IL34118
the other (RLI2) is at the output of the speaker
amplifier. On the transmit side, one level detector
(TLI2) is at the output of the microphone amplifier,
while the other (TLI1) is at the hybrid output.
Outputs RLO1 and TLO1 feed a comparator, the
output of which goes to the Attenuator Control Block.
Likewise, outputs RLO2 and TLO2 feed a second
comparator which also goes to the Attenuator Control
Block. The truth table for the effects of the level
detectors on the Control Block is given in the section
describing the Control Block.
the gain of the receive attenuator is reduced, and the
gain of the transmit attenuator is increased such that
their sum remains constant. Changing the voltage at
VLC changes the voltage at C
T
(see the Attenuator
Control Block section), which in turn controls the
attenuators.
The volume control setting does not affect the
maximum attenuator input signal at which notice able
distortion occurs.
The bias current at VLC is typically 60 nA out of
the pin, and does not vary significantly with the VLC
voltage or with V
CC
.
BACKGROUND NOISE MONITORS
DIAL TONE DETECTOR
The purpose of the background noise monitors is
to distinguish speech (which consists of bursts) from
background noise (a relatively constant signal level).
There are two background noise monitors - one for
the receive path and one for the transmit path.
Refering to Figure 2, the receive background noise
monitor is operated on by the RLI1-RLO1 level
detector, while the transmit background noise
monitor is operated on by the TLI2-TLO2 level
detector. They monitor the background noise by
storing a dc voltage representative of the respective
noise levels in capacitors at CPR and CPT. The
voltages at these pins have slow rise times
(determined by the external RC), but fast decay times.
If the signal at RLI1 (or TLI2) changes slowly, the
voltage at CPR (or CPT) will remain more positive
than the voltage at the non-inverting input of the
monitor’s output comparator. When speech is
present, the voltage on the noninverting input of the
comparator will rise quicker than the voltage at the
inverting input (due to the burst characteristic of
speech), causing its output to change. This output is
sensed by the Attenuator Control Block.
The 36 mV offset at the comparator’s input keeps
the comparator from changing state unless the speech
level exceeds the background noise by
≈4.0
dB. The
time constant of the external RC (≈4.7 seconds)
determines the response time to background noise
variations
The dial tone detector is a comparator with one
side connected to the receive input (RXI) and the
other input connected to V
B
with a 15 mV offset (see
Figure 3). If the circuit is in the receive mode, and the
incoming signal is greater than 15 mV (10 mVrms),
the comparator’s output will change, disabling the
receive idle mode. Tthe receive attenuator will then
be at a setting determined solely by the volume
control.
The purpose of this circuit is to prevent the dial
tone (which would be considered as continuous
noise) from fading away as the circuit would have the
tendency to swich to the idle mode. By disabling the
receive idle mode, the dial tone remains at the
normally expected full level.
Figure 3. Dial Tone Detector
AGS
The AGS circuit affects the circuit only in the
receive mode, and only when the supply voltage
(V
CC
) is less than 3.5 volts. As V
CC
falls below
3.5 volts, the gain of the receive attenuator is
reduced. The transmit path attenuation changes such
that the sum of the transmit and receive gains remains
constant.
The purpose of this feature is to reduce the power
(and current) used by the speaker when a line-
powered speakerphone is connected to a long line,
where the available power is limited. By reducing the
speaker power, the voltage sag at V
CC
is controlled,
preventing possible erratic operation.
VOLUME CONTROL
The volume control input at VLC (Pin 13) is
sensed as a voltage with respect to V
B
. The volume
control affects the attenuators only in the receive
mode. It has no effect in the idle or transmit modes.
When in the receive mode, the gain of the receive
attenuator will be +6.0 dB, and the gain of the
transmit attenuator will be -46 dB only when VLC is
equal to V
B
. As VLC is reduced below V
B
,
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