Aircraft Communication Topic 6 pa system

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A radio communication system requires the use of two audio transducers:
 a microphone to convert audio signals to electrical impulses for use by the
transmitter,
 a speaker to convert the electrical impulses from the receiver back into audio
signals.
The transmitter portion of an airborne VHF or HF communication transceiver requires a
microphone audio input and a means to activate the transmitter.
The transmitter may be activated or keyed by a push‑to‑talk (PTT) switch located
directly on the microphone housing.
The receiver portion of the VHF or HF transceiver provides:
 a 500‑ohm (typical) low‑level audio output to be used by the headphones,
 sometimes an eight ohm (typical) high‑level audio output for loudspeaker
operation in cases where an external isolation amplifier is not used.
AV2220 - Aircraft Communication Systems Chapter 3 1

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The airborne navigation receivers, such
as the ADF, VOR/ILS, Marker Beacon
and DME, provide audio output signals
to be reproduced by the headphone
and/or loudspeaker.
Integrating the audio signals from
these systems into an audio control
unit allows the pilot to control the
desired audio inputs and outputs to
and from the respective avionic
equipment.
In larger aircraft, one each audio
control unit is usually found positioned
at the pilot and copilot side consoles
and one at the flight engineer's console
if the aircraft is equipped with a third
crewmember station.
Baker Model M1035 Audio Control Unit

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The Baker Model M1035 shown has
provisions for:
 up to eleven low‑level receiver
audio inputs
 four transmitter key and
microphone audio outputs,
 selection for crew interphone or
cabin public address (PA).
Also, an option is available to provide
automatic selection of the
corresponding receiver audio when the
transmitter key and microphone audio
is selected for a particular transceiver. Baker Model M1035 Audio Control Unit

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Inside the M1035:
 All communication, VOR/ILS
navigation and DME inputs from 10
to 300 milliwatts are leveled to less
than a 3‑dB change in output.
 ADF and Marker Beacon inputs are
not leveled so that the output is
proportional to the input.
 The selected receiver inputs are
processed through an internal audio
mixing circuit and isolation amplifier
before being output to the cockpit
speaker and headphones.
 A 1,020‑Hz filter (FILT) may be
selected for coded ADF or VOR
station identification signals.

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The audio control unit is an integral
component in the operation of aircraft
communication and navigation
systems.
 It provides audio selection,
 Isolates and mixes the outputs
from the various receivers to the
speaker or headphones.
 The audio outputs from the
various receivers must be
isolated from each other so
that one receiver's audio
output will dissipate power
only to the speaker and not to
the output stages of the other
receivers.

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In a schematic diagram of a typical
audio control unit:
 Isolation is obtained through the use
of a mixing circuit in the audio
control unit,
 It provides a high resistance
isolation between the various
receiver audio outputs when
several receivers are connected to
the same speaker.
 The audio mixer matches the output
impedance to the receiver's
termination impedance for maximum
power transfer, and sends the mixed
audio signals to an isolation amplifier
to increase the audio gain before
being output to the cockpit speaker.
Schematic diagram of a typical Audio Control Unit

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In the event the isolation amplifier fails,
or there is a power loss to the audio
control unit:
 Selection of the emergency function
allows continued use of transmit and
receive functions.
In the emergency mode:
 All inputs bypass the internal
isolation amplifier and are connected
directly to the headphones on a
priority basis
 Only one receiver's audio may be
listened to at any one time.
A speaker volume control is provided
which adjusts the gain of the isolation
amplifier.

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The navigation and communication
control heads have volume controls to
adjust the individual receiver's audio
output level to the audio control unit.
 Once the individual volume controls
are set to approximately the same
level, the speaker volume control can
be used as a master level control.
The receiver audio output provided to
the speaker should be disabled or
muted when a microphone is keyed:
 to prevent retransmission of received
audio and to prevent possible audio
feedback.

