IL34118N Просмотр технического описания (PDF) - IK Semicon Co., Ltd

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IL34118N

Voice Switched Speakerphone Circuit

IK Semicon Co., Ltd 

SIMPLIFIED BLOCK DIAGRAM

IL34118

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) simultaneously. 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 IL34118 provides the necessary level detectors,

attenuators, and switching control for a properly

operating speakerphone. The detection sensitivity and

timing are externally controllable. Additionally, the

IL34118 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 control 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 (see Figure 1), and the input signal

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

prevent distortion. That maximum

2013, January, Ver. 02

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