Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to wired broadcasting systems
and seeks to provide an improved form thereof.
Wired broadcasting systems comprise two general types.
In the first type a plurality of television signals are distri-
buted between a central station and each of a plurality of sub-
scribers over a single signal path, usually in the form of a
coaxial cable, the plurality of signals being frequency distinc-
tive. Commonly the range of frequencies employed extends through-
out the VHF spectrum, for example, from about 40-300 ~Hz. In the
second type, a plurality of television signals are distributed
between a central station and each of a plurality of subscribers
over separate signal paths usually in the form of twisted pairs
of conductors contained within a common cable. The desired sig-
nal is selected for reception by establishing a connection between
the subscribers equipment and that pair of conductors carrying the
signal required. Commonly the signals all have the same nominal
carrier frequency somewhere in the range 2-20 MEIz.
Both types of known sys-tem referred to above suffer
from various disadvantages. With the first type the transmission
losses are relatively high due to the frequencies involved and
intermodulation in the repeater amplifiers is a problem due to
the large number of signals which have to be amplified simultan-
eously. With the second type crosstalk between the separate
signal paths within the common cable is a limitation and in
systems where a large number of programmes are distributed the
immense number of physical connections between the various con-
ductors introduce maintenance problems.
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It is an object of the pxesent invention to provide a
wired broadcasting system in which the disadvantages of the
presently known systems are minimised.
Accordingly the invention provides a broadcasting system
conveying signals by a signal path between a central station and
a plurality of subscribers, comprising in combination, a common
optical fibre in said signal path carrying signals to said plura-
lity of subscribers from said central station, said fibre extend-
ing between an electro-optical transducer at said central station
producing a light beam and photosensitive detector means at a
reception position near the subscribers station, transmission
means at the central station modulating the light beam for trans-
mission through said optical fibre, said transmission means
including modulation means producing a light beam incorporating
a high frequency carrier modulated with video broadcast signals,
conventional television receivers at the subscriber stations
responsive to receive said high frequency carrier modulated with
video broadcast signals, detection means at said reception
position responsive to said photo-sensitive detector to convert
said light beam into demodulated high frequency carrier radio
wave signals modulated with video broadcast signals, and means
coupling said demodulated signals from said reception position
to said subscriber stations in a form suitable for direct applic-
ation to said conventional television receivers without further
si~nal processing.
If desired, the optical fibre may be arranged to extend
over the whole length of the transmission path between said
central station and each of the plurality of subscribers. In
such an arrangement, a plurality of optical fibres may be provided
extending over the whole length of the transmission path between
the central station and each of the plurality of subscribers, each
subscriber being able to select a desired programme signal on any
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one o~ the optical fibres by deriving the si~nals from a photo-
sensitive detector associated with the said one optical fibre.
Alternatively, the optical fibre may extend between the
central station and a distribution point from which signals may
be conveyed to a~ relatively small group of subscribers over convent-
ional conductive paths.
In a alternative arrangemen-t an optical fibre may be
arranged to extend between each of the plurality of subscribers
and a programme exchange at which input signals for each subscribers
optical fibre may be selected from one of a~plurality of available
programme sources. Control of each subscribers programme selection
means at the programme exchange may be
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effected either over a conductive signalling path associated
with his optical fibre or over an auxiliary optical fibre. The
signals transmitted over the optical fibres may comprise a light
beam modulated in respect of video frequency signals, one or more
high frequency modulated carrier waves of differing carrier fre-
quency or a combination of video frequency signals and one or
more high frequency modulated carrier waves~ Each optical fibre
may carry two or more light beams of different frequency (color)
each modulated as described above.
Further features and advantages of the present invention
will become apparent from the following description of some embod-
iments thereof given by way of example only with reference to the
accompanying drawings in which:
Figure 1 is a block schematic diagram of one form of
wired broadcasting system in accordance with the present inven-
tion;
Figure 2 is a block schematic diagram of a second form
of wired broadcasting system in accordance with the present inven-
tion;
Figure 3 is a block schematic diagram of a third form
of wire broadcasting system in accordance with the present inven-
tion, and
Figure 4 is a block schematic diagram of a fourth form
of wired broadcasting system in accordance with the present inven-
tion.
