ts34118 Taiwan Semiconductor Company, Ltd. (TSC), ts34118 Datasheet - Page 7

• Manufacturer Part Number: ts34118
• Description: Voice Switched Speakerphone Circuit
• Manufacturer: Taiwan Semiconductor Company, Ltd. (TSC)

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 10K
(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 V
and VT
specification being defined as they are in the Electrical Characteristics. The output
RXOL
XOL
impedance is 10 until the output current limit typically 2.5mA) is reached.
Figure 3. Attenuator Input Stage
TS34118
7-20
2004/09 rev. B