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An audio mixing circuit, similar to that used for receiver audio, is provided for the
audio sidetone outputs from the VHF and HF communication transceivers.
The purpose of the sidetone output is:
 to provide a transmitter audio modulation sample to the aircraft audio system
 to allow the crew members to monitor the aircraft's radio transmissions and
 to permit the speaking crew member to listen to and adjust the volume of his
voice when speaking into the microphone.
Sidetone audio is usually only provided to the headphones;
 if sidetone is desired at the speaker, a sidetone level control must be provided to
prevent feedback from occurring due to reamplification of the audio signal.
Sidetone audio is also available from the cabin public address system so that PA
announcements may be heard in the cockpit.

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Most audio control units also provide microphone audio and keying signals to an
external interphone amplifier for crew interphone communication (intercom) when
using an oxygen mask microphone or boom headset microphone.
Either carbon element or dynamic microphones may be used; however the later
requires the use of a microphone preamplifier.
Oxygen mask and boom mics are keyed from a remote switch located on the control
yoke.
 The yoke switch is sometimes a two‑position switch to allow individual keying of
either the radio transmitters (or PA), or the crew interphone function.
The audio system is wired to prevent a transmitter from keying when the interphone
or PA is in operation.

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Other features of a typical audio control unit include provisions for:
 selecting "Ramp Hailing" from a wheel‑well speaker using an external PA amplifier
 selecting service interphone operation to communicate with ramp personnel via an
external jack and interphone amplifier.
Cabin handsets may also be connected to provide a service interphone with flight
attendants.
 Call lights are usually installed with cabin handsets to alert the selected station.
On most corporate aircraft, UHF Radio Telephones are installed to provide:
 voice communication over the telephone network via a ground operator
 crew‑to‑cabin interphone operation if more than one telephone handset is
installed

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Aircraft often carry a radiotelephone system which is somewhat similar to the portable
cellular phone available for cars.
 It employs radio signals to permit telephone calls to be made from the aircraft in
flight.
The frequencies used are 450‑500 MHz in the UHF band.
The antenna used is a Marconi antenna of a slightly different shape and size compared
to a VHF communication antenna.

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Call System
The call system is used as a means for various crewmembers to gain the attention of
other crewmembers and to indicate that interphone communication is desired.
 Attention is gained through the use of lights and aural signals (chimes or horn).
 The system can be activated from the cockpit, either flight attendant station, or from
the external power receptacle.
Passengers may also use the system to summon an attendant, through the use of
individual call switches at each seat.
The cockpit may be called by either flight attendant station, or by the ground crew.
 The ground crew may only be called by the cockpit.
 Flight attendants may be called by the cockpit, the other attendant station, or by any
passenger seat or lavatory.
 Master call lights in the passenger cabin identify the source of incoming calls to the
attendants.

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Call System (cont’d)
Good communication between the flight crew and the passengers is extremely
important in airline flying. There are four levels of priority assigned to the passenger
address system.
 Announcements by the pilot have first priority
 Announcements by the flight attendants.
 Prerecorded announcements follow as third level
 Boarding music.
A chime is produced when the pilot turns on the "fasten seat belt" or "no smoking"
signs.
Prerecorded emergency announcements may be initiated by the pilot or by a flight
attendant, and these messages are initiated automatically in the event of a cabin
depressurization.

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In Boeing 757, the call
system allows the cockpit
crew, flight attendants and
ground personnel to indicate
that interphone
communication is desired.
The cockpit crew can initiate
calls through the pilots' call
panel and are alerted through
call lights and chimes.
Boeing 757 crew call system

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Block diagram of Boeing 777 ground crew call system

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In the Boeing 777, flight crew
and the ground crew use the
ground call system to alert
each other.
The system supplies aural and
visual signals in the flight deck
and in the nose wheel well
area.
The flight crew can select a
call code which will sound a
horn in the nose wheel well.

