Studio Technologies IFB Plus 2001 - Manual (Page 40)

Issue 7, November 2001 Model 2 User Guide
Page 40 Studio Technologies, Inc.
continuing to prove that comparators are
about the handiest “building block” avail-
able! The comparators perform three
major functions. The first function prevents
an access station from initiating an inter-
rupt on a channel that is already in the
interrupt state. This allows only one ac-
cess station at a time to interrupt a chan-
nel. This function is operationally helpful,
as well as being required by the audio bus
arrangement. If, for example, two access
stations connect to the bus at once, the
audio level of the two signals would each
drop by 6dB. This is due to the fact that
the outputs would load each other, creat-
ing a voltage divider. The operating state
of the interrupt channels is determined
by monitoring the status lamp voltages.
A lamp voltage is nominally 10V when
an interrupt channel is idle, 14.5V when
active. The 12V reference is used by the
comparators as the lamp voltage “slicing”
level. Another section of comparator is
used to “mask” the lamp signal when the
access station has initiated the interrupt.
This prevents an oscillation condition—
can you figure out what it is?
The second function the comparators
perform is to activate the interrupt (talk)
buses. When the access station’s interrupt
logic lines go to the interrupt active state
the comparator outputs sink current,
pulling the interrupt buses to the low
state. The buses are normally high, and
get pulled low when interrupt is required.
The third comparator function, if config-
ured upon installation using a slide switch,
causes the Model 2’s monitor output to
mute whenever the access station is
initiating an interrupt. This is an important
feature, preventing feedback if the access
station is located in close proximity to the
monitor loudspeaker. Two sections of
comparator monitor the internal interrupt
active logic signals. The 6V reference is
used as the slicing level. The outputs of
the comparators are connected in parallel,
allowing current sinking whenever one or
both of the channels are active. The com-
parators have open collector outputs, thus
allowing the direct parallel connection—
pretty handy, huh!) The paralleled com-
parator outputs are connected, via the
configuration switch, to the monitor mute
bus. The bus is normally high, and gets
pulled low by the comparators when
monitor muting is required.
Two analog switches perform a dual role,
provide voltage level shifting, and imple-
menting a delay function. The switches
convert the access station’s interrupt
active logic signal, which is referenced to
system common, to a signal referenced
to VREF. The analog switches which
connect audio to the buses need VREF
as their reference. A combination of a
diode, resistor, and capacitor form a
delay-on-turn off function. This allows the
SSM-2400-series analog switches on the
Model 2 to quietly disconnect the access
station audio buses from the interrupt
channel prior to the access station releas-
ing its connection to the bus. This pre-
vents a nice loud click from reaching the
interrupt output. Only a delay on turn off
is implemented. A fast off-to-on turn on
is required, placing the access station
audio on the bus prior to the analog
switch on the Model 2 quietly ramping
to the on state.
Model 32 Talent Amplifier
General Description
The Model 32 Talent Amplifier is a self-
contained module which allows head-
phones or ear pieces to be driven with