6

4743D–CORD–03/06

U4082B

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. The outputs RLO1 and

TLO1 feed a comparator, whose output is fed 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 is given in the section “Attenuator Control Block”

on page 8.

3.4

Background Noise Monitors

This circuit distinguishes 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

the other for the transmit path. The receive background noise monitor is operated by the

RLI1-RLO1 level detector, while the transmit background noise monitor is operated by the

TLI2-TLO2 level detector (Figure 3-2).

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-invert-

ing input of the monitor's output comparator. When speech is present, the voltage on the

non-inverting input of the comparator will rise more quickly 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.

Figure 3-2.

Level Detectors

The 36 mV offset at the comparator's input keeps the comparator from changing state unless the

speech level exceeds the background noise by approximately 4.0 dB. The time constant of the

external RC (approximately 4.7s) determines the response time to background noise variations.

V

S

-

+

(TLI2) RLI1

(17) 26

0.1 µF

Level detector

(TLO2) RLO1

15

+

-

-

-

+

+

36 mV

-

+

350

Ω

Signal input

4 µA

0.1 µF

Signal input

Level detector

Background noise monitor

(RLI2) TLI1

(20) 23

TLO1

24

(19)

To attenuator

control block

-

+

Comparator

C2 (C1)

C4 (C3)

(CPT)

CPR

27

(16)

100 k

Ω

(18)25

2 µF

4 µA

47 µF

2 µF

(RLO2)

56 k

Ω

33 k

Ω

V

B

350

Ω

5.1 k

Ω

V

B

V

B

5.1 k

Ω