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There is a pilot call switch on
the APU service and shutdown
panel. When the ground crew
operates this switch:
 The audio control panels
FLT call lights come on
 A message is shown on
EICAS
 A chime sounds through
the aural warning
speakers

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Public Address System
The public address or passenger
address (PA) system allows cockpit
crewmembers and flight cabin
attendants to make announcements
throughout the cabin to the
passengers.
Cockpit crewmembers can make
announcements through any
microphone and respective audio
selector panel (ASP).
Announcements are heard through
speakers located in the cabin and in
the lavatories.
Passenger address system configuration

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Public Address System (cont’d)
An audio selector panel (ASP) is
installed at the captain, first officer,
and observer stations.
 Each panel controls an independent
crew station audio system and allows
the crewmember to select the
desired radios, navigation aids,
interphones, and PA system for
monitoring and transmission.
Transmitter selectors on each ASP
select one radio or system for
transmission by that crewmember.
Any microphone at that crew station
may then be keyed to transmit on the
selected system.

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Receiver switches select the systems to
be monitored.
 A combination of systems may be
selected.
Receiver switches also control the
volume for the headset and speaker at
the respective crew stations.
The cockpit crewmembers can make
announcements using a PA hand
microphone or by using any standard
microphone and the respective audio
selector panel.

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Flight attendants make announcements using PA hand microphones located at their
stations.
The attendants can also use the PA system to play recorded music for passenger
entertainment.
The passenger entertainment system (PES) consists of a multi-track tape player
providing boarding music.
PA announcements from any station override all tape player outputs.

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Boeing 757 passenger address system

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Public Address System Operation
The cabin PA system receives microphone audio and keying from the audio control unit
when this function is selected.
 It outputs a high-level audio to the cabin speakers and provides a sidetone output
back to the audio control unit.
An audio input is usually provided for a cabin entertainment system, such as boarding
music from a tape player or audio from an onboard television.
Most cabin PA units also have a built-in tone generator which provides an alert signal
to the passengers when the no-smoking or fasten-seat-belts switches are activated in
the cockpit.
The inputs to the PA amplifier are usually arranged on a priority basis with the pilots'
control given first priority, the stewards' control given second priority, and cabin
entertainment system as the last priority.

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Public Address System Operation (cont’d)
The input relays in the PA amplifier are interconnected so that the pilot has full control
in overriding inputs provided to the PA from the steward stations and cabin
entertainment system.
 The steward's push-to-talk switch has second priority control to disable only the cabin
entertainment input to the PA amplifier.
When any one of the input control wires are grounded by selecting PA (or Cabin) on
an audio control unit, or by selecting a cabin entertainment switch:
 A corresponding relay is activated in the PA unit which applies operating power and the
input signal to the amplifier.
 The amplified audio output is then applied to the cabin loudspeaker system.
When the aircraft is on the ground, the flight/ground switch is closed.
 Resulting in the addition of a gain control signal to the amplifier for reducing the PA
audio output to the cabin speakers.

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When the aircraft is airborne:
 The gain control signal is removed to restore the cabin PA audio output to a sufficient
level to compensate for the higher ambient noise level.
All audio leads between the audio control units, interphone amplifiers, PA amplifier,
communication transceivers, navigation receivers, and other audio equipment, must be
shielded to prevent the occurrence of audio interference due to inductive cross-coupling
with other wiring.
 The audio wires be 22-gauge twisted-pair and shielded, rather than coaxial cable, so
that the return current is not dependent on the conductivity of the shield.
The copper-braided shield covering is intended only to reduce inductive coupling of
unwanted signals, such as 400 Hz AC from the aircraft electrical system inverters.
 Shields should be grounded at one point to prevent loop currents, and this common
ground point is usually provided at the audio junction box or terminal strip.

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In the Boeing 777, PA function sends announcements to the passenger cabin which
uses these components:
 Passenger address/cabin interphone (PA/CI) controller
 Ambient noise sensors (ANS)
 Speaker drive modules (SDM)
 Zone management units (ZMU)
 Cabin system control panel (CSCP)
 Cabin system management unit (CSMU)
Announcements come from the flight crew, the cabin attendants, or a prerecorded
announcement machine (PRAM). The PA function also supplies:
 Boarding music
 Video entertainment audio
 Chimes

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Cabin interphone and passenger address functions

