U4084B Datasheet, PDF (9/26 Pages) TEMIC Semiconductors – Low-Voltage, Voice-Switched Circuit for Hands-Free Operation

U4084B
Introduction
General
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) simul-
taneously. 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).
Today, the only practical and economical solution is to de-
sign the speakerphone in half-duplex mode, i.e., only one
person speaks at a time, while the other listens. To achieve
this, a circuit able to detect who is talking, to switch-on
the appropriate path (transmit or receive) and to switch-
off (attenuate) the other path is necessary. In this way, the
loop gain is maintained less than unity. The circuit has to
detect quickly a change from one speaker to the other and
to switch the circuit accordingly. Due to its speech-level
detectors, the circuit operates in a “hands-free” mode,
eliminating the need for a “push-to-talk” switch.
The handset has the same loop as the speakerphone.
Oscillations do not occur because the gains are consider-
ably lower, and there is almost no coupling from the
earpiece to the mouthpiece (the receiver is normally held
at a person’s ear).
The U4084B provides the level detectors, attenuators,
and switching control necessary for proper operation of
the speakerphone. The detection sensitivity and timing
are externally controllable. Additionally, the U4084B
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.
Transmit and Receive Attenuators
TI, TO and RI, RECO
The attenuators are operating complementary, i.e., when
one is at maximum gain (+6.0 dB), the other is at maxi-
mum attenuation (–46 dB), and vice versa, i.e., both are
" ^{never completely on or off. The sum of their gains re-}
mains constant (within a nominal error band of 0.5 dB)
at a typical value of –40 dB (see figure 8). The attenuators
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 approximately 100 kHz.
W ^{The input impedance of each attenuator (TI and RI) is}
nominally 10 k (see figure 3). To prevent distortion the
input signal should be limited to 350 mV_rms. The maxi-
mum recommended input signal is independent from the
volume control setting. The diode clamp on the inputs
W ^{limits the input swing, and therefore the maximum nega-}
tive output swing. The output impedance is less than 10
until the output current limit (typically 2.5 mA) is
reached.
12627
11 kΩ
V_B
RI 18
5 kΩ
95 kΩ _CT
TI 6
11
Figure 3. Attenuator input stage
The attenuators are controlled by the signal output of the
control block which is measurable at the C_T pin (Pin 11).
When the C_T pin is at +240 mV w.r.t. V_B, the circuit is in
receive mode (the receive attenuator is at 6.0 dB). When
the C_T pin is at –240 mV w.r.t. V_B, the circuit is in transmit
mode (the transmit attenuator is at 6.0 dB). The circuit is
in idle mode when the C_T voltage is equal to V_B causing
the attenuators’ gain to be half-way between their ful-
ly-on and fully-off position (–20 dB each). Monitoring
the C_T voltage (w.r.t. V_B) is the most direct method of
monitoring the circuit’s mode.
The attenuator control has seven inputs: two from the
comparators operated by the level detectors, two from the
background noise monitors, volume control, dial-tone
detector, and AGC. They are described as follows:
TELEFUNKEN Semiconductors
9 (26)
Rev. A1, 31-Jan-97
Preliminary Information