Panasonic EURO 4 Chassis, Technical Manual

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3DQDVRQLF
8
Chapter 1 Common Circuits
3.1.1. Microphone ON/OFF Control
This control is used for models which do not have a
seperate headphone output path fed from the MSP
IC2101, this results in the headphone terminal being
in series with the loudspeakers. This configuration
means that when headphones are connected the
loudspeakers are disconnected.
This control line is used to mute the microphone input
signal to IC2201, IC2221 when the headphones are
used, which as just mentioned disconnects the
loudspeakers. If the microphone input signal was not
muted when the headphones were connected, the
microphones would feedback external sounds from
the
surrounding area, which would then be
reproduced in the headphones, mixing with the user
selected audio signal.
The microphone ON/OFF control is fed from the
headphone terminal (located on the M-Board) and fed
to the Z-Board via connectors M7 and Z3 pin 1.
Once on the Z-Board the control line is fed to the base
of two muting transistors Q2201 and Q2202.
Transistor Q2201 being used to mute the right
microphone audio signal and Q2202 muting the left.
Where the headphone terminal is not in use the
control line is LOW biasing the muting transistors
Q2201, Q2202 OFF, allowing the microphone signal
to be input to IC2201 and IC2221, where the
processing discussed in the previous section takes
place.
When headphones are connected the control line fed
to the muting transistors Q2201, Q2202 goes HIGH.
This high level fed via resistor R2246, results in the
muting transistors being biased ON, pulling the
microphone input signals to IC2201, IC2221 to
ground, preventing the microphone audio signal being
passed to the headphones.