Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to aircraft intercommunication
(intercom) systems.
Aircraft and avionics systems face a hostile
electromagnetic environment which presents numerous sources of
electrical noise. Such noise can be produced by external
radiation, induction on transmission lines, coupling between
antennas, coupling by difference in ground potential, cross talk,
conduction among power supply lines and injection by transmitters
and receivers.
~o Noise interference is especially serious at audio
frequencies as modulation on the power supply line (400 Hz) will
immediately be picked up by the audio lines and amplified by
the intercom system, rendering monitoring of audio frequenc~es
difficult. Interference with communications among operators (pilot~
navigator, flight engineers, etc.) is obviously a potential flight
safety hazard.
This very important opera-tional problem compounds the
normal problem of intercom system maintenance since it requires
maintenance actions to respond to operators grievances. ~owever,
in practice, it has been found that:
a) there is a large difference between the number of
malfunctions recorded duxing flight operations and bench tests;
b) this difference is much more important for the
intercom system than for any other avionics system; and
c) the number of maintenance actions showing "no
fault" found accounts for about 10% of the manhours expended.
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The above factors are indicative of the noise and intermit-
tent electromagnetic interference problem a~fecting aircraft
intercom sys-tems. It is clear that, bo~h from flight opera~ions
and maintenance aspects, it would be of great benefit to resolve
this problem.
In existing intercom systems, various corrective measures
are resorted to to alleviate the problem of noise including
shielding, grounding, conductors twisting, balancing and isolation
transformers. All these techniques must, to achieve conclusive
results, often be combined which makes for complexity, high cost
and additional weight. As an indication of the magnitude of the
problem, it might be noted that some aircraft installations have
over a thousand feet of audio lines.
The number of wiring links required for the interconnection
between the reeeivers and the transmitters to the control sets are
repeated for eaeh system. Thus, if there are M receivers and
transmitters and N control sets, there are MxN interconnecting
links. One ean easily appreciate the disadvantages both in terms
of weight and volume. Furthermore, the maintenance aspeet suffers
from such ~aried and diverse eleetrical aircraft wiring.
A review of a typical aircra~t installation makes the
potential drawbacks self evident. A VHF (Very High Frequency)
communication system aircraft installation is typically as
follows:
Following the VHF audio ~rom the navigator's intercom
eontrol/monitor to the transciever, a shielded wire goes through a
panel assembly diseonneet connector, a braeket assembly disconnect
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connector -to a connector on a junction box (JB) to two terminal
posts inside the JB, then to a connector on the JB which is wired
to a connector on the JB through the JB out another connector
to the control connector to a volume control, back out of the
control through the same connector to the transceiver connector.
The path for the microphone audio "high" is equally
complicated. It runs from the navigator intercom control box
connector through shielded wire to the panel assembly disconnect
to the bracket assembly disconnect to a connector on the JB to
two terminal posts in the JB, then out of a connector on the JB
to a connector on the JB 7 out of the JB to a connector on the
transceiver.
There are two main reasons for taking the receiver and
microphone audio signals to the junction box. They are:
a) to distribute the signals to the other intercom
; control sets; and
b) to terminate or "load'l these lines with a
termination resistor.
All these wires, interconnectors and junction boxes are
subject to noise sources.
The present invention provides a means of alleviating
the above-described problem of electrical noise interference in
an aircraft intercom system. Basically, in accordance with the
present invention, the problem is avoided by replacing the
existing aircraft audio lines wiring connections from the
communication and navigation sets to the control sets of the intercom
system by an interconnection system utilizing fiber optic lines~
Fiber-optic lines are virtually immune to radio-frequency
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(R.F.) interference and are much lighter in weight than wires.
Thus, in accordance with a broad aspect of the invention, there is
provided, in an aircraEt intercom system having a plurality of radio trans-
ceivers, radio receivers and control sets, the improvement wherein audio sig-
nals from the radio transceivers and radio receivers to the control sets, and
audio signals from the control sets to the transceivers, are multiplexed for
transmission, converted to optical signals for transmission via fiber optic
lines, demodulated back to a~dio signals and then distributed via demultiplex-
ing means.
The optical signals may, of course, be in the infrared portion of the
spectrum and the term "light" used herein includes infrared as well as visible
li~ht.
