Lexicon Prime Time 93 - Page 29

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For example, a metallic sound can be heard as a result of comb filtering
among the recirculating delays. More generally, the frequency response of
the recirculating delayed signal becomes distorted as a result of these fil-
tering effects.
DELAY
LINE
MASTER
OUTPUT
FIG. 3.6 SCHEMATIC REPRESENTATION OF RECIRCULATION AND ROLLOFF
M.93 incorporates three methods of combatting these effects. The first is
that of setting the delays to prime number values, identified by the PRIME
indicator. Relatively prime delays insure that there are no common factors
and consequently a minimum number of repetitive delay patterns which contrib-
ute to unnatural
side effects. Second, a small amount of automatic VCO
cycling randomizes the delays to break up repetitive delay patterns. However,
the FREQUENCY must be low enough and the DEPTH must be shallow enough so as
not to introduce
any audible
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doppler
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or flanging effects. Finally, the in-
vert function on the input signal at the OUTPUT MIXER enables a Schroeder*
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all-pass
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filter to be constructed. This is a technique of providing smooth
frequency response through a system of recirculated and delayed signals. It
turns out that the input invert function provides a qualitative improvement
even though the exact amplitudes prescribed by Schroeder are not duplicated.
See FIG 3.7. These methods all contribute to a smoother, more realistic
simulation of room ambiance.
*Schroeder,
M.R.,
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Natural Sounding Artificial Reverberation
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, Journal of the
A.E.S., Vol.lO, No.3,
(July 1962), p.219.
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