Studio Technologies IFB Plus 2005 - Manual (Page 34)

Issue 8, December 2005 Model 2 User Guide
Page 34 Studio Technologies, Inc.
from initiating an interrupt on a channel that
is already in the interrupt state. This allows
only one access station at a time to inter-
rupt a channel. 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 6 dB. This is due to the fact
that the outputs would load each other, cre-
ating a voltage divider. The operating state
of the interrupt channels is determined by
monitoring the status lamp voltages. A lamp
voltage is nominally 10 V when an interrupt
channel is idle, 14.5 V when active. The 12 V
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 per-
form 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 initiat-
ing 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 6 V 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 chan-
nels are active. The comparators have open
collector outputs, thus allowing the direct
parallel connection—pretty handy, huh!)
The paralleled comparator outputs are con-
nected, 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 capaci-
tor form a delay-on-turn off function. This
allows the SSM-2404 analog switches on
the Model 2 to quietly disconnect the access
station audio buses from the interrupt chan-
nel prior to the access station releasing its
connection to the bus. This prevents 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 ramp-
ing to the on state.
Model 32 Talent Amplifier
General Description
The Model 32 Talent Amplifier is a self-
contained module which allows headphones
or ear pieces to be driven with IFB channel
1 or 2 audio from a Model 2 Central Control-
ler. The unit contains circuitry to extract DC
power from the three conductors that carry
power and two channels of audio. Along
with a power amplifier integrated circuit, the
unit contains a channel select switch, power
LED, and level control. The Model 32 con-
tains two 3-pin XLR-type connectors, one
male and one female. The female connector