KK34118
SIMPLIFIED BLOCK DIAGRAM
FUNCTIONAL DESCRIPTION
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
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 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 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 earpiece
to the mouthpiece is almost non-existent (the receiver is
normally held against a person’s ear), oscillations don’t
occur.
The
KK34118
provides the necessary level detectors,
attenuators, and switching control for a properly
operating speakerphone. The detection sensitivity and
timing are externally controllable. Additionally, the
KK34118
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
The transmit and receive attenuators are
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
±0.1
dB) at a
typical value of -40 dB. 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 -3.0 dB
(from max gain) frequency of
≈100
KHz. The input
impedance of each attenuator (TXI and RXI) is
nominally 10 kΩ (see Figure 1), and the input signal
should be limited to 350 mVrms (990 mVp-p) to
prevent distortion. That maximum
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KODENSHI [ KODENSHI KOREA CORP. ]
