Studio Technologies IFB Plus 2001 - Manual (Page 33)

Model 2 User Guide Issue 7, November 2001
Studio Technologies, Inc. Page 33
A DC signal is derived by the rectifier, and
fed back into the control section. The
circuit implements a true dual-slope pro-
cessor, first compressing in an approxi-
mately 4 to 1 input-to-output ratio, and
then going to infinite compression, i.e.,
hard limiting. This circuit ensures that the
talent will get even, intelligible interrupt
audio. Even screaming directors will have
a difficult time fooling this circuit!
Current to Voltage Converter/Summing
Amplifiers
The output of each SSM-2120 section is
fed to a section of operational amplifier.
Each op-amp is implemented as a current
to voltage converter, as the ‘2120 provides
only a current output. The op-amps get
double duty as they sum (combine) the
interrupt audio with program audio from
the program level control. The output of
the op-amps is the composite audio for
the channels. The composite audio for
each channel is sent to five points: talent
amplifier output, line output, telephone
interface 2, level meter, and monitor out-
put select switch.
Internal Microphone
The Model 2 contains an internal electret
microphone, along with front-panel
switches and control circuitry. A micro-
phone preamplifier is created using one
section of low noise operational amplifier.
Power is provided to operate the FET
preamp in the mic. The microphone audio
signal is capacitive coupled to the op-
amps’s non-inverting input. To provide
optimal voice transmission, the capacitor
was selected to roll-off low frequency
input signals; the 3dB down point is
approximately 105Hz. Two fixed resistors
set the gain of the preamp. The output of
the op-amp is capacitive coupled to two
sections of SSM-2400-series analog switch,
part of the interrupt audio routing circuit.
Voice Operated (VOX) Interrupt
To the Model 2’s designers, the VOX cir-
cuitry 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 compli-
cated, it’s just that they don’t often work
very well!
Audio enters the VOX circuit via a 3-position
source select switch. The switch selects
between audio from the two telephone
interfaces, and the auxiliary audio input.
The auxiliary audio input circuit is identical
to those used by the program inputs. The
output of the source switch connects to
one section of SSM-2400-series analog
switch and the input of a bandpass filter.
The bandpass filter allows signals in the
400 to 1400Hz band to pass, while restrict-
ing all others. This frequency range was
selected to best isolate voice signals, which
is the desired VOX interrupt audio source.
The 24dB per octave slope of the filter is
quite effective in removing signals that
could cause false voice detection. It was
felt that providing an excellent bandpass
filter would be the key to accurate voice
detection. While this filter is more compli-
cated than most VOX circuits might use,
we think it provides the desired outcome.
The output of the filter is sent to one section
of integrated circuit operational amplifier
configured as a synchronous half-wave
rectifier. The output of the rectifier is a DC
representation of the energy in the voice
band of the input signal.