Technical description
HF SSB Transceiver 9323/9360/9390/9780 Technical Service Manual 4-9
Voice mute
1 04-02976 Sheets 1, 2 and 3
The audio at the output of IC204/A on the Rx/Exciter PCB [1 04-02972
Sheet 2] is connected via P201 pin 9 (Rx Audio) to the Microprocessor and Audio
PCB pin 9 (see Sheet 2). It is then coupled via C206 to the input of IC307A (see
Sheet 3), which operates as a squaring amplifier. At the open collector output, the
squared signal across load resistor R352 charges C343 via D308 during the
negative excursions. The charge is transferred to C344 by V303 during the
positive excursions. The resultant DC voltage on C344 is proportional to the
frequency of the audio (i.e. voltage rises as frequency increases). The squaring
amplifier is disabled in transmit by applying a DC offset from IC207 pin 14
(Sheet 2) via D307 and R365 to IC307/B pin 5.
IC304/B and its associated components function as an LPF with a cut-off
frequency of approximately 10 Hz. The output from IC304/B is a DC voltage
varying at the syllabic rate of the received speech.
IC307/C and IC307/D form a window comparator. The window width is adjusted
by the mute sensitivity preset R357. The divider network R359 and R360 together
with C348 averages the output of IC304/B to provide the reference voltage for the
window comparator. If the output from IC304/B rises or falls below this reference
by the amount set by R357, then the open collector outputs of IC307/C and
IC307/D will discharge C349, applying a low to the input of comparator IC307/B
pin 6.
The second input of comparator IC307/B pin 7 is set to 4 V by resistor divider
R363 and R364. When input pin 6 falls below 4 V, the comparator output at pin 1
goes high (MUTE DET line). This indicates to the microprocessor IC101, via
IC111 pin 5 (see Sheet 1), that speech has been detected.
Mute detection timing is controlled by discharging C349 through R361 for a fast
attack and charging C349 via R362 for a slow release (about 3 seconds).
When Voice Mute mode is selected by the Voice Mute button on the control
panel [1 04-02974], the microprocessor IC2 on the control head sends a signal
via the external I
2
C bus to the microprocessor IC101 on the Microprocessor and
Audio PCB [1 04-02976 Sheet 1]. The microprocessor IC101/A sends a
command via the local I
2
C bus to IC114 to latch the mute switch pin 19 low. This
applies a low to IC206/D pin 12 (see Sheet 3), which opens the mute gate and only
allows a very low level of audio to pass via R330 (when fitted) to the volume
control circuit.
When voice is detected by the mute circuit, a high is applied to IC111 pin 5
(MUTE DET line). The interrupt pin (pin 13) momentarily goes low, informing
the microprocessor to read the input ports of IC111. This will indicate to the
microprocessor that the mute detect line is high. The microprocessor then sets the
mute switch IC114 pin 19 high and closes the mute gate IC206/D (see Sheet 3).
The audio is then passed to the volume control circuit.
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