Studio Technologies ISS - Manual (Page 30)

ISS User Guide Issue 3, June 1990
Studio Technologies, Inc. Page 31
ISS
integrated circuits U8 and U3. Analog
switch U9 selects which input, left or right,
gets sent to the simulator cards.
The left and right audio output signals from
the Type I and Type II simulator cards come
into the Crossfade Card via two sections of
op amp U5, which serve as inverting sum-
ming junctions to combine the signals from
the two simulator cards. The outputs of the
summing amplifiers each connect to two
points in the circuit: summing with the line
input signal going into the VCA, and sum-
ming with the output of the VCA going into
another op amp section of U5.
The crossfade action occurs by the cancel-
lation process that is directly proportional to
the output level of the VCA. The VCA output
level is determined by a 0 to 800mV control
voltage whose source will be discussed
later. A control voltage of 800mV gives
maximum attenuation; 0mV gives unity gain
(0 attenuation). When 0mV is fed to a VCA,
the summed signal from the simulator cards
is cancelled out; when 800mV is fed to the
VCA the line input signal is canceled out.
The null adjusts, trim pots R6 and R25, are
set at the factory to provide the best attenu-
ation at the extreme end of the crossfade.
An incorrectly set null adjustment would
give cross talk between the inputs and the
outputs of the stereo simulator cards.
Logic Control: The logic and crossfade
control voltage circuitry is quite straight
forward. Four logic input signals, entering
via the ribbon cable bus, give the functional
commands to the Crossfade Card. These
signals request simulate from the left chan-
nel, simulate from the right channel,
crossfade at FAST, and crossfade at
IMMEDIATE.
The “heart” of the circuitry is a simple
resistor/capacitor circuit created by C7 and
R7, and 5V and 10V reference voltages
created by R31, R32 and R38. We add to
this three op amps acting as comparators
(sections of U7 and U11) which allow the
RC circuit to swing between 5V and 10V.
The 5V to 10V swing is controlled in direc-
tion (up or down) and speed. Two sections
of NAND gate U1 provide a logic high when
simulate from left or simulate from right is
requested. This logic high (15V) moves the
voltage on the RC circuit from 5V to 10V.
Since the capacitance is fixed, the speed at
which the RC circuit goes from 5V to 10V is
dependent on the resistance in the RC
circuit. The FAST/SLOW and IMMEDIATE
speed logic signals determine if additional
resistance is placed in parallel with R7,
decreasing the time constant and speeding
up the crossfade. The additional resistance
is placed in or out of the RC circuit using
two sections of analog switch U2. When
simulate from left or simulate from right is
not requested, the RC circuit moves back to
5V; the speed is again determined by the
status of the speed logic signals.
The 5V to 10V ramp signal is sent to two
places in the circuitry. A section of op amp
U11 provides a linear conversion, taking the
5V to 10V swing and changing it to a 0V to
800mV swing. This voltage swing provides
the VCAs with the crossfade voltage. The
circuit is designed so as to “lock” within a
few mV of 0V so as to keep the VCA pre-
cisely at unity gain. Another section of op
amp U11 converts the 5V to 10V swing to a
0V to 15V swing. This swing is used to light
the crossfade status lights. Note that two
sections of analog switch U2 control the
simulate from left and simulate from right
LEDs.