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SL34118D

Voice Switched Speakerphone Circuit

System Logic Semiconductor 

SL34118

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.

BACKGROUND NOISE MONITORS

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

VOLUME CONTROL

The volume control input at VLC (Pin 13) is

sensed as a voltage with respect to VB. 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

VB. As VLC is reduced below VB,

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 CT (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 VCC.

DIAL TONE DETECTOR

The dial tone detector is a comparator with one

side connected to the receive input (RXI) and the

other input connected to VB 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 (VCC)

is less than 3.5 volts. As VCC 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 VCC is controlled,

preventing possible erratic operation.

SLS

System Logic

Semiconductor

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