Lexicon DC-1 Supplementary Manual page 29

DC-1 Theory
and Design
Pro Logic requires phase accuracy. Com-
mon azimuth errors cause ghost dialog in
all channels unless the azimuth error is
corrected.
Auto Azimuth and Auto Balance fea-
tures in the DC-1 allow accurate decod-
ing of matrix surround.
26
The DC-1 decoder is unusual in a number of ways. First of all, it is entirely
digital. (Most surround decoders advertise that they are digital because
there is a digital delay line for the surround channel but the matrix and the
logic decoding are done in analog.) Because the DC-1 is all digital, we can
use some of the digital memory to delay all the output channels by 10
milliseconds — about the same as the acoustic delay you get in the front row
of a theater. (The surround channels are delayed by an additional 15 to 30
milliseconds.) This delay allows plenty of time for the DC-1 to determine the
direction of sounds and adjust the matrix before the sounds are sent to the
amplifiers. This substantially improves dialog and effects cancellation, as is
immediately apparent from the spread of ambient material or music, even
in the presence of dialog. The DC-1 can also sense and continuously correct
both balance and azimuth errors in the incoming material. All the time the
film is playing, the DC-1 is checking balance and azimuth, keeping the
dialog perfectly centered. The result is superior steering. An added benefit
is that the DC-1 needs no front panel input balance control; the user need
not bother with this adjustment. You can check the quality of the balance
and azimuth in a tape or disc if you wish by turning the Auto Azimuth/
Balance parameter off and observing any changes in the location of dialog
and effects.
As of this writing, the film industry is moving from a 4 channel audio
standard to a 5.1 channel standard. The new standard is similar to the old,
but has two surround channels instead of one, and an additonal low
bandwidth channel for a subwoofer. Once one has heard some good
examples of stereo surround channels the mono surround channel of Dolby
Surround is no longer satisfying. As most of the information in the sur-
rounds is ambient information and music, stereo significantly increases
listening enjoyment. As listeners become accustomed to placing the sur-
round speakers out to the sides instead of behind them, stereo surrounds
become even more important.
The decorrelation circuit required in previous THX decoders overcomes
some of the limitations of the mono surround channel. While it does not give
directionality to the signal, it at least increases the sense of spaciousness and
reduces the tendency of the surround speakers toward localization. Arti-
facts from the decorrelation itself are usually mild. However, when an event
is specifically steered to the surrounds the decorrelation can diffuse the
signal too much. For example, if a jet flies from the front to the rear, the
sound should not sound sharp and well localized in the front, and then
disappear into a diffused mush in the rear.
Lexicon has developed a new technique for decoding a stereo surround
signal from a standard matrix encoding. This yields a 4-2-5 encoding/
decoding process. When the source film was originally a 5.1 channel film the
resulting 4-2-5 decoding is much closer to the original if the 5.1 Two Channel
effect is used to create a 5.1 channel original. 5-2-5 or 5-2-7 encoding/
decoding is possible, and the match to the original film can be very close
indeed.
Lexicon