The invention will now be further described in conjunction with the
accompanying drawings, in which:
Figure 1 is a simplified block diagram of the basic layout of a
; system according to tlle invention;
Figure 2 is a block diagram showing the functional arrangement of a
junction terminal used in the system of Figure l;
Figure 3 is a partly block, partly schematic diagram of a multiplex/
demultiplex unit as used in the terminal shown in Figure 2;
Figure 4 is a schematic diagram of a synchronization unit as used
in the terminal shown in Figure 2, and
Figure 5 is a schematic diagram of a modulator/demodulator as used
in the terminal shown in Figure 2~
Turning now to Figure 1, the basic system layout according to the
invention is shown. The system is assumed to have L radio transmitters/recei~ers(transceivers) 10 and M-L radio receivers 12; i.e. the total number of radio
receivers and radio transceivers is hl. ~11 the audio lines 13 from and to the
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radio transceivers 10 and from the radio receivers and all the audio lines 14
to and from the intercom control sets 15 (N in number) are multiplexed and
demultiplexed through two audio junction terminals 18 and 19. If redundant
audio junction terminals are desired for reliability purposes, they can be
coupled via optical couplers 20 known in the art, e.g. "access" or "star" type
couplers. Light may be distributed into different distribution links to a
number of terminals. In Figure 1, light is shown conducted to four different
junction terminals.
Audio signals from the M radio transceivsrs and radio receivers
10, 12 are fed into junction terminal 18 at connection means 21. The multi-
plexer 23 sequentially samples the inputs at 21 and drives an optical modulator
25 which produces an optical output on fiber-optic line 27. The signals on
riber-optic line 27 are received by the optical demodulator 30 in terminal
19 which produces an audio output which is distributed by demultiplexer 32
to its M x N output lines 33. These output lines 33 are connected as inputs to
the N control sets 15. Any extra audio output lines can be connected to the
monitor 26 as in any typical intercom.
~he monitor, which is conventional and not directly related to the
invention, is an integral part of aircraft avionics intercom systems. It
provides for the junction of audio signals when the number of inputs is too
large to be
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handled by the control sets. In Figure 1 the inputs to the
monitor are any extra audio output lines from the junction
terminal. Audio lines can be selected one-by-one from the
monitor by the operator (pilot, navigator...). The selected
audio is sent to the control set.
The control sets are connected via lines 38 to
junction terminal lg where the lines are sequentially examined
by a multiplexer 40 which feeds an optical modulator 42 whose
output on fiber optic line 43 is received by optical demodulator
45 in terminal 18. The demodulator 45 feeds a demultiplexer
46 which provides LxN outputs for the transceivers 10. That
is, each of the N control set has to be connected to each of
the L transmitters (in the transceivers) so L.xN outputs are
required from demultiplexer 4~.
Turning now to Figure 2, the layout of a junction
terminal, e.g. terminal 18, is shown in more detail. Electrical
inputs 13 to the multiplexer 23 are taken from electrical
connector Jl(21). Similarly, the outputs 50 o-E the demultiplexer
46 are taken to the same electrical connector Jl(21), or a
similar one (standard electrical connection). The output 51
of the multiplexer 23 is connected via electrical bus 52 to
the input 54 of a preamp 55. The preamp 55 feeds an amplifier
57 via, if required, an AGC circuit 56. The amplifier 57
drives an optical modulator comprising, a light emitting
diode (LED) 58 whose light output is coupled to an optical
fiber (not shown) at optical connectox J3.
Optical signals are received at optical junction J2 and
detected by an optical demodulator compri~ing a photodiode
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60. After preamplification at 61, AGC at 62, if re~uired,
and ampllfication at 63, the electrical signal passes via
bus 52 and line 64 to the input of demultiplexer 46.
Demultiplexer 46 directs the signals from line 64 to the
appropriate output line S0.
The multiplexer 23 and demultiplexer 46 are controlled
by a clock 65, binary counters 66, 67 and sync. unit 68 (comprising
phase locked loop 70, delay compensation circuit 71 and level
detector 72) as will be discussed in more detail later.
Figure 3 is a more detailed diagram of the multiplex/
demultiplex unit. Power supply, ground and all audio inputs and
outputs go through the connector Jl. Analog audio signals,
typically from 2 to 7 volts and of 100 to 150 mW, are the inputs
to channels Sl to S15 of the multiplexer 23. The switching
; from one channel to the next is controlled by a binary counter
66, which divides a reference pulse given by the crystal oscillator
clock 65. The output 51 of the multiplexer 23 is sent to the
optical modulator which will be later described in connection
with Figure 5.
