MC33218A Datasheet, PDF (18/28 Pages) Motorola, Inc – Voice Switched Speakerphone with Microprocessor Interface

Freescale SMeC3m32ic18oAnductor, Inc.
Figure 29. CT Attenuator Control Circuit
VB
RT
CT
CT
MC33218A
Voltage Clamps
I1
42 µA
I2
42 µA
Rx
Tx _{Control Circuit}
AGC
To
Attenuators
Tx Background
Rx ^Monitors
Tx–Rx Comp
µP Interface
Dial Tone Det.
Referring to Figure 3, 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 30. Level Detector
Signal ^C
Input
R
TLI
(RLI)
VB
500 Ω
2.0 µA
TLO
(RLO)
1.0 µF
External Component Values are
Application Dependent.
Background Noise Monitors
The purpose of a background noise monitor 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. Referring to Figure 32, each is
operated on by a level detector, which provides a dc voltage
representative of the combined speech and noise level. The
peaks, valleys, and bursts, which are characteristic of
speech, will cause that 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, 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. Whenever 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
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 to
occur.
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 8. 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.
Dial Tone Detector
When a speakerphone is initially taken off–hook, the dial
tone signal will switch the circuit to the receive mode.
However, since the dial tone is a continuous signal, the
MC33218A will consider it as background noise, rather than
speech, and would switch from receive to idle, causing the
dial tone sound to fade. The dial tone detector prevents the
fading by disabling the receive background noise monitor.
The dial tone detector is a comparator with one side
connected to the receive attenuator input (RXO), and the other
input connected to VB with a –20 mV offset (see Figure 31). If
the circuit is in the receive mode, and the incoming signal has
peaks greater than 20 mV (14 mV rms), the comparator’s
output will change, keeping the circuit from switching to the
idle mode. The receive attenuator will then be at a gain
determined solely by the volume control. NOTE: The dial tone
detector is not a frequency discriminating circuit.
Figure 31. Dial Tone Detector
To Rx
Attenuator
RXO
To Attenuator
Control Circuit
20 mV
VB
18
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