Studio Technologies IFB Plus 2001 - Manual (Page 32)

Issue 7, November 2001 Model 2 User Guide
Page 32 Studio Technologies, Inc.
The nominal –2dBu signal is capacitive
coupled to an operational amplifier config-
ured as a variable attenuator. The cou-
pling capacitors are nonpolar type,
allowing for DC voltages of unknown
polarity to be received on the program
inputs. The output of the op-amps have
nominal signal levels of –10dBu and are
direct coupled to the program select
switches. Small capacitors in the feedback
loop of the op-amps ensures stability at
the less than unity gain implementation.
Program Select, Program Muting, and
Program Level Control
Each IFB channel contains a group of six
program select switches. These switches
allow one or more of the program sources
to be selected from among the four pro-
gram inputs and the receive audio from
the two telephone interfaces. During the
design phase, the switches were carefully
specified to allow more than one program
source to be selected at one time. This
feature can prove invaluable in special
applications. The output connections
of the switch sections are connected to
operational amplifiers configured as unity
gain summing amplifiers.
The output of the summing amplifiers are
connected to sections of SSM-2400-series
analog switch, providing program audio
muting during interrupt. These switches
perform a click free off-to-on and on-to-off
transition by internally generating a ramp
control signal. This ensures a very “clean”
transition from program audio to interrupt
audio and vice versa. Optionally, by add-
ing resistors in parallel with the analog
switches, program “dimming” rather than
muting can be implemented.
The output of each analog switch is
connected to a potentiometer. The
potentiometers exhibit a log (audio) taper,
setting the level of program audio relative
to the fixed nominal –10dBu internal level
of the interrupt audio.
Interrupt Audio Routing and Summing
Two identical circuits control interrupt
audio routing. The composite interrupt
audio signals are derived from two opera-
tional amplifiers configured as summing
amplifiers. Four audio sources can enter
the summing junctions: internal micro-
phone audio, access station audio, voice
activated (VOX) interrupt audio, and pro-
gram audio. All audio sources are con-
trolled by sections of SSM-2400-series
analog switch. These switches ensure
clean, click-free audio transitions. The
outputs of the interrupt summing op amps
are connected to the compressor/limiter
circuits. Although program audio can
enter the interrupt audio summing amplifi-
ers, Model 2s are not factory-set to pro-
vide this function. The standard audio
routing combines program audio with
interrupt audio after the compressor/
limiter. Resistors can be added to the
Model 2’s circuit board (after removing
two others) to route program audio into
the interrupt summing amplifiers.
Compressor/Limiters
Interrupt audio, and optionally program
audio, is processed using two compres-
sor/limiter circuits, one for each channel.
Another Analog Devices integrated circuit,
the SSM-2120, is used in creating the
compressor/limiters. The ‘2120 provides—
along with some discrete resistors, capaci-
tors and diodes—everything required to
implement two channels of studio-quality
dynamic range control. Signal from each
of the interrupt summing amplifiers enters
the ‘2120’s control and rectifier sections.