4

4743D–CORD–03/06

U4082B

3.

Introduction

3.1

General

The fundamental difference between the operation of a speakerphone and a handset is that of

half duplex versus full duplex. The handset is full duplex since conversation can occur in both

directions (transmit and receive) simultaneously. A speakerphone has higher gain levels in both

paths, and attempting to converse in full-duplex mode results in oscillatory problems due to the

loop that exists within the system. The loop is formed by the receive and transmit paths, the

hybrid and the acoustic coupling (speaker to microphone).

The only practical and economical solution used to date is to design the speakerphone to oper-

ate in half-duplex mode. That is, only one person speaks at a time, while the other listens. To

achieve this, a circuit is required which can detect who is talking, switch on the appropriate path

(transmit or receive), and switch off (attenuate) the other path. In this way, the loop gain is main-

tained less than unity. When the talkers exchange functions, the circuit must quickly detect this,

and switch the circuit appropriately. By providing speech-level detectors, the circuit operates in a

“hands-free” mode, eliminating the need for a “push-to-talk” switch.

The handset has the same loop as the speakerphone. Oscillations do not occur because the

gains are considerably lower and the coupling from the earpiece to the mouthpiece is almost

nonexistent (the receiver is normally held against a person's ear).

The U4082B provides the necessary level detectors, attenuators, and switching control for a

properly operating speakerphone. The detection sensitivity and timing are externally controlla-

ble. Additionally, the U4082B provides background noise monitors (which make the circuit

insensitive to room and line noise), hybrid amplifiers for interfacing to Tip and Ring, the micro-

phone amplifier, and other associated functions.

15

VB

Output voltage

≈ V

16

CPT

An RC at this pin sets the time constant for the transmit background monitor

17

TLI2

Transmit level detector input on the microphone/speaker side

18

TLO2

Transmit level detector output on the microphone/speaker side, and input to the transmit background

monitor

19

RLO2

Receive level detector output on the microphone/speaker side

20

RLI2

Receive level detector input on the microphone/speaker side

21

RI

Input receive attenuator and dial tone detector. Maximum input level is 350 mVrms. Input impedance is

approximately 10 k

Ω

22

RECO

23

TLI1

Transmit level detector input on the line side

24

TLO1

Transmit level detector output on the line side

25

RLO1

Receive level detector output on the line side and input to the receive background monitor

26

RLI1

Receive level detector input on the line side

27

CPR

An RC at this pin sets the time constant for the receive background monitor

28

GND

Ground

Table 2-1.

Pin Description (Continued)

Pin

Symbol

Function