3.1.4. Audio.
AUDIO INTEGRATION
In most large passenger aircraft, with two or more flight crew members and many different radio systems an Audio Integration system is fitted which fulfils the following requirements:
Radio system selection
Flight interphone
Service interphone
Cabin interphone
Radio System Selection
Facilities must be provided for the various crew members to select any particular system. The choice of system is dictated by the immediate operational requirements. Any one time, however, these requirements could require more than one facility to be monitored. The audio integration system feeds the selected receiver outputs to the telephone and also feeds the microphone inputs to the selected transmitters.
Flight Interphone
To enable the various crew members to communicate with each other while being able to monitor different radio systems, intercommunication facilities are provided.
Service Interphone
It is often necessary that communication should exist between flight deck and various points on the aircraft, for maintenance purposes, engine starting and towing etc. Facilities are therefore provided for ground crew positions to connect to the flight interphone circuit.
Cabin Interphone
Facilities are provided by the same audio equipment for different cabin crew positions to have an interphone link. A switchable link is available for the cabin crew circuit to be connected to the flight interphone circuit.
Audio Selector Panel
The audio selector panels enable the flight crew to transmit or receive on the radio communication systems; communicate using the flight and service interphone systems or make announcements over the PA system. The aircraft VHF navigation, DME, marker beacon and ADF receivers may also be monitored.
One audio selector panel is fitted at each crew position. A typical audio selector panel is shown in figure 3.2.
The CAA has a requirement for "Hot Mic". Hot Mic is a term, which has been adopted for a microphone which is permanently live - irrespective of selector panel or column switch selections. It takes the form of an additional amplifier which is mounted inside the selector panel. This amplifier has its own regulated power supply, derived from the aircraft 28V dc supply to power the hot mic when no transmitter is keyed.
Fig. 3.1. TYPICAL AUDIO INTEGRATING SYSTEM
Fig. 3.2. Audio control panel (ACP) В737.
To block unwanted signals, V or R can be selected on the V/B/R (Voice/Both/Range) switch. With the switch in V, all DME audio signals are shorted to ground, and the voice/range filter is connected to pass VOR and ADF voice-type signals and block VOR and ADF range-type signals. With the switch in R, the voice/range filter is connected to pass VOR, DME and ADF range-type signals and block voice-type signals. With the range switch in B, the voice/range filter is not connected.
Control Wheel Mic Switch
Both pilots control wheels have an int/mic switch, spring loaded to the OFF position from the MIC position and latched in the INT position until either MIC or OFF is selected.
The purpose of the switch is to provide PTT input for the boom or mask microphones.
Fig. 3.3. CONTROL WHEEL MIC SWITCH
In the MIC position, MIC audio is directed to the selected communication system. In the INT position, MIC audio is connected directly to the flight interphone system without selecting INT on the ASP.
PASSENGER ADDRESS
The Passenger Address (PA) system is used by crew members to make announcements over a network of speakers/earphones throughout the cabin. The PA system also provides amplification and reproduction of pre-recorded announcements and music from a tape reproducer.
Flight crew make announcements over the PA system through audio selector panels at each crew members station. Cabin crew make announcements from handheld microphones usually located at the forward and aft attendants panels.
A basic passenger address system consists of a PA amplifier, a tape reproducer, microphones and speakers. Audio inputs from the flight crew, cabin crew and tape reproducer are prioritised by the PA amplifier. The audio with the highest priority is amplified and distributed to the cabin speakers and to all earphones (interrupting In Flight Entertainment audio where necessary).
The first priority is announcement from the flight compartment, the second priority is announcement from an attendant panel, the third priority is prerecorded announcements and the fourth priority is boarding music.
All priority switching is controlled by grounding keying circuits through PTT switches.
Speakers are located throughout the passenger cabin, at each attendants station, galley areas and in each lavatory. Each speaker is connected to the PA system through coupling transformers so as to present a load that matches the output of the PA amplifier.
The positioning of attendant's handheld microphones is fairly critical in order to avoid acoustical feedback. Attendant speakers are sometimes muted during attendant announcements to avoid feedback.
Fig. 3.4. TYPICAL PASSENGER ADDRESS SYSTEM
The PA amplifier gain circuits are connected to an engine oil pressure switch which causes the audio level of the amplifier to be raised approximately 6dB whenever an engine is started. The increased audio level is provided to compensate for the increase in ambient noise level.
In most aircraft the PA amplifier contains an electronic chime. Operation of the NO SMOKING SIGN, FASTEN SEAT BELT sign, or any of the attendant, passenger or lavatory call switches causes chime signals to be generated within the amplifier. The chime audio is superimposed on the audio selected by the priority system and distributed to the speakers.