TS34118

Voice Switched Speakerphone Circuit

7/23

Version: A08

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 con-version 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 required 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 compiling from the earpiece to the mouthpiece is almost non existent (the receiver is normally held

against a person’s eat.) oscillations don’t occur.

The TS341118 provides the necessary level detectors, attenuators, and switching control for a properly operating

speakerphone. The detection sensitivity and timing are externally controllable. Additionally, the TS34118 provides

background nodule monitor’s which make the circuit insensitive to room and line noise, hybrid amplifier, and other

associated functions, please refer to the Block Diagram (Figure 2) when reading the following sections.

Attenuators

The transmit and receive attenuators are complementary in function. i .e. when one is at maximum gain (+6.0dB), the

other is at maximum attenuation (-4.5dB), and vice verse. They sum of their gains remains constant (within a nominal

error hand 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 a speakerphone.

The attenuators are non-inverting, and have a –3.0dB (from max gain) frequency of 100KHz. The input impedance of

each attenuator (TXI and RXI) is nominally 10Kς (see Figure 3), and the input signal should be limited to 350mVrms

(990mVp-p) to prevent distortion. That maximum recommended input signal is independent of the volume control

setting. The diode clamp on the inputs 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 10ς until the output current limit typically 2.5mA) is reached.

Figure 3. Attenuator Input Stage