The input 6~ to the demultiplexer 46 is the demodulated
optical signal received from the optical demodulator to also be
discussed in connection with Figure 5. See also Figure 2.
Switching of the demul~iplexer 46 from one channel to the next is
controlled by the binary counter 67, which divides a reference
pulse received from the synchronisation unit, to be discussed in
detail in connection with Figure 4. The required D.C. voltage
for all units is obtained from voltage regulators VRl and VR2.
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The crystal clock is conventional and here comprises a 1 M~lz crystal
CRl, resistors Rl-R4, capacitor Cl and amplifier Ul.
rne binary dividers 66 and 67 are of similar construction including
four stages connected as shown.
Pin 9 of Illultiplexer 23 is used as the synchronization input from
the master clock oscillator 65. This input is sent through the line and
will be detected by the synchronizatlon unit of the next j~mction terminal.
The multiplexer and demultiplexer may be integrated circuits. Pins
10-11, D13J C14 and lNH15 are the same for multiplexer 23 and demultiplexer
4-6 and are used by the logic structure of the integrated circuits. Pin
12 is connected to ground.
Suitable multiplexers/demultiplexers may comprise COS/MOS type CD
4067 integrated circuits.
Line 80 from pin 11 of counter 67 provides pulses at 1/16 th the
clock rate for use by the synchronization unit, Figure 4.
Referring to Figure 4, the synchronization unit receives pulses
from the optical demodulator on line 64 for comparison with a reference
voltage, on line 81, applied via resistor R5 to the non-inverting input of
level detector 82, line 64 being connected to the inverting input. The
delay equali~ation circuit ~C2, R9, R10, Ll, U2, Rll) makes compensation for
phase propagation delays both over the transmission line and the întegrated
circuits. It is followed by a comparator, U3 controlled by Rl2, which
reestablishes, if required, the original shape of the clock pulse. Finally
a phase locked
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loop circuit 83, comprising a phase detector 84, a filter 85 and
a voltage controlled oscillator 86, whose characteristics are
controlled by R13, R14, R15, R16, R17, C3 and C4, ensures final
synchronisation of the signal.
Figure 5 illustrates a simple light emitting diode
modulator and a photodiode detector demodulator used to optically
process the multiplexed signals in between the junction
terminals. The modulator and demodulator could also be of
integrated circuit types. The multiplexed signal on l:ine 51
is fed via transformer Tl, capacitor C5 and variable resistor
R18 to the base of transistor Ql. Transistor Ql drives the
light emitting diode Vl, whose light output is coupled to
the fiber optic link by optical coupler J3.
The photcdiode V12 of the demodulator receives light
from optical coupler J2. The electrical output of V12, passing
through capacitor C6, is amplified by amplifiers UlA and UlB (and
associated resistors and capacitors) and then sent, through
transformQr T2, to the demultiplexer unit.
As mentioned above, the multiplexers/demultiplexers may
comprise COS/MOS type CD 4067 integrated circuits but of course
other suitable circuits may be used if desired. A number of
other components may also comprise integrated circuits and,
without limiting the invention thereto in any way, th~ following
examples have been found suitable:
Figure 3
Ul - LM149/LM349
66 - SN5493J
67 - SN5493~
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~.~85~1~22
23, 2~ - CD4067
Figure
82 - L~741LN
U2 - UA702DC/RC702
U3 - NS3302L/SN72711
U~ - MC140463AL
! Figure 5
Ul - MLM149 or LM349
The intercom system according to the invention has a
number of advantages, including the following:
a) reduction of noise usually picked up by the audio
transmission lines, relays and splices;
b) reduction of crosstalk;
c) reduction of the number of maintenance actions and noise
problems;
d) reduction of wiring weight and volume; and
e) system fle~ibility and adaptability.
The reduction of noise makes the intercom system
according to the invention more "airworthy".
The system is relatively simple and very adaptable to
various types of aircraft intercom systems. System modifications
are not required as the multi-point fiber-optic junction
terminals will interface with any analog audio signal.
The fiber-optics junction terminal is relatively simple
and eliminates the need to have a large number of aircraft
electrical terminals and junction boxes. The simple module/unit
design allows adaptation to specific systems or aircraft
requirements.
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Obviously, some modifications and variations in the
system are possible in light of particular requ.irements. It is
therefore to be understood that within the scope of the attached
claims the invention may be practiced with some variations to
the system specifically described.
