MC33219A Datasheet, PDF (16/28 Pages) Motorola, Inc – Voice Switched Speakerphone

Freescale SMeCm33i2c1o9Anductor, Inc.
FUNCTIONAL DESCRIPTION
Introduction
The fundamental difference between the operation of a
speakerphone and a telephone handset is that of
half–duplex versus full–duplex. The handset is full duplex,
meaning conversation can occur in both directions (transmit
and receive) simultaneously. This is possible due to both
the low sound level at the receiver, and the fact that the
acoustic coupling from the earpiece to the mouthpiece is
almost non–existent (the receiver is normally held against a
person’s ear). The loop gain from the receiver to the
microphone and through the circuit is well below that
needed to sustain oscillations.
A speakerphone, on the other hand, has higher gain levels
in both the transmit and receive paths, and attempting to
converse full duplex results in oscillatory problems due to the
loop that exists within the speakerphone circuit. The loop is
formed by the hybrid, the acoustic coupling (speaker to
microphone), and the transmit and receive paths (between
the hybrid and the speaker/microphone). The only practical
and economical method used to date 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 (in
reality, who is talking louder), 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 “hands–free”
mode, eliminating the need for a “push–to–talk” switch.
The MC33219A provides the necessary circuitry to
perform a voice switched, half duplex, speakerphone
function. The IC includes transmit and receive attenuators,
pre–amplifiers, level detectors and background noise
monitors for each path. An attenuator control circuit
automatically adjusts the gain of the transmit and receive
attenuators based on the relative strengths of the voice
signals present, the volume control, and the supply voltage
(when low). The detection sensitivity and timing are
externally controllable. Please refer to the Block Diagram
(Figure 2) when reading the following sections.
Transmit and Receive Attenuators
The transmit and receive attenuators are complementary,
performing a log–antilog function. When one is at maximum
gain (≈ 6.7 dB), the other is at maximum attenuation
(≈ –46 dB); they are never both fully on or fully off. Both
attenuators are controlled by a single output from the
Attenuator Control Circuit which ensures the sum of their
gains will remain constant at a typical value of – 40 dB.
Their purpose is to provide the half–duplex operation
required in a speakerphone.
The attenuators are non–inverting, and have a usable
bandwidth of 50 kHz. The input impedance of each
attenuator (TXI and RXI) is nominally 100 kΩ (see Figure 24),
and the input signal should be limited to 300 mVrms (850 mV
p–p) to prevent distortion. That maximum recommended
input signal is independent of the volume control setting. Both
the input and output are biased at ≈ VB. The output
impedance is <10 Ω until the output current limit (see specs)
is reached.
Figure 24. Attenuator Input Stage
VB
TAI
(RAI)
10 k
90 k
VB
The attenuators are controlled by the single output of the
Attenuator Control Circuit, which is measurable at CT (Pin 7).
When the circuit detects speech signals directing it to the
receive mode (by means of the level detectors described
below), an internal current source of 90 µA will charge the CT
capacitor to a voltage positive with respect to VB (see
Figure 25). At the maximum volume control setting, this
voltage will be approximately 150 mV, and the receive
attenuator will have a gain of 6.7 dB. When the circuit detects
speech signals directing it to the transmit mode, an internal
current source of 50 µA will take the capacitor to
approximately – 100 mV with respect to VB (the transmit
attenuator will have a gain of 6.7 dB). When there is no
speech present in either path, the current sources are shut
off, and the voltage at CT will decay to be equal to VB. This is
the idle mode, and the attenuators’ gains are nearly halfway
between their fully ON and fully OFF positions (– 25 dB for the
Rx attenuator, –16 dB for the Tx attenuator). Monitoring the
CT voltage (with respect to VB) is the most direct method of
monitoring the circuit’s mode, and its response.
The inputs to the Attenuator Control Section are six: The
Tx–Rx comparator operated by the level detectors, two
background noise monitors, the volume control, the dialtone
detector, and the AGC circuit. These six functions are
described as follows.
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