Studio Technologies Gen II - Manual (Page 12)

Issue 3, July 1991 Generation II User Guide
Page 14 Studio Technologies, Inc.
buffer. The compressor attack time is
speeded by a charge pump, which reduces
transient distortion that is often associated
with compandors.
Input Low-pass Filter
The compressed signal connects to an anti-
aliasing low-pass filter. Three sections of op
amp form a 6-pole, 20kHz Butterworth low-
pass filter. This reduces the possibility of
audio frequencies aliasing with the BBD’s
clock signal.
BBD Clock
An oscillator integrated circuit provides
three signals for the BBD: clock 1, clock 2,
and a voltage reference. Clock 1 and clock
2 are identical square wave signals 180
degrees apart in phase. The frequency of
clock 1 and clock 2 is set by a resistor-
capacitor (RC) combination, in association
with an FET-controlled frequency select
circuit. In the music mode, the FET is bi-
ased to the non-conducting state. In the
music & voice mode, a shorter delay is
required and the FET is turned on, reducing
the time constant of the RC circuit and
raising the clock frequency. The clock is set
for 64±1kHz in the music mode and
switches to approximately 119kHz in the
music & voice mode.
Output Low-pass Filter
The time delayed audio enters a 5-pole,
20kHz Butterworth low-pass filter created
using three sections of operational amplifier.
This filter removes clock signal from the
BBD’s output signal.
Expander
The signal was compressed prior to being
delayed. The expander portion of the
compandor integrated circuit is used to
recreate the original dynamic range. Again
a charge pump is used, this time in the
compandor rectifier. The audio signal has
now been delayed and is ready to go to
work!
Randomizing Network
The delayed signal enters a proprietary
randomizing circuit which is in the form of a
sealed module. Frankly, the great sound of
the Generation II’s simulated stereo is
created by this network. The randomizing
circuit modifies the delayed signal to ensure
that the peaks and dips in the soon to be
created comb filter do not fall on objection-
able harmonics, preserving the natural
sound of the audio. The output of the net-
work connects to both the front panel and
trim stereo intensity potentiometers.
Summing/Gain and Difference/Gain
The simulated left and right signals are
created by two sections of op amp. The left
channel is created by summing the output
of the front panel or trim potentiometer
stereo intensity control with the direct input
signal. This creates a comb filter which
breaks the audio signal into hundreds, or
thousands, of little pass-bands. The right
channel is created by producing the differ-
ence between the output of the front panel
or trim potentiometer stereo intensity control
and the direct input signal. Again, combs
are created, but with every comb 180 de-
grees out of phase with those created in the
left channel. These combs are what give the