30
Multi-Channel Music & Cinema Systems
Theory and Design
Lexicon
low frequency information from the front and surround channels with the
LFE channel. This facilitates optimum placement for imaging and smooth-
ness, protects smaller speakers, and uses amplifiers more efficiently.
Re-Equalization is provided as a control to compensate for the fact that the
soundtracks in films sound "bright" when listened to in a home environ-
ment. This results from a combination of the way we perceive sound in large
halls vs. small rooms, and the theater equalization which has become
standardized throughout the movie industry. This problem can be more
pronounced with discrete soundtracks due to the addition of three full-
range channels in the mix (Center, Left Surround and Right Surround). The
re-equalization control restores the sound to its natural balance, and re-
duces listening fatigue dramatically by reducing excessive high frequen-
cies.
As the surround channel in a Dolby stereo film is monaural, the signal is
neither spacious nor enveloping. In a theater, the quantity and placement of
surround speakers compensates for this; in the home, arrays of 12-18
speakers are generally impractical. To diffuse the perceived sound, the
processor (in the decorrelated mode) provides a decorrelation algorithm
which, after the monaural signal is split in two, alters the phase/time
relationships between channels slightly. This eliminates the mono effect
from the rear channel and creates an enveloping soundfield. When the
"Stereo Surround" parameter is selected, the rear channels become stereo-
phonic, and the decorrelation is unnecessary. This setting is recommended.
Although Dolby Digital and DTS have discrete surround channels, not all
of the surround information on these channels is stereophonic. Rear chan-
nel information is often mono. Raindrops, for instance, are typically re-
corded in mono. When played back through the array of surround speakers
in a theater, there is no need for left channel and right channel raindrops, as
the speaker system presents this sound as an enveloping environmental
effect. The Home THX Adaptive Decorrelation circuitry is active when
surround channel information is predominanatly mono. When stereo ef-
fects are present, decorrelation is not used. The result is consistently
spacious surround sound and dramatic stereo effects.
Timbre Matching compensates for the difference in characteristics of
sounds coming from different locations. In real life, timbre differences help
us differentiate sounds which originate from the sides and rear from those
originating from the front. Some people, however, find that these natural
changes in timbre are undesirable in a film. Timbre-Matching provides
equalization to the surround channels, reducing the perceived change in
character of sounds which are panned from the screen into the surrounds.
In Logic 7, stereo surround 7 channel
decoding features full-range stereo ef-
fects in side and rear speakers.
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