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DC-1 Theory
and Design
DC-1 Digital Controller
The Subwoofer Crossover feature enables the subwoofer speaker to repro-
duce only frequencies in the low bass range, leaving the front, center, sides,
and rears to reproduce frequencies above 80 Hz. Dolby Digital adds
another challenge to bass reproduction in home theaters, as bass frequen-
cies can be mixed into all five channels, as well as the LFE channel. Adapted
for Dolby Digital, this multichannel crossover is designed to blend low
frequency information from the front and surround channels with the LFE
channel. This facilitates optimum placement for imaging and smoothness,
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 Dolby Digital 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
reduces 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
DC-1 (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 stereophonic, and the
decorrelation is unnecessary. This setting is recommended.
Although Dolby Digital has discrete surround channels, not all of the
surround information on these channels is stereophonic. Dolby Digital rear
channel information is often mono. Raindrops, for instance, are typically
recorded 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 effects 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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