Aircraft Communication Topic 6 pa system

AV2220 - Aircraft Communication Systems Chapter 3 1

AVIONICS AVIONICS TECHNOLOGYTECHNOLOGYIntroductionIntroduction

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.


AVIONICS AVIONICS TECHNOLOGYTECHNOLOGY

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



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.




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.




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.




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



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.




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.




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.



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.



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



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.


AVIONICS AVIONICS TECHNOLOGYTECHNOLOGYPassenger Address SystemPassenger Address 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.

Call System



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.

Call System (cont’d)



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




Block diagram of Boeing 777 ground crew call system



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.





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





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.

Public Address System

Passenger address system configuration



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.

Public Address System (cont’d)




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.





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.





Boeing 757 passenger address system



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.

Public Address System Operation



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.

Public Address System Operation (cont’d)



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.




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




Cabin interphone and passenger address functions



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




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.




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.



AVIONICS AVIONICS TECHNOLOGYTECHNOLOGYInterphone SystemInterphone System

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.



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.



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.



The intercom and interphone system for a corporate jet



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.



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.

Flight Interphone System



Flight interphone system configuration



Boeing 757 flight interphone system



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.

Flight Interphone System (cont’d)



Boeing 777 flight and cabin/service interphone systems



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.

Flight Interphone System (cont’d)



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.

Cabin/Service Interphone System



Service interphone system configuration



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.

Cabin/Service Interphone System (cont’d)



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.

Cabin Management System



Cabin management system block diagram



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.

Cabin Management System (cont’d)



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.




There are three zone management units (ZMU). Each ZMU controls an area of the cabin: ZMU-1 controls zone 1 ZMU-2 controls zone 2 ZMU-3 controls zone 3

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.


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Aircraft Communication Topic 6 pa system