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MC34118

VOICE SWITCHED SPEAKER PHONE CIRCUIT

Contek Microelectronics 

MC34118

LINEAR INTEGRATED CIRCUIT

PARAMETER

Transmit-Receive Switching Threshold

Sink Current at RLO1, RLO2, TLO1, TLO2

Closed Loop Gain( HTO- to HTO+)

TYP VALUE

(Ta=25 C)

1

4mA

0dB

TYP CHANGE

+-0.02

-10

0.001

UNIT

%/ C

nA/ C

%/ C

FUNCTIONAL DESCRIPTION

INTRODUCTION

The fundamental difference between 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 date is to design the speakerphone to function in a half duplex mode- ie. 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 hands-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 ear piece to the mouthpiece is almost non-existent ( the receiver is normally

held against a person s ear), oscillations do not occur. The Contek MC34118 provides the necessary level detectors,

attenuators, and switching control for a properly operating speakerphone. The detection sensitivity and timing are

externally controllable. Additionally, the Contek MC34118 provides background noise monitors which make the circuit

insensitive to room and line noise, hybrid amplifier for interfacing to Tip and Ring, the microphone amplifier, and other

associated functions. Please refer to the Block Diagram when reading the following sections.

ATTENUATORS

The transmit and receive attenuators are complementary in function, ie. when one is at maximum gain(+6dB), the

other is at maximum attenuation(-46dB), 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 +-0.1dB) at a typical value of -40dB( see Figure 10). Their

purpose is to control the transmit and receive paths to provide the half-duplex operation required in speakerphone.

The attenuators are non-inverting, and have a -3dB(from max gain) frequency of ~100kHz. The input impedance of

each attenuator( TXI and RXI) is nominally 10k W, and the input signal should be limited to 350 mVrms( 990mvp-p) to

prevent distortion. That maximum recommended input signal is independent of control setting. The diode clamp on

the inputs limits the input swing, and therefore the maximum negative output swing. This is the reason for VRXOL and

VTXOL specification being defined as they are in the Electrical Characteristics. The output impedance is < 10 W until

the output current limit ( typically 2.5mA) is reached.

The attenuators are controlled by the signal output of the Control Block, which is measurable at the CT pin ( pin 14).

When the CT pin is at +240mV with respect to VB, the circuit is in the receive mode(receive attenuator is at +6dB).

The circuit is in an idle mode when the CT voltage is equal to VB causing the

attenuators gains to be halfway

between their fully on and fully off positions(-20 dB each). Monitoring the CT voltage(with respect to VB) is the most

direct method of monitoring the circuit s mode. The inputs to the Control Block are 7: 2 from the comparators operated

Contek Microelectronics Co.,Ltd.

7

CONTEK

http://www.contek-ic.com E-mail:sales@contek-ic.com

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