GB1586543A - Wired broadcasting systems - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO WIRED BROADCASTING SYSTEMS (71) We, COMMUNICATIONS PAT ENTS LIMITED, a British Comapny of Carlton House, Lower Regent Street, London, SW1Y 4LS, do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: The present invention relates to broadcasting systems and in particular to broadcasting systems incorporating optical fibre signal transmission links.

Considerable progress has been made in the development of fibre optics technology for application on HF systems with respect to both the understanding of the mechanisms involved and in the reduction of expense. There remain, however, severe doubts about the application of fibre optics to wide band frequency division multiplexing (F.D.M. or stacked carrier) tapoff systems, because of difficulties in operating an electrooptical transducer such as a light emitting diode with several F.D.M. channels present simultaneously, due to non-linearity, and no device exists which can adequately provide the optical power per channel required for such tap-off systems.

Fibre optics technology is however rapidly becoming a viable alternative to the copper based technology of switched television systems which employ star networks.

It is an object of the present invention to provide a wired broadcasting system using optical fibres in which system a wide range of facilities can be economically provided to subscribers.

According to the present invention, there is provided a broadcasting system comprising at least one programme exchange at which television signals are made available, and a plurality of subscriber units each linked to a respective programme selector at the exchange and provided with means for controlling the said - selector, wherein the subscriber unit is linked to the exchange by a cable incorporating at least one optical fibre for transmitting television signals to the subscriber unit from the exchange and at least one electrical conductor for carrying selector control signals from the subscriber unit to the exchange.

Preferably the cable comprises two optical fibres to enable the transmision of two television signals to the subscriber unit and two electrical conductors. Power for the subscriber units selector control means may be be supplied from the exchange on the conductors, and stacked carrier radio programmes may also be transmitted from the exchange on the conductors.

Preferably the selector control means comprises means for generating F.S.K. signals which are transmitted to the exchange, demodulated and used to control a switching matrix which forms the programme selector.

Two F.S.K. signals of different frequency may be generated to control the selection of programmes on the two optical fibres.

The electrical conductor or conductors may be in the form of steel support wires, which support wires may be brassed.

The exchange may be in the form of one of a series of kiosks spaced apart along a programme distribution trunk cable. The trunk cable preferably comprises optical fibres for carrying the distributed television signals.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 illustrates a signal combining and splitting unit located at a distribution kiosk of a wired broadcasting system; Fig. 2 is a circuit diagram of a F.S.K. demodulator provided at the kiosk; Fig. 3 illustrates dial access equipment with which each subscriber is provided; Fig. 4 illustrates a signal combining and splitting unit forming a part of the subscriber equipment as illustrated in Fig. 3; and Fig. 5 is a circuit diagram of a subscribers F.S.K. signal generating equipment.

In the broadcasting system to which the accompanying drawings relate, HF television signals are made available at each distribution kiosk. Each kiosk serves a number of subscribers and the drawings show some of the equipment provided in respect of one subscriber.

Each subscriber is connected to his respec tive kiosk by two optical fibres contained within a single jacket which also contains two conductors in the form of brassed steel sup- port wires. The support wires carry low fre quency power and stacked carrier F.M. radio signals from the kiosk to the subscriber and also carry F.S.K. television programme selec- tion signals from the subscriber to the kiosk.

The F.S.K. selection signals control two switching matrixes at the kiosk to select two of the television programmes made available at the kiosk. The selected signals are then coupled to respective ones of the two optical fibres by suitable light sources, - for example light emitting diodes. Further details of suit able electro-optical devices are described in our British Patent No. 1,486,421.

Referring to Figs. 1 and 2, which do not show the optical fibres or associated television signal selectors, the support wires 1, 2 of a subscribers link to the kiosk are connected at the kiosk to a balun 3 (Fig. 1). Stacked carrier FM radio signals having frequencies of from 10MHz to 20MHz are coupled from a source 4 to the support wires through a winding 5 on balun 3.

A 30 volt 50 Hz power source is applied via terminals 6, 7 to a further balun 8, the supply being centre tapped to earth. The baluns 3 and 8 are interconnected, and a wind ing 9 on balun 8 couples subscriber generated F.S.K. selection signals from the support wires to terminals 10, 11.

