IL34118
ATTENUATOR CONTROL BLOCK
The Attenuator Control Block has the seven
inputs described above:
- Tthe output of the comparator operated by RLO2
and TLO2 (microphone/speaker side) - designated
C1.
- The output of the comparator operated by RLO1
and TLO1 (Tip/Ring) side) - designated C2.
- The output of the transmit background noise
monitor - designated C3.
- The output of the receive background noise
monitor - designated C4.
- The volume control.
- The dial tone detector.
- The AGC circuit.
The single output of the Control Block controls
the two attenuators. The effect of C1-C4 is as
follows:
Inputs
Output
C1
C2
C3
C4
Mode
Tx
Tx
1
X
Transmit
Tx
Rx
Y
Y
Fast Idle
Rx
Tx
Y
Y
Fast Idle
Rx
Rx
X
1
Receive
Tx
Tx
0
X
Slow Idle
Tx
Rx
0
0
Slow Idle
Rx
Tx
0
0
Slow Idle
Rx
Rx
X
0
Slow Idle
X = Don’t Care; Y = C3 and C4 are not both 0
A definition of the above terms:
1) “Transmit” means the transmit attenuator is
fully on (+6.0 dB), and the receive attenuator is
at max. attenuation (-46 dB).
2) “Receive” means both attenuators are controlled
by the volume control. At max. volume, the
receive attenuator is fully on (6.0 dB), and the
transmit attenuator is at max. attenuation (-
46 dB).
3) “Fast Idle” means both transmit and receive
speech are present in approximately equal
levels. The attenuators are quickly switched
(30 ms) to idle until one speech level dominates
the other.
4) “Slow Idle” means speech has ceassed in both
transmit and receive path. The attenuators are
then slowly switched (1 second) to the idle
mode.
5) Switching to the full transmit or receive modes
from any other mode is at the fast rate (30 ms).
A summary of the truth table is as follows:
1) The circuit will switch to transmit if: a) both
transmit level detectors sense higher signal levels
relative to the respective receive level detectors
(TLI1 versus RLI1, TLI2 versus RLI2), and b) the
transmit background noise monitor indicates the
presence of speech.
2) The circuit will switch to receive if: a) both
receive level detectors sense higher signal levels
relative to the respective transmit level detectors, and
b) the receive background noise monitor indicates the
presence of speech.
3) The circuit will switch to the fast idle mode if
the level detectors disagree on the relative strengths
of the signal levels, and at least one of the
background noise monitors indicates speech. For
example, refferring to the Expanded Logic Diagram
(Figure 8), if there is sufficient signal at the
microphone amp output (TLI2) to override the
speaker signal (RLI2), and there is sufficient signal at
the receive input (RLI1) to override the signal at the
hybrid output (TLI1), and either or both background
monitors indicate speech, then the circuit will be in
the fast idle mode. Two conditions which can cause
the fast idle mode to occur are a) when both talkers
are attempting to gain control of the system by talking
at the same time, and b) when one talker is in a very
noisy environment, forcing the other talker to
continually override that noise level. In general, the
fast idle mode will occur infrequently.
4) The circuit will switch to the slow idle mode
when a) both talkers are quiet (no speech present), or
b) when one talker’s speech level is continuously
overriden by noise at the other speaker’s location.
The time required to switch the circuit between
transmit, receive, fast idle and slow idle is determined
in part by the components at the C
T
pin (Pin 14). A
schematic of the C
T
circuitry is shown in Figure 4 and
operates as follows:
- R
T
is typically 120 kΩ, and C
T
typically 5.0
µF.
- To switch to the receive mode, I
1
is turned on (I
2
is off), charging the external capacitor to
+240 mV above V
B
. (An internal clamp prevents
further charging of the capacitor.)
- To switch to the transmit mode, I
2
is turned on (I
1
is off) bringing down the voltage on the capacitor
to -240 mV with respect to V
B
.
- To switch to idle quickly (fast idle), the current
sources are turned off, and the internal 2.0 kΩ
resistor is switched in, discharging the capacitor
to V
B
with a time constant = 2.0 KΩ x C
T
.
- To switch to idle slowly (slow idle), the current
sources are turned off, the switch at the 2.0 kΩ
resistor is open, and the capacitor discharges to
V
B
through the external resistor R
T
with a time
constant = R
T
x C
T
.
5
INTEGRAL [ INTEGRAL CORP. ]