In the wired broadcasting system shown in Figure 1 a
central station 1 is connected to each of a plurality of sub-
scribers 2, 3, by means of an optical fibre 4 extending between
an electro-optical transducer 5 and a photo-sensitive detector
6,7. The electro-optical transducer 5 may comprise a light
emitting diode or an injection laser the light output of which is
controlled in
5~
response to the output signals of a modulation amplifier 8 to
which the video and/or high frequency carrier wave signals are
applied on the output line 9. In a case of the electro-optical
transducer 5 being a light emitting diode the diode may be formed
as a Burras diode and have as its active constituent zinc or
aluminium doped gallium arsenide. In some applications the mod-
ulation characteristic of the electro-optical transducer may be
unacceptably non-linear. In this event the modulation signal
applied thereto may be such as to cause it to operate in a pulsed
mode. The photo-sensitive detectors 6,7 may comprise PN photo-
diodes associated with low noise pre-amplifier devices.
The signals applied to the input line 9 may comprise
video frequency signals, one or more high frequency modulated
carrier waves of different carrier frequency or a combination of
video frequency signals and one or more high frequency modulated
carrier waves. In the event that said signals are high frequency
modulated carrier waves the output signals provided by the photo-
sensitive detectors 6,7 may be of a form suitable for direct
application to the television receivers 10,11 if these are of a
type designed for use in high frequency wired broadcasting systems.
There may be a modest advantage in utilising video frequency sig-
nals and in this event the output signals from the photo-sensitive
detectors may be applied directly to the post detector stages of
the receivers lO,ll.
In the system shown in Figure 2 a central station 12 is
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provided with an electro-optical transducer 13 modulated by the
output signals of a modulation amplifier 14 which receives its
input signals over the line 15. The resulting modulated light
signals are passed over an optical fibre 16 to distribution sta-
tions one of which is ~hown at 17. Each distribution station is
provided with a photo-sensitive detector 18, output signals from
which are passed to a launching amplifier 19 from which a conduc-
tive network 20 extends to the receivers 21,22. In this system
it is convenient to arrange that the modulation signals applied
to the line 15 comprise high frequency modulated carrier waves.
In this event, the launching amplifier 19 and receivers 21,22 may
be of the kind presently employed in high frequency wired broad-
casting systems.
Turning now to Figure 3, the system shown therein is
arranged to provid~ two television programmes each distributed
over separate optical fibre lines 23,24. The signals applied to
each of these lines are provided by electro optical transducers
25,26 having associated therewith corresponding modulation ampli-
fiers 27,28 arranged to receive input signals over the lines 29,
30 and arranged within a central station. The separate optical
fibres extend to each subscriber 31,32. Each subscriber is pro-
vided with programme selecting means 33,34 to which each of the
optical fibres are connected and by means of which signals may be
derived from a photo-sensitive detector associated with that
optical fibre which carries the desired programme signal. The
desired signal from the programm~selection device 33,34 is then
applied to the associated television receiver 35,36.
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The arrangement of Figure 4 is an alternative sys-tem to
that shown in Figure 3. In this arrangement an optical fibre
37, 38, 39, 40 is arranged to extend between a programme exchange
41 and each of a plurality of subscriber installations 42, 43, 44,
45. The signals applled to each optical fibre are derived from a
corresponding programme selection device 46, 47, 48, 49 each of
which comprises means whereby an electro optical transducer may
be made responsive to one of the programmes available at the pro-
gramme exchange 41 and generate a light beam for transmission to
the subscriber. Each programme selection device is arranged to
be capable of selecting programmes from any one of the programme
ampli~iers 50, 51, 52, 53 which receive their input signals over
corresponding lines 54, 55, 56, 57. Each subscriber installation
comprises a photo-sensitive detector 58, 59, 60, 61 from which
signals may be applied to the associated television receiver 62,
63, 64, 65. Each subscriber installation is provided with a pro-
gramme selector control means 66, 67, 68, 69 which are connected
to the corresponding programme selection device 46, 47, 48, 49
over a control signal path 70, 71,'72, 73 associated with that
subscribers optical fibre. The control signal path may comprise
one or more conductors or an auxiliary optical fibre.
The programme capacity of any of the systems described
above may be increased by arranging that each optical ibre
carries two or more light beams of a different frequency (colour).
Each light beam may be modulated in respect of one or more than
one television or other signal. The light beams may be coupled
into and out o the optical fibre by means of systems of di-
chroic
mirrors. If ~imu]taneous reception of the separate li~ht
beams is not requ:ired the photo-sensitive detector ma~ be
rendered responsive to the appropriate li~ht beam by means
of a ~uitable colour filter.
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