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Multi-Channel Music & Cinema Systems
Theory and Design
Lexicon
Lexicon decoders are unusual in a number of ways. First of all, our
processors are 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 our proces-
sors are 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 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 mate-
rial or music, even in the presence of dialog. Lexicon processors can also
sense and continuously correct both balance and azimuth errors in the
incoming material. All the time the film is playing, the processor is checking
balance and azimuth, keeping the dialog perfectly centered. The result is
superior steering. An added benefit is thatLexicon processors need 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.
The film industry continues to move 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 additional low frequency effects
channel for subwoofer playback. 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 surrounds is
ambient information and music, stereo significantly increases listening
enjoyment. As listeners become accustomed to placing the surround speak-
ers 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 Logic 7 technology for decoding a stereo surround
signal from a standard matrix encoding. This yields a 4-2-5 encoding/
decoding process. If a 5 .1 channel soundtrack is recorded onto a 2-channel
medium-such as HiFi VHS tape or mini-disc using our 2-Channel
downmixing effect, the resulting two-channel mix will be encoded with all
5.1 channels, which is much closer to the original. When played back
through a matrix decoder using Pro Logic, a four-channel signal is pre-
sented, including all the surround information and low frequency informa-
Auto Azimuth and Auto Balance fea-
tures allow accurate decoding of matrix
surround.
Pro Logic requires phase accuracy. Com-
mon azimuth errors cause ghost dialog in
all channels unless the azimuth error is
corrected.
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