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IL34118DWT

Voice Switched Speakerphone Circuit

IK Semicon Co., Ltd 

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.

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. Referring 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

non-inverting 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. The 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

switch 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

AGC

The AGC 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.

2013, January, Ver. 02

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