X-2 Technical Service Manual Technical Description 4-7
Publication No: 15-02047
4.2.8 Mute
Squaring Amplifier
The audio at the output of IC303/A
[D7]
(pin 1) is connected to the input of
IC308/A
[E7]
which operates a squaring amplifier. The squared output from
pin 2 charges C333 via D307
[D8]
during the negative excursions and the
charge is transferred to C334
[F8]
by V304 during the positive excursions. The
resultant DC voltage on C334 is proportional to the frequency of the audio.
Low Pass Filter
IC309/B
[E9]
and its associated components function as a low-pass filter with a
cut-off frequency of approximately 10 Hz. The output from IC309/B is a
DC voltage varying at the syllabic rate of the received speech.
Window Comparator
IC308/B
[E10]
and IC308/C form a window comparator where the window
width is adjusted by the mute sensitivity preset R358
[E9]
. The divider
network R360 and R361, together with C338, averages the output of
IC309/B to provide the reference voltage for the window comparator. If the
output from IC309/B rises above or falls below this reference by the amount
set by R358, then the ORed outputs of IC308/B and IC308/C will discharge
C339 applying a low to the input of comparator IC308/D on pin 10.
The second input at pin 11 of comparator IC308/D is set to 4 V by resistor
dividers R365 and R366. When input pin 10 falls below 4 V, the comparator
output at pin 13 goes high and indicates to the microcontroller IC403, via
input pin 14, that speech has been detected.
Mute detection timing is controlled by discharging C339 through R362 for a
fast attack, and charging of the capacitor via R363 for a slow release (about
3 seconds).
Control
104-02907 Sheet 2 and 3
When mute mode is selected by the mute switch on the front panel, a low is
applied to input pin 29 on the microcontroller (IC403). The microcontroller
sends via the I
2
C bus to IC405, instructions to latch output pin 21 (mute-
out) low. This applies a low to the IC305/A pin 13, opening the mute gate
and breaking the audio path to the volume control.
When speech is detected by the mute circuit, a high is applied from the
comparator IC308/D pin 13 to the microcontroller (IC403) pin 14. The
microcontroller, via the I
2
C bus, sets pin 21 of IC405 high. This closes the
mute gate IC305/A and passes the received audio on to the volume control.
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