MC34118

LINEAR INTEGRATED CIRCUIT

UNISONICTECHNOLOGIESCO.,LTD

8 of 12

www.unisonic.com.tw

QW-R108-007,D

FUNCTIONAL DESCRIPTION(Cont.)

BACKGROUND NOISE MONITORS

To distinguish speech (consists of bursts) from background noise (a relatively constant signal level) is the main

purpose of the background noise monitors. There is one background noise monitor for the receive path and another

one for the transmit path. The receive background noise monitor is operated on by the RLI1-RLO1 level detector, as

same as the transmit background noise monitor is operated on by the TLI2-TLO2 level detector. By storing a DC

voltage representative of the respective noise levels in capacitors at CPR and CPT, the background noise can be

monitored. At these pins, the voltages have slow rise times which determined by the external RC, but fast decay

times. The voltage at the non-inverting input of the monitor’s output comparator won’t be more positive than the

voltage at CPR/CPT when the signal at RLI1/TLI2 changes slowly. But while speech is present, it will rise quicker

than the voltage at the inverting input, causing its output to change. The term’s output is sensed by the attenuator

Control Block.

VOLUME CONTROL

At VLC (pin 13) is the volume control input which is sensed as a voltage with respect to VB. Only in the receive

mode the volume control affects the attenuators. In the receive mode, the receive attenuator’s gain will be +6dB, and

the transmit attenuator’s gain will be -46dB when VLC is equal to VB. When VLC is reduced below VB, the receive

attenuator’s gain is reduced, and the transmit attenuator’s gain is increased, but the sum of them tow remains

constant. Out of the pin VLC, the bias current is typically 60nA.

DIAL TONE DETECTOR

This detector is a comparator with one side connected to RXI and one side input connected to VB with a 15mV

offset. Under this situation: the circuit is in the receive mode, and the incoming signal is greater than 15Mv, the

comparator’s output will change, disabling the receive idle mode. Then receive attenuator will be at a setting only

determined by the volume control.