IL34118
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) simultaneousiy. 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 IL34118 provides the necessary level
detectors, attenuators, and switching control for a
properly operating speakerphone. The detection
sensitivity and timing are externally controllable.
Additionally, the IL34118 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
2
INTEGRAL [ INTEGRAL CORP. ]
