MC34118

LINEAR INTEGRATED CIRCUIT

UNISONICTECHNOLOGIESCO.,LTD

10 of 12

www.unisonic.com.tw

QW-R108-007,D

FUNCTIONAL DESCRIPTION(Cont.)

Definition of the above is as follows:

Trems

Definition

Transmit

The transmit attenuator is fully on: +6dB. And the receive attenuator is at max. attenuation :-46dB

Receive

Both attenuators are controlled by the volume control. At max. Volume the receive attenuator is fully

on: +6dB, and the transmit attenuator is at max.attenuation:-46dB.

Fast Idle

Both transmit and receive speech are present in approximately equal levels.

The attenuators are quickly switched to idle until one speech level dominates the other.

Slow Idle

Speech has ceased in both transmit and receive paths. The attenuators are then slowly switched to

the idle mode.

A summary of the truth table is:

1. Under all the below situations the circuit will switch to transmit :

(1) Both transmit level detectors sense signals relative to the respective receive level detectors

TLI1 versus RLI1, TLI2 versus RLI2

(2) The transmit background noise monitor indicates the presence of speech.

2. Under all the below situations the circuit will switch to receive

(1) Both receive level detectors sense higher signal levels relative to the respective transmit level detectors

(2) The receive background noise monitor indicates the presence of speech.

3. Under all the below situations the circuit will switch to the fast Idle mode:

(1) If the level detectors disagree on the relative strengths of the signal levels

(2) At least one of the background noise monitor indicates speech.

There are two conditions which can cause the fast idle mode to occur:

(A) When both talkers are attempting to gain control of the system by talking at the same time

(B) When one talker is in a very noisy environment, forcing the other to continually over-ride that noise level. In

general, the fast idle mode will occur infrequently.

4. Under any of the below situations the circuit will switch to the slow idle mode

(1) Both talkers are quiet( no speech present)

(2) When one talker’s speech level is continuously over-ride by noise at the other speaker’s location.

CT pin (pin14) determine the time required to switch the circuit between transmit, receive, fast idle, and slow

idle.

A schematic of the CT circuitry operates as follows:

*

RT =120kΩ (typ.), and CT =5µF (typ.).

*

Switching to the receive mode

I1 is turned on; I2 is off,

Charging the external capacitor to +240mV above VB

An internal clamp prevents further charging of the capacitor).

*

Switching to the transmit mode

I2 is turned on; I1 is off,

Bringing down the voltage on the capacitor to -240mV with respect VB.