MC33219A Datasheet, PDF (17/28 Pages) Motorola, Inc – Voice Switched Speakerphone

Freescale SMeCm33i2c1o9Anductor, Inc.
Figure 25. CT Attenuator Control Circuit
VB
RT
CT
CT
MC33219A
Voltage Clamps
I1
90 µA
I2
50 µA
Level Detectors
There are two identical level detectors: one on the receive
side and one on the transmit side (refer to Figure 26). Each
level detector is a high gain amplifier with back–to–back
diodes in the feedback path, resulting in non–linear gain,
which permits operation over a wide dynamic range of
speech levels. Refer to the graphs of Figures 6, 7 and 8 for
their DC and AC transfer characteristics. The sensitivity of
each level detector is determined by the external resistor and
capacitor at their input (TLI and RLI). The output charges an
external capacitor through a diode and limiting resistor, thus
providing a DC representation of the input AC signal level.
The outputs have a quick rise time (determined by the
capacitor and an internal 500 Ω resistor), and a slow decay
time set by an internal current source and the capacitor. The
capacitors on the two outputs should have the same value
(±10%) to prevent timing problems.
Referring to Figure 2, the outputs of the two level detectors
drive the Tx–Rx comparator. The comparator’s output state
depends on whether the transmit or receive speech signal is
stronger, as sensed by the level detectors. The Attenuator
Control Circuit uses this signal, along with the background
noise monitors, to determine which mode to set.
Figure 26. Level Detector
Signal ^C
Input
R
TLI
(RLI)
VB
External Component Values are
Application Dependent.
500 Ω
2.0 µA
TLO
(RLO)
1.0 µF
To
Attenuators
Control Circuit
AGC
Tx _Background
Rx ^Monitors
TX–Rx Comp.
Vol. Control
Dial Tone Det.
Background Noise Monitors
The purpose of the background noise monitors is to
distinguish speech (which consists of bursts) from
background noise (a relatively constant signal). There are
two background noise monitors: one for the receive path and
one for the transmit path. Refering to Figure 27, each is
operated on by a level detector, which provides a DC
voltage representative of the combined speech and noise
level. However, the peaks, valleys, and bursts, which are
characteristic of speech, will cause the DC voltage (at CP2
or RLO) to increase relatively quickly, causing the output of
the next amplifier to also rise quickly. If that increase
exceeds the 36 mV offset, and at a speed faster than the
time constant at CPT (CPR), the output of the last
comparator will change, indicating the presence of speech
to the attenuator control circuit. This will keep the circuit in
either the transmit or the receive mode, depending on which
side has the stronger signals. When a new continuous signal
is applied, the time constant at CPT (CPR) determines how
long it takes the circuit to decide that the new sound is
continuous, and is therefore background noise. The system
requires that the average speech signal be stronger than the
background noise level (by 6.0–7.0 dB) for proper speech
detection.
When only background noise is present in both paths, the
output of the monitors will indicate the absence of speech,
allowing the circuit to go to the idle mode.
AGC Circuit
In the receive mode only, the AGC circuit decreases the
gain of the receive attenuator when the supply voltage at
VCC falls below 3.5 V, according to the graph of Figure 5.
The gain of the transmit path changes in a complementary
manner.
The purpose of this feature is to reduce the power (and
current) used by the speaker when the speakerphone is
powered by the phone line, and is connected to a long
telephone line, where the available power is limited.
Reducing the speaker power controls the voltage sag at VCC,
reduces clipping and distortion at the speaker output, and
prevents possible erratic operation.
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