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Additional Information
[6] THX Surround EX
In 1999, a new surround system was launched
simultaneously with the release of the movie “Star Wars
Episode I”. “Dolby Digital Surround EX” is a new movie
sound track that greatly enhances the sense of spatial
expression and the positioning of the surround channel
sound. The result is 360 degrees of movement and moving
sound effects that seem to pass right over the listener’s
head.
This system was developed jointly by THX and Dolby
Laboratories, fusing THX’s idea of improving spatial
expression and achieving a uniform 360 degree sound
positioning with Dolby Laboratories’ matrix encoding
technology. Emphasis was placed on compatibility with
the existing system Dolby Digital 5.1-channel, and the new
“surround back (SB) channel” was added to achieve
improvements over the conventional 5.1-channel system
in terms of the positioning of the sound at the rear, the
acoustic image of sound moving from the two sides to the
back as well as sound moving from the front to the center
rear with the multi surround speaker systems used in
movie theaters, thereby enabling various types of surround
sound.
The surround back channel signal is a matrix-encoded
signal inserted into both the Dolby Digital SL (surround left)
and SR (surround right) channels. Upon playback, the
signals are decoded by a high precision digital matrix
decoder within the Dolby Digital decoder into the SL, SR
and SB channels and output as 6.1 channels of signals.
With the AVR-4806, the signals further undergo Home THX
Cinema processing to achieve a THX Surround EX system.
Even without the proper environment for playing the SB
channel, Dolby Digital Surround EX signals are 100%
compatible with existing 5.1-channel playback systems, so
they can be played as such. In this case, the SB channel
signal is produced as a monaural signal from both the SL
and SR channels, so none of the signal components are
missing. The effects specific to THX Surround EX (the
sense of spatial expression and the positioning of the
sound), however, are the same as with conventional 5.1-
channel surround systems.
THX and Ultra 2 are trademarks or registered trademarks of
THX Ltd. Surround EX is a jointly developed technology of
THX and Dolby Laboratories, Inc. and is a trademark of
Dolby Laboratories, Inc. Used under authorization. All rights
reserved.
Audyssey MultEQ XT
There are several factors that can degrade the sound from
even the best loudspeakers in a listening room. One of the
most important is the interaction of sound from the
loudspeakers with large surfaces such as walls, the floor, and
the ceiling in the room. Even with careful loudspeaker
placement and acoustical treatments, there are significant
problems that are caused by room acoustics. These include
reflections from nearby surfaces and standing waves that are
created between large parallel surfaces in the room.
In a home theater the situation is further complicated because
there are several listening locations. The effects of room
acoustics on the sound arriving at each person’s ears are very
different and the result is a listening experience that is
degraded in a different way for every person in the room. It is
not uncommon to have variations in two adjacent seats that
are as large as 10 dB, particularly in the frequency range below
250 Hz.
The solution to this problem is to apply room correction after
precisely measuring how each loudspeaker interacts with the
room. Because the room causes variations in the frequency
response of the loudspeakers that are so large from seat to
seat, it is important to measure each loudspeaker at several
locations in the listening room. This should be done even if
there is only one listener. Measurement at a single location is
not representative of the acoustical problems in the room and
will, in most cases, degrade overall performance.
Audyssey MultEQ XT is the only technology that can achieve
room correction for multiple listeners in a large listening area.
It does so by combining the data collected at several points in
the room from each loudspeaker and then applying correction
that minimizes the acoustical effects of the room and is
matched to the frequency resolution of human perception
(known as psychoacoustics). Furthermore, MultEQ XT
correction is applied both in frequency and time domains and
so there are no artifacts (such as smearing of sound or modal
ringing) that are sometimes associated with traditional
methods of room equalization.
In addition to correcting frequency response problems over a
wide listening area, Audyssey MultEQ XT provides a
completely automated sound system set-up process. It
identifies how many loudspeakers are connected to the
amplifiers and whether they are full-range, satellites, or
subwoofers. If there is a least one subwoofer connected,
Audyssey MultEQ XT determines the optimum crossover
frequency between each satellite and the subwoofer(s). It
automatically checks the polarity of each loudspeaker and
alerts the user if there are any that may be wired out-of-phase
relative to the others. It measures the distance to each
loudspeaker from the main listening position and adjusts the
delays so that sound from each loudspeaker arrives at the
same time. Finally, Audyssey MultEQ XT determines the
playback level of each loudspeaker and adjusts the volume
trims so that all levels are equal.
