SL34118D System Logic Semiconductor, SL34118D Datasheet - Page 2

• Manufacturer Part Number: SL34118D
• Description: Voice Switched Speakerphone Circuit
• Manufacturer: System Logic Semiconductor

SL34118
INTRODUCTION
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
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 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 “hand-free” mode,
eliminating the need for a “push-to-talk” switch.
the
considerably lower, and since the acoustic coupling
from the earpiece to the mouthpiece is almost non-
The fundamental difference between the operation
The handset, by the way, has the same loop as
speakerphone.
and
receive)
But
since
simultaneousiy.
SIMPLIFIED BLOCK DIAGRAM
FUNCTIONAL DESCRIPTION
the
gains
are
A
existent (the receiver is normally held against a
person’s ear), oscillations don’t occur.
detectors, attenuators, and switching control for a
properly operating speakerphone. The detection
sensitivity and timing are externally controllable.
Additionally, the SL34118 provides background noise
monitors which make the circuit insensitive to room
and line noise, hybrid amplifiers for interfacing to Tip
and Ring, the microphone amplifier, and other
associated functions.
ATTENUATORS
complementary in function, i.e., when one is at
maximum gain (+6.0 dB), the other is at maximum
attenuation (-46 dB), 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
to control the transmit and receive paths to provide
the half-duplex operation required in a speakerphone.
3.0 dB (from max gain) frequency of
input impedance of each attenuator (TXI and RXI) is
nominally 10 k
should be limited to 350 mVrms (990 mVp-p) to
prevent
0.1 dB) at a typical value of -40 dB. Their purpose is
The SL34118 provides the necessary level
The
The attenuators are non-inverting, and have a -
transmit
distortion.
(see Figure 1), and the input signal
and
receive
SLS
That
System Logic
Semiconductor
attenuators
100 KHz. The
maximum
are