SL34118

System Logic

Semiconductor

SLS

SIMPLIFIED BLOCK DIAGRAM

FUNCTIONAL DESCRIPTION

INTRODUCTION

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)

simultaneousiy.

A

speakerphone has higher gain levels in both paths,

and attempting to converse full duplex 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 data is to design the

speakerphone to function in a half duplex mode - i.e.,

only one person speaks at a time, while the other

listens. To achieve this requires a circuit 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 maintained

less than unity. When the talkers exchange function,

the circuit must quickly detect this, and switch the

circuit

appropriately.By

providing

speech

level

detectors, the circuit operates in a “hand-free” mode,

eliminating the need for a “push-to-talk” switch.

The handset, by the way, has the same loop as

the

speakerphone.

But

since

the

gains

are

considerably lower, and since the acoustic coupling

from the earpiece to the mouthpiece is almost non-

existent (the receiver is normally held against a

person’s ear), oscillations don’t occur.

The

SL34118

provides

the

necessary

level

detectors, attenuators, and switching control for a

properly operating speakerphone. The detection

sensitivity and timing are externally controllable.

Additionally, the SL34118 provides background noise

monitors which make the circuit insensitive to room

and line noise, hybrid amplifiers for interfacing to Tip

and Ring, the microphone amplifier, and other

associated functions.

ATTENUATORS

The

transmit

and

receive

attenuators

are

complementary in function, i.e., when one is at

maximum gain (+6.0 dB), the other is at maximum

attenuation (-46 dB), and vice versa. They are never

both fully on or both fully off. The sum of their gains

remains constant (within a nominal error band of

±0.1 dB) at a typical value of -40 dB. Their purpose is

to contro l the transmit and receive paths to provide

the half-duplex operation required in a speakerphone.

The attenuators are non-inverting, and have a -

3.0 dB (from max gain) frequency of

≈100 KHz. The

input impedance of each attenuator (TXI and RXI) is

nominally 10 k

Ω (see Figure 1), and the input signal

should be limited to 350 mVrms (990 mVp-p) to

prevent

distortion.

That

maximum