TS34118CS28

Taiwan Semiconductor

7

Version: E1608

APPLICATION INFORMATION

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 nonexistent (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 versa. 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 10kHz. The input impedance of

each attenuator (TXI and RXI) is nominally 10kΩ (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

The output impedance is 10Ω until the output current limit typically 2.5mA) is reached.

Figure 3. Attenuator Input Stage

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

gain to be halfway between their fully on and fully off positions (-20dB 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 seven, 2 from the comparators operated by the level detectors, 2 from the

background noise monitors, the volume control, the dial-tone detector, and the AGC circuit. These seven inputs are

described below.