Lexicon DC-1 Supplementary Manual page 8

DC-1 Digital Controller
To overcome this problem, Schroeder played his recordings through a pair
of loudspeakers in an anechoic chamber, using a special electro-acoustic
technique (developed by Atal, Schroeder, Damaske and Mellert) to elimi-
nate crosstalk between the listener's ears. Normally each speaker is heard
by both ears but Schroeder's system canceled the sound reaching the right
ear from the left speaker and vice-versa. Provided that the listener held his
head in exactly the right spot, the sound had all the excellent localization
properties of earphones but was properly located outside the listener's
head.
This technique, generically known as interaural crosstalk cancellation, was
found to work well even without the anechoic chamber, and music pre-
sented this way can be quite enjoyable. Lexicon incorporated this technique
in many products as "Panorama". Dwane Cooper coined the name
"transaural reproduction" for similar effects. This allowed the first direct
comparisons of specific halls. From these studies Schroeder concluded that
the best halls were all characterized by having large differences in the sound
between the two ears in the dummy head. Very simply, the best halls gave
the most stereo. But there is more to it than that. We have continued
laboratory work, using both Panorama and headphones and have utilized
our skill in reproducing different types of reverberation to study the effects
of reflected energy at all time delays and angles. Our results have shown
that differences between the ears are created by sound arriving from the
sides, but the optimum angle is a function of frequency. Below 700Hz the
optimum angle is completely from the side (or lateral). As the frequency
rises the optimum angle becomes a cone centered on a line drawn between
the ears. The interior angle of the cone rises from 0° at 700Hz to about 30°
at 1000Hz, to about 60° at 2000Hz.
This angular dependence has some very interesting consequences for the
perception of spaciousness, envelopment, and reverberance. Most home hi-
fi systems have the front loudspeakers separated by at least ±30°. Such a
separation is effective for frequencies above 1000Hz, and the front speakers
alone create substantial spaciousness, at least at the frequencies which
convey the most speech (and music) information. In the best seats in concert
halls the orchestra is at least this wide. The high frequency direct sound is
inherently spacious, even without the aid of reflected energy. Yet it is clear
that halls increase the spaciousness of the sound. With careful listening both
in actual halls and in the laboratory, it becomes apparent that the increased
spaciousness is perceived primarily at low frequencies and in the gaps
between notes. Hall spaciousness can occasionally also be heard when a
note is held, particularly during a brass chord with a little vibrato, but this
perception is relatively rare. Spaciousness and reverberance are never
heard at the attacks of notes. In the best halls these are always clear and dry.
DC-1Theory
and Design
Sound from the side is vital to listener
comfort and involvement . . . It must
really be from the side!
In an anechoic chamber, this sideways
sound is missing . . . and music sounds
unpleasant.
In an ordinary room, the room supplies
these directions, and the sound is
cramped, but tolerable. The overall im-
pression, however, may be muddy due to
unwanted frontal reflections.
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