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be selected at a time. A light in each push button comes on when that
button is selected.
Fig. 41 -1 Audio Control Panel
Thirteen circular-shaped knobs are dual function control knobs. To select
audio, push on-push off switches are used. Turning the same knob
adjusts the audio volume level. Any number of knobs may be turned on
at the same time. A light in each knob turns on when that knob is selected.
When a microphone selector push button is selected, the audio for that
system is automatically enabled.
[3] The receive/transmit-intercom (R/T - I/C) switch is a 3 position
switch. In the R/T position, a press-to-talk (PTT) is sent to the
communication system that has been selected by the mic selector
switches. In the I/C position, the boom and mask microphone jacks are
connected directly to the flight interphone system. The R/T- I/C switch is
connected in parallel to the PTT switch on the control wheel. The
BOOM/MASK switch allows selection of the boom microphone jack or
the mask microphone jack as the source of audio sent to the selected
communication system.
[4] The voice range filter (V/B/R) switch controls the filtering of
received navigation system audio. In the V position, only voice audio
will be heard. In the R position, only range audio (Morse identifier ) will
be heard. In the B position, both voice and range audio will be heard. The
ALT/NORM switch is used to select alternate operation of the system.
When ALT is selected on the captain's, or observer's ACP, it is directly
connected to VHF-1. When ALT is selected on the first officer's ACP, it
is connected directly to VHF-2.
[5] The remote electronic unit (REU) manages communication between
the 3 flight compartment stations, the service and flight interphone, and
all related communication, radio navigation, and recorder systems. The
REU determines which selections have been made on the audio control
panel (ACP), and then sends the appropriate signals to and from the
selected system. Inside the REU case are 6 printed circuit cards. Three
identical station cards, which respond to inputs from the three audio
control panel (ACP), are mounted into the motherboard. A fourth card,
the audio accessory unit (AAU) card, contains circuitry dedicated to
audio accessory functions such as the service interphone system. The
fifth card is an interface between the motherboard and the external
connector. The sixth card is the motherboard.
[6] Figure 41-2 shows the control wheel microphone switch.
Fig. 41-2 control wheel microphone switch
The purpose of interphone/microphone switch is to provide press-to-talk
(PTT) input for the boom or oxygen mask interphones. The switch is a 3
position switch on the outboard horn of the captain's and the first
officer's control wheel. In the MIC position, microphone audio is
directed to the selected communication system. In the INT position,
microphone audio is connected directly to the flight interphone system
without changing the audio control panel (ACP) microphone selection.
The switch is spring loaded to the OFF position from the MIC position
and is latched in the INT position.
[7] Fig 41-3 shows the block diagram of the audio control system. Dual
power sources supply 28v dc to the REU. The REU provides 15 v dc to
each audio control panel (ACP). The ACP sends switch selections to the
REU in the form of a data word. The data word is used in the REU to
send microphone audio to the selected systems and to send selected audio
back to the flight compartment speakers and headsets. PTT signals are
sent through the REU to the selected communication system.
Fig 41-3 audio control system block diagram
[8] Warning audio from the GPWS is sent through the REU to the flight
compartment speakers and headsets. A discrete from the DFCS activates
the altitude alert tone generator (located inside the REU), which
transmits an audio tone over the flight compartment speakers. All audio
from the DFCS and GPWS will be heard in the flight compartment
regardless of switch selection on the ACP.
[9] The REU sends all microphone audio, selected listen audio, and
warning audio to the cockpit voice recorder. An alternate mode of
operation provides emergency communication capability during power
loss. The alternate mode connects the captain and observer to VHF-1 and
the first officer to VHF-2. Any position may enter the alternate mode of
operation without affecting the other position.
Questions to Lesson 41:
1. What does the digital audio control system provide ?
2. What does the REU manage ?
3. What does the ACP contain ?
4. Where does the ACP allow received audio navigation signal to be
　
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