Fig. 2 shows one F.S.K. demodulator for connection at its input 12 to terminal 10. The circuitry is such that the demodulator of Fig.

2 accepts F.S.K. signals at 88KHz whereas the other demodulator (not shown) accepts signals at 136KHz. The subscriber is equipped with two selectors for providing selection con trol signals at the respective frequencies as will be described below. The acceptance fre quency of the demodulator can be controlled by adjustment of capacitor 13.

The F.S.K. input to the demodulator is converted to DC dialling pulses which appear at terminal 14 and a reset pulse which appears at terminal 15. The dialling pulses clock a shift register the outputs of which control a television programme selection matrix (not shown), and the reset pulse resets the shift register so that the switching matrix assumes a datum condition.

Referring now to Fig. 3, the subscriber equipment is connected to the optical fibre and support wire cable such that the selected signals on the optical fibres 16, 17 excite respective transducers 18, 19. The transducers may be for example photo-diedes which pro vide HF television signals to conventional HF receivers one of which is indicated by numeral 20.

The receiver 20 is associated with a selection keyboard 21. The keyboard 21 controls one of two generators in F.S.K. generator unit 22 to provide F.S.K. signals to filter unit 23 over line 24. A similar keyboard (not shown) associated with the other receiver causes F.S.K. signals to be applied to the filter 23 over line 25.

The filter is connected to the support wires 1, 2 and separates the incoming FM radio and power signals to provide an FM radio output at terminal 26 and to energise a DC pack 27 powering the F.S.K. generator. The filter 23 also couples the generated F.S.K.

signal to the support wires 1, 2 so that programme selection signals can be transmitted to the kiosk.

Fig. 4 shows details of the filter 23 and DC pack 27 of Fig. 3. The support wires 1, 2 are connected to a push-push rejector 28 feeding a matching transformer 29. The transformer provides a balanced FM radio output at terminals 30, 31 and couples the powering signal to a directional coupler 32. The power supply is rectified by rectifier bridge 33 to provide positive and negative supplies to keyboard 21 at terminals 34, 35, positive and ground supplies to the other keyboard (not shown) at terminals 36, 37, and positive and negative supplies to the F.S.K. generator unit 22 at terminals 38, 39.

The directional coupler 32 receives F.S.K.

signals from the two generators of generator unit 22 at terminals 40, 41 respectively, a common terminal 42 also being connected to the generator unit 22 and an intermediate voltage source defined by capacitors 43, 44 connected across the rectifier bridge 33.

Referring now to Fig. 5, one of the two F.S.K. generators of unit 22 is shown. Terminals common to Figs. 4 and 5 bear the same reference numerals. The keyboard 21 of Fig.

3 is connected to terminal 45 of Fig. 5. The circuit of Fig. 5 comprises a timer chip 46 of known type tuned to the desired frequency by suitable selection of its associated resistors.

Low loss graded index fibres are now available which are suitable for use as trunk lines connecting exchange kiosks providing spatialdivision multiplexing (S.D.M.) is used, i.e.

every television HF channel is assigned to a separate fibre. With such an arrangement it is not necessary to use bulky high-power light sources which would otherwise be necessary for wideband F.D.M C.A.T.V. systems to achieve a high quality signal-to-noise performance along an extended trunk. Using low power light sources, optical fibre trunk repeaters are only required at typically 1 to 2km intervals.

The spacing between exchange kiosks in high subscriber density areas is however typically only 300 m. Thus, to take full advantage of the optical trunk performance, there is a requirement for optical tap-off units at intermediate kiosks along the trunk.

One of the prime disadvantages of fibres and one that makes application to final distribution network of F.D.M C.A.T.V systems so difficult, is that it is not easy to tap the main light signal. This is particularly true for all glass fibres, but the latest glass fibres which have soft silicon cladding are more amenable to tapping. It is possible to produce optical taps for trunk application which have transfer losses of 13 dB and introduce an insertion loss to the main light signal of only half a dB. This is only roughly twice the insertion loss of high grade electrical tap-off units. Accordingly although the embodiment of the invention described herein refers to the kiosk receiving programme signals from a conductive trunk, in many applications the trunk network could advantageously comprise optical fibre cables.

