MC34118 Unisonic Technologies, MC34118 Datasheet - Page 7

• Manufacturer Part Number: MC34118
• Description: Voice Switched Speaker Phone Circuit
• Manufacturer: Unisonic Technologies

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UTC MC34118 LINEAR INTEGRATED CIRCUIT
FUNCTIONAL DESCRIPTION
INTRODUCTION
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 UTC 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 UTC 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 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 10kΩ, 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 < 10Ω until the output current limit( typically 2.5mA) is reached.
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 by the level detectors, 2 from the background noise monitors, the volume control, the dial-tone detector,
and the AGC circuit. These 7 inputs are described below.
UTC
The attenuators are controlled by the signal output of the Control Block, which is measurable at the CT pin( pin 14).
The fundamental difference between operation of a speakerphone and a handset is that of half-duplex versus
The transmit and receive attenuators are complementary in function, ie. when one is at maximum gain(+6dB),
UNISONIC TECHNOLOGIES CO., LTD.
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