Studio Technologies IFB Plus 2005 - Manual (Page 28)

Issue 8, December 2005 Model 2 User Guide
Page 28 Studio Technologies, Inc.
Audio enters the VOX circuit via a
3-position source select switch. The switch
selects between audio from the two tele-
phone 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 con-
nects to one section of SSM-2404 analog
switch and the input of a bandpass filter.
The bandpass filter allows signals in the 400
to 1400 Hz band to pass, while restricting all
others. This frequency range was selected
to best isolate voice signals, which is the de-
sired VOX interrupt audio source. The 24 dB
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 complicated 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.
A capacitor connected to the output of the
rectifier sets the VOX release time. The
larger the capacitor value, the longer an
absence of voice-band energy is required
for the DC output to fall below the VOX
threshold. The DC voltage from the rectifier
is connected to one section of integrated
circuit comparator. A reference voltage, cre-
ated by two resistors, sets the comparator’s
switching threshold. A resistor from the out-
put of the comparator to the reference input
provides positive feedback, i.e., hysteresis.
This adds stability to the circuit, preventing
minor variations in the DC input voltage from
causing rapid switching. Since the output of
the comparator is an open-collector type, a
pull up resistor is required. The output of the
comparator is a “voice present” logic signal.
It goes to the logic low state whenever a
voice signal is detected. This logic signal is
connected to one section of the VOX output
select switch. The switch connects the VOX
logic signal to the circuitry associated with
the interrupt logic, program muting, and
status LEDs.
The “voice present” logic signal is also
connected to an inverter gate. The inverted
signal controls the VOX audio analog switch,
mentioned previously. The analog switch
goes to the on state whenever an input
audio signal is detected. The output of the
analog switch is routed by the VOX output
select switch to the interrupt summing bus
for either channel 1 or channel 2.
Telephone Interfaces
The Model 2 contains two telephone inter-
face circuits. Although somewhat similar,
each has unique characteristics. Interface 1
is designed to receive audio from the out-
side world. Interface 2 can receive audio,
as well as send IFB channel 1 or 2 audio.
In addition, interface 2 contains circuitry to
automatically answer a ringing telephone
line. Each interface contains a mode switch,
allowing the interface to be set to the tele-
phone line or standard audio mode. The
telephone line mode optimizes the interface
for connection to a standard loop start tele-
phone line. This mode is sometimes referred
to as “wet,” a telephone company term
for DC current flowing through a line. The
standard audio mode optimizes the inter-
face for connection to a circuit that does not
have DC current flowing in it. This mode is
sometimes referred to as “dry,” a telephone
company term for a line with no DC current
flowing through it.