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The airline can configure the passenger
cabin into as many as six PA areas for
announcements.
The PA/CI controller receives all audio
inputs and selects the input with the
highest priority.
 It digitizes the audio and sends it to
the SDMs.
 The SDMs convert the digital audio
back to analog. Each SDM can drive
one or two speakers.
These are the PA audio priorities:
 Flight deck announcements
 Direct access announcements when
the attendant selects a direct access
switch on the cabin attendant
handsets (CAH) panel
 Handsets announcements when the
attendant dials a PA dial code from a
CAH
 Prerecorded announcements
 Video entertainment audio
 Boarding music

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The PA/CI controller pauses prerecorded announcements, entertainment audio, and
video entertainment when a higher priority announcement is in progress.
Chimes are superimposed over existing audio so both are heard at the same time. The
PA function generates chimes:
 For passenger to attendant calls fro, either the passenger seat or lavatory
 For cabin interphone calls
 When passenger information signs go on or off
There are three ways to control PA volume:
 By the configuration database
 Automatically
 Manually
The configuration database defines the normal reference level for each speaker in
flight.

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Automatic control adjusts the normal reference level due to flight conditions or
ambient noise levels.
These are the flight conditions:
 Engine start
 Airborne
 High airspeed
 Decompression
The attendants can also make manual adjustments from the CSCP or a CACP.
The PA/CI controller has two identical circuits for the PA function and two identical
circuits for the CI function.
 Each has a primary and alternate circuit.
 The attendant switch panel if a primary circuit fails.

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Interphone systems is not radio systems,
 They use audio signals to permit communication between various points in and
around the aircraft.
The two systems operate in a similar manner; the difference is:
 who uses the systems
 where the phone jacks are located
The intercom system is used for voice communications from one point to another
within the aircraft.
Large aircraft have intercom systems so that the cockpit crew can communicate
with the cabin crew and vice versa.
On small airplanes, the intercom is used to communicate within the cockpit area
and is needed because of noise in the cockpit area.

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The interphone system permits conversation between the cockpit and someone
outside the aircraft, usually maintenance or service personnel.
The operation of intercom and interphone systems is the same.
 Phone jacks are available at different locations where a handset or headset can be
connected.
 The handset or headset contains a microphone, a small speaker and a
push‑to‑talk switch (PTT).
 The phone jacks and wiring are connected to an audio amplifier so that the
volume can be controlled.
 Switches are available to select the desired system and a ringing system like that
of a telephone is used for alerting the other party.
On larger aircraft, a passenger address (PA) system is included so that
announcements can be made to the passengers by the flight crew or cabin crew.

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There are two interphone systems in the aircraft:
 The flight interphone system
 The cabin/service interphone system
The flight interphone system permits the flight crew members on the flight
deck to communicate with each other and with audio communications systems
and ground crew members.
The service interphone permits communication between the pilots, ground
crew, and maintenance personnel.
Jacks for plug-in microphone and headsets are at various locations on the
airplane.
When the service interphone switch is ON, the service and flight interphone
systems are connected together.

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The intercom and interphone system for a corporate jet

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In figure above, external interphone jacks are located in:
 The nose-wheel area
 The avionics equipment bay area
 The aft fuselage near the auxiliary power unit (APU)
These external jacks permit communication between the cockpit and
maintenance personnel at these locations outside the aircraft.

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Flight Interphone System
The flight interphone system provides the flight crew members on the flight deck
to communicate with each other and with:
 Audio communication systems
 Ground crew members
The flight interphone system (FIS) is an independent communications network.
The primary purpose is to provide private communication between cockpit
crewmembers without intrusion from the service interphone system.
The ground crew may also use flight interphone through a jack at the external
power receptacle.
Ground personnel are able to communicate on the FIS through a jack located on
the APU ground control panel.

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Flight interphone system configuration

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Boeing 757 flight interphone system

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Flight Interphone System (cont’d)
In Boeing 777, switches on the audio control panels (ACPs) permit selection of
the following types of audio:
 Communication transceiver audio
 Navigation receiver audio
 Cabin interphone audio
 Passenger address (PA) audio
 Flight interphone audio
 SATCOM audio
Hand microphones, boom microphones, or oxygen mask microphones can be
connected through the audio management unit (AMU) to the radio transceivers,
cabin interphone system, or PA system.
 Functions selected on the ACP go digitally to the AMU.