WHAT WE CLAIM IS: 1. A broadcasting system comprising at least one programme exchange at which television signals are made available, and a plurality of subscriber units each linked to a respective programme selector at the exchange and provided with means for controlling the said selector, wherein the subscriber unit is linked to the exchange by a cable incorporating at least one optical fibre for transmitting television signals to the subscriber unit from the exchange and at least one electrical conductor for carrying selector control signals from the subscriber unit to the exchange.

2. A broadcasting system according to claim 1, wherein said cable comprises two optical fibres to enable the transmission of two television signals to the subscribers unit, and two electrical conductors.

3. A broadcasting system according to claim 1 or 2, comprising means for powering the subscriber units selector control means from the exchange via a said conductor.

4. A broadcasting system according to claim 1, 2 or 3, comprising means for transmitting stacked carrier radio programmes from the exchange on the conductors.

5. A broadcasting system according to any preceding claim, wherein said selector means comprises means for generating F.S.K. signals which are transmitted to the exchange, means for demodulating the F.S.K. signals at the exchange, and a switching matrix which forms the programme selector controlled by the demodulating means.

6. A broadcasting system according to claim 5 as dependent upon claim 2, wherein two F.S.K. signals of different frequency are generated to control the selection of programmes on the optical fibres.

7. A broadcasting system according to any preceding claim, wherein the exchange is in the form of one of a series of kiosks spaced apart along a programme distribution trunk cable.

8. A broadcasting system according to claim 7, wherein said trunk cable comprises optical fibres for carrying the distributed television signals.

9. A broadcasting system according to any preceding claim, wherein the or each electrical conductor comprises a steel support wire.

10. A broadcasting system according to claim 9, wherein the or each support wire is brassed.

11. A broadcasting system substantially as hereinbefore described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (11)

**WARNING** start of CLMS field may overlap end of DESC **. at intermediate kiosks along the trunk. One of the prime disadvantages of fibres and one that makes application to final distribution network of F.D.M C.A.T.V systems so difficult, is that it is not easy to tap the main light signal. This is particularly true for all glass fibres, but the latest glass fibres which have soft silicon cladding are more amenable to tapping. It is possible to produce optical taps for trunk application which have transfer losses of 13 dB and introduce an insertion loss to the main light signal of only half a dB. This is only roughly twice the insertion loss of high grade electrical tap-off units. Accordingly although the embodiment of the invention described herein refers to the kiosk receiving programme signals from a conductive trunk, in many applications the trunk network could advantageously comprise optical fibre cables. WHAT WE CLAIM IS:

1. A broadcasting system comprising at least one programme exchange at which television signals are made available, and a plurality of subscriber units each linked to a respective programme selector at the exchange and provided with means for controlling the said selector, wherein the subscriber unit is linked to the exchange by a cable incorporating at least one optical fibre for transmitting television signals to the subscriber unit from the exchange and at least one electrical conductor for carrying selector control signals from the subscriber unit to the exchange.

2. A broadcasting system according to claim 1, wherein said cable comprises two optical fibres to enable the transmission of two television signals to the subscribers unit, and two electrical conductors.

3. A broadcasting system according to claim 1 or 2, comprising means for powering the subscriber units selector control means from the exchange via a said conductor.

4. A broadcasting system according to claim 1, 2 or 3, comprising means for transmitting stacked carrier radio programmes from the exchange on the conductors.

5. A broadcasting system according to any preceding claim, wherein said selector means comprises means for generating F.S.K. signals which are transmitted to the exchange, means for demodulating the F.S.K. signals at the exchange, and a switching matrix which forms the programme selector controlled by the demodulating means.

6. A broadcasting system according to claim 5 as dependent upon claim 2, wherein two F.S.K. signals of different frequency are generated to control the selection of programmes on the optical fibres.

7. A broadcasting system according to any preceding claim, wherein the exchange is in the form of one of a series of kiosks spaced apart along a programme distribution trunk cable.

8. A broadcasting system according to claim 7, wherein said trunk cable comprises optical fibres for carrying the distributed television signals.

9. A broadcasting system according to any preceding claim, wherein the or each electrical conductor comprises a steel support wire.

10. A broadcasting system according to claim 9, wherein the or each support wire is brassed.

11. A broadcasting system substantially as hereinbefore described with reference to the accompanying drawings.