Studio Technologies IFB Plus 2005 - Manual (Page 27)

Model 2 User Guide Issue 8, December 2005
Studio Technologies, Inc. Page 27
to be selected from among the four program
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 con-
figured as unity gain summing amplifiers.
The output of the summing amplifiers are
connected to sections of SSM-2404 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 adding resistors in
parallel with the analog switches, program
“dimming” rather than muting can be imple-
mented.
The output of each analog switch is con-
nected to a potentiometer. The potentiom-
eters exhibit a log (audio) taper, setting the
level of program audio relative to the fixed
nominal –10 dBu internal level of the inter-
rupt audio.
Interrupt Audio Routing and Summing
Two identical circuits control interrupt audio
routing. The composite interrupt audio sig-
nals are derived from two operational ampli-
fiers configured as summing amplifiers. Four
audio sources can enter the summing junc-
tions: internal microphone audio, access
station audio, voice activated (VOX) interrupt
audio, and program audio. All audio sources
are controlled by sections of SSM-2404
analog switch. These switches ensure clean,
click-free audio transitions. The outputs of
the interrupt summing op-amps are con-
nected to the compressor circuits.
Compressor Circuits
Interrupt audio is processed using two com-
pressor circuits, one for each channel. Laser-
trimed voltage-controlled amplifier integrated
circuits are used to create the compressor
circuits. They provide—along with some op-
erational amplifier integrated circuits, discrete
resistors, capacitors and diodes—everything
required to implement two channels of dy-
namic range control. The circuit implements
true studio-quality compressors, ensuring
that the talent will get even, intelligible inter-
rupt audio. Even screaming directors will
have a difficult time fooling these circuits!
Internal Microphone
The Model 2 contains an internal electret
microphone, along with front-panel switches
and control circuitry. A microphone preampli-
fier is created using one section of low noise
operational amplifier. Power is provided to
operate the FET preamp in the mic. The mi-
crophone audio signal is capacitive coupled
to the op-amp’s non-inverting input. To pro-
vide optimal voice transmission, the capacitor
was selected to roll-off low frequency input
signals; the 3 dB down point is approximately
105 Hz. Two fixed resistors set the gain of the
preamp. The output of the op-amp is capaci-
tive coupled to two sections of SSM-2404
analog switch, part of the interrupt audio
routing circuit.
Voice Operated (VOX) Interrupt
To the Model 2’s designers, the VOX circuitry
is the most interesting as it combines analog
and digital functions to perform a function
that, historically, has not worked very well in
most equipment. Including a VOX circuit was
contemplated with some trepidation. Don’t
get us wrong, it’s not that most VOX circuits
are unusually complicated, it’s just that they
don’t often work very well!