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Boeing 777 flight and cabin/service interphone systems

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Flight Interphone System (cont’d)
The AMU uses new technology digital signal processing for clear sound quality.
The AMU sends the selected audio to and from the flight deck.
Each flight crew member’s station has a jack outlet for a boom
microphone/headset and headphones.
There can be an optional fourth ACP for the second observer.
Interphone/radio push-to-talk (PTT) switches are on each pilot’s glareshield and
control wheel for the boom and oxygen mask microphones.

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Cabin/Service Interphone System
The service interphone system provides intercommunication between the cockpit,
flight attendants, and ground personnel.
Cockpit crewmembers communicate using either a separate handset (if installed)
or their respective audio selector panel and any standard microphone.
The Boeing 757 cabin interphone system (CIS) permits intercommunication
between the cockpit and flight attendant stations.
Cockpit crew members communicate on the CIS through their audio select panel.
The flight attendants communicate between flight attendant stations or with the
cockpit using any of the handsets in the cabin.
The system is a party line similar to the Boeing 737 system.

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Service interphone system configuration

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Cabin/Service Interphone System (cont’d)
The service interphone system consists of additional internal and external jacks
connected to the cabin interphone system for use by maintenance personnel.
The flight attendants communicate between flight attendant stations or the
cockpit using any of the attendant handsets.
The system is a party line, in that anyone who picks up a handset/microphone is
automatically connected to the system.
External jacks for use by maintenance or service personnel can be added to the
system by use of the service interphone switch.

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Cabin Management System
In Boeing 777, the cabin management system (CMS) is an integrated system that
combines many cabin and passenger functions.
 It controls the cabin interphone, passenger address, passenger entertainment,
passenger service, and cabin lighting functions.
 It also provides for monitor and control of many cabin functions.
The passenger entertainment system (PES) is complex in that it allows 10 tape-deck
channels, four movie audio channels, and the PA channel to be fed to each
of the individual seats.
 This is done by a time-multiplexing system.
The passenger can select the channel that is heard over the stethoscope-type
headset.

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Cabin management system block diagram

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Cabin Management System (cont’d)
The cabin management system
controls these function:
 Cabin interphone
 Passenger address
 Passenger entertainment
 Passenger services
 Cabin lighting
 Monitor and control of many cabin
functions
The integration of these functions
permits control, monitoring and test of
the system from a central location.
Software controls the CMS which uses
a configuration database to define the
cabin interior configuration.
Interior configuration changes are easy
to do by modifying the configuration
database.
The configuration database also makes
possible for airlines to customize
entertainment systems to their needs.

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The configuration database generator
(CDG) is a menu-driven database
editor that runs on a personal
computer (PC).
The CDG changes the database, then
after that, the operator loads the
database into the cabin management
system through the cabin system
control panel (CSCP).
The CSCP stores many databases and
operational software on a mass storage
device.
The CSCP is used by flight attendants
for CMS functions and by maintenance
persons for test and program functions.
The passenger address/cabin
interphone (PA/CI) controller controls
the passenger address (PA) and cabin
interphone (CI) functions.
The entertainment multiplexer
controller (EMC) receives passenger
entertainment signals and sends them
to the passenger cabin.

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There are three zone management
units (ZMU). Each ZMU controls an
area of the cabin:
 ZMU-1 controls zone 1
Each ZMU connects to the overhead
electronic units (OEU) and the seat
electronic units (SEU) in its control
zone.
Each ZMU also connects to one cabin
area control panel (CACP) and up to
five cabin attendant handsets (CAH).
The functions of the ZMU:
 Analog to digital and digital to analog
audio conversion for the cabin
interphone function
 Receive and send RF signals for the
passenger entertainment function
 Control the passenger service
selections from the SEUs and cabin
light selections from the CACP
The ZMUs use the configuration database
to determine the proper state of each
light.
 They then interface with the OEUs to
control the lights.

Aircraft Communication Topic 6 pa system

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