intelligible,  the high frequencies will
reproduce clearly, the bass will not build up
excessively within the room, etc. 
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How THX Works
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requirement for accurate multi-channel sound
reproduction
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Front Channel Loudspeakers
- Conventional two-channel speaker designs are
constructed to present a good solid image over a
45 to 60 degree angle
THX LCR front channel speakers improve dialogue
intelligibility and imaging in two ways.
First, they provide wide horizontal dispersion.
This allows for people sitting off-axis to hear
full and flat frequency response
The second technique is to control the speakers'
vertical dispersion
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Surround Channel Loudspeakers
The purpose of a surround channel is to surround
you with sound
Decorrelation takes care of Left/Right
spaciousness
The total energy radiated by a speaker in all
directions is called its "Power Response". The
Home THX Diffuse Surround Speaker is required to
have a flat power response. This means that the
total energy radiated by the speaker (in front,
above, below, behind, and to the sides) must
together average a flat frequency response. With
the total speaker energy being flat, a listener
anywhere within the surround speaker's null area
will hear flat frequency response.
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Subwoofer Loudspeakers
All THX subwoofers are designed to precisely
match LCR speakers through the Electronic
Crossover in a Home THX Controller or Receiver
THX Subwoofers must have an accurate in-room
response down to 20 Hz. Approved systems must be
able to play 105 dB sound pressure level or
louder without distortion.
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Features THX Certified Controllers
Patented circuitry to manage and deliver
cinema-quality sound
THX has developed patented circuitry designed to
address the tonal and spatial differences between
cinema
Six Important Controllers Used in THX
Re-Equalization
Timbre Matching
Adaptive Decorrelation
Bass Management Electronic Crossover
Bass Peak Level Manager
Loudspeaker Position Time Synchronization
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Re-Equalization
An important step in THX Theatre processing is to
correct the over-bright sound of movie
soundtracks
Movies are mixed in cinemas with a controlled
high frequency roll-off, called the "X-Curve".
This curve, an international standard, is part of
every mixing and cinema's playback system because
high frequency roll-off is appropriate for sound
sources that are not close
THX uses a special Re-Equalization Curve,
designed for home environments, to restore the
correct tonal balance of a movie soundtrack
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THX Dolby Pro LogicFrequency Response Plots
Figure 1 Front (Left) Channel Frequency
Response Dolby Pro Logic (white), THX
Re-Equalization (blue)
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Timbre Matching
The first correction is to match the sound
character, or Timbre, of the Surround Channels to
the Front Channels in order to ensure smooth
sound movement from front to surrounds. It also
realistically places the audience inside the
movie experience and not "in-between" two
competing sound fields
The second correction is to match tone. As sound
sources move around the listener (from front to
side and behind) the sound can vary in perceived
character as much as 8 to -3 dB, even with
matched and equalized speakers. This is because
sound quality is actually shaped by the human
head and outer ear
In a cinema, the speakers are all around your
head and are equalized to match the front
channels
Surround Channels have to compensate for the fact
that fewer speakers typically, two - are used
in a theatre system, these are the following
Timbre Matching Circuit corrects the surround
tonal balance to match the front channels - so
what you hear is one smooth, continuous sound
field.
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Surround Channel Frequency ResponseTimbre
Matching
Dolby Pro Logic (white), THX Timbre Matching
(blue)
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Adaptive Decorrelation
Adaptive Decorrelation compensates for monophonic
surround channels in Dolby Pro Logic
the THX Decorrelation Circuit subtly adjusts the
time and phase of one surround channel against
the other. This constantly changing time and
phase differences means that your left ear and
right ear never hear identical signals
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Bass Management Electronic Crossover
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Bass Management Electronic Crossover
The first job of a controller is to properly
decode the soundtrack
Once decoded, the multi-channel signals are
processed to correct for tonal and spatial errors
THX Electronic Crossover circuit allows for main
speakers by sending the deepest, loudest bass
signals to a subwoofer. This improves the
frequency response, lowers distortion, and
increases dynamic range, via a technique called
bi-amplification
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Bass Peak Level Manager
The loudest part of any movie soundtrack is the
bass pulsing out of the subwoofer With a Dolby
Digital soundtrack, the subwoofer must reproduce
even more bass from the LFE channel. To reduce
the chance that your home subwoofer will overload
from this large amount of bass
Loudspeaker Position Time Synchronization
The Home THX Loudspeaker Position Time
Synchronization circuit allows you to digitally
adjust the apparent position of each speaker in
your system for the best multi-channel imaging
and smoothest frequency response.