GB2168226A - Distribution apparatus - Google Patents

Abstract

A hybrid cabled T.V. distribution system has features of switched star and "tree and branch" systems. Basic fare subscribers may receive simultaneously a basic group of channels fed at U.H.F. Premium subscribers can additionally control a switch at a star-point to select premium channels. The switches have superheat converters to convert channels supplied at V.H.F. to a predetermined U.H.C. channel, using a programmable local oscillator. The frequency plan has all V.H.F. frequencies and a pilot reference frequency supplied to the switch phase-locked to multiples of the nominal U.H.F. channel spacing. The local oscillator is phase-locked to the pilot so the U.H.F. signals are all synchronised and intermodulation products minimised. In an alternative arrangement, channels supplied at V.H.F. are simultaneously converted to a common I.F. on separate signal paths. A switch matrix controlled by the subscriber selects one of the I.F. signals which is then converted to a predetermined U.H.F. channel for supply to the subscriber.

Description

SPECIFICATION Distribution apparatus The present invention relates to distribution apparatus and particularlyto selective signal distribution apparatusforselecting at least one of a plurality of frequency division multiplexed signals for distribution to a user. The invention is particularly, though not exclusively, applicable to wide band cabled T.V.

distribution systems including distribution starpoints. Cabled T.V distribution systems fall into two main types. The distribution system most commonly used hitherto is that known as "tree and branch". The tree and branch system provides for connecting each subscriber, via various tree and branch connections, to a trunk cable so that all the services provided on the trunk cable are fed along the subscriber's cable into the subscriber's residence. With a tree and branch system any selection by the subscriber of the various channels supplied on the subscriber's cable is done by selection equipment in the subscriber's residence.

Furthermore, if anytarriffs are to recharged by the cable companyforthe reception of particularchan- nels, this must be achieved by encoding those channels on the cable so that only a subscriber with the right forum of decoder can receive them.

The other main type of cabled distribution system is the star system in which each subscriber is connected by a subscriber's cable to a distribution star-point.

Then, switching arrangements can be provided at the star-pointto supply only selected services and channels on to the subscriber's cable. This selection can be made bythe subscriber using a controller in the subscriber's residence which signals along the subscriber's cableto the switching arrangement at the star-pointto make the desired selection ofthe available channels.

In one aspect, the present invention provides a selective signal distribution apparatus which may be useful as the switching arrangement at the distribution star-point of a star-type cable distribution system.

Howeverthe selective signal distribution apparatus of the present invention is not exclusively for this purpose.

According to one aspect ofthe present invention, there is provided selective signal distribution apparatusforselecting at least one of a plurality of frequency division multiplexed signals for distribution to a user, the plurality of f.d.m. signals being modulated on respective predetermined carrierfre quencies,the apparatus comprising at least one superhetfrequency converter simultaneously receiving all the plurality off.d.m. signals, a programmable local oscillator responsive to selection control signals to generate a respective local oscillator frequency supplied to the superhetfrequency converterto convert a selected one of the f.d.m. signal carrier frequenciesto a predetermined differentcarrierfre- quency, and band-pass filter means to pass only the signal band about said predetermined different carrier frequencyfordistrihution to the user. There is thus provided a convenient form of distribution apparatus or switch enabling the selection of one of a number of f.d.m. signals.

In a preferred embodiment, said predetermined different carrierfrequency is one of a set of such carrierfrequencies at a predetermined nominal frequency spacing, and supply means are provided to supply said plurality off.d.m. signals all at respective carrier frequencies which are multiples of said nominal spacing.

Conveniently, the supply means is arranged to supply a reference frequency at a multiple of said nominal spacing and to phase-lockthe carrierfrequencies of said plurality of f.d.m. signals to the reference frequency and the programmable local oscillator is also arranged to phase-lockthe local oscillatorfrequencies to the reference frequency. The advantage of thins arrangement will become apparent later herein.

It may be appreciated that a plurality of f.d.m.

signals may be composite television signals including vision signals modulated on respective vision carrier frequencies in the V.H.F. band and the predetermined different carrier frequency may be the vision carrier frequency of a pre-selected standard U.H.F. television channel. Then the distribution apparatus may be used, for example in the distribution star-point of cable system, to select one of a number oftelevision signals at different V.H.F. frequencies on a single cable for supply art a predetermined U.H.F. television channel on a subscriber's cable.

In another aspect, the present invention provides a wide band cabled T.V. distribution system comprising a head station, at least one distribution star-point serving a number of subscribers, trunk cables connecting the head station to the star-pointto carry to the star-point a plurality of television channels including a number of channels supplied as f.d.m.VHF signals, a respective subscriber cable connecting each subscriber to the star-point, and selective distribution means at the star-point including, for each of at least a proportion of the subscribers, at least one superhet frequency converter simultaneously receiving all channels of said number, a programmable local oscillator responsiveto selection control signalsfrom a controller operable by the subscriberto generate a respective local oscillator frequency supplied to the superhetfrequencyconverterto convert a selected one of said number of channels to a predetermined U.H.F. channel and band-pass filter means to pass onlytheT.V. channel converted to said predetermined U.H.F. channel to the subscriber cable ofthe respective subscriber.

In a preferred example, said predetermined U.H.F.

channel is a standard U.H.F. channel having a standard U.H.F. vision carrier frequency, and the V.H.F. vision carrierfrequencies of said number of channels are all arranged to be multiples ofthe nominal spacing ofthe standard U.H.F. channels. The standard U.H.F. channels are those used as standard broadcast channels, for U.H.F. television signals. In the United Kingdom,the nominal spacing ofthese standard U.H.F. channels is 8MHz. Accordinglyforthe United Kingdom, the various V.H.F. vision carrier frequencies mentioned above may be multiples of 8MHz.The advantage ofthis arrangement arises because the local oscillatorfrequency required to convert one ofthe V.H.F. vision carrier frequencies to a standard U.H.F. vision carrierfrequency is, when in the U.H.F. television band, exactly atthe vision carrier frequency of one ofthe standard U.H.F. channels. This advantagewill be explained morefully later herein.

Conveniently, the head station is arranged to supply a reference frequency on the trunk cables to the star-point at a multiple of said nominal U.H.F. channel spacing and to phase-lockthe V.H.F. vision carrier frequencies of said number of channels to said referencefrequencyandthe programmable local oscillators are arranged to generate said local oscillatorfrequencies also phase-locked to the reference frequency. In this way, all the frequencies generated in the distribution system are phase-locked to one another.

Preferably, each of the V.H.F. channels is spectrum inverted and each ofthe programmable local oscillators is arranged to generate local oscillatorfrequencies which are above said predetermined U.H.F.

channel frequency. It will be appreciated that with a local oscillator frequency above the intended U.H.F.

channel frequency, spectrum inversion oftheV.H.F.

channels to be converted is required so that the resultant U.H.F. channel is correctly orientated with the sound carrier on the correct side ofthe vision carrier.

In one preferred embodiment ofthe invention, the plurality ofT.V. channels carried to the star-point comprise a predetermined basic group of channels andfurtheradditional channels, and there is at the star-point continuous distribution means to supply said predetermined basic group only of channels simultaneously on to the respective subscriber cable of each of at least a first group ofthe subscribers who receive only said basic group of channels, said proportion ofthe subscribers constituting a second group and said numberofV.H.F.channels including said further additional channels. This arrangement provides a hybrid form of distribution system which has features of both the tree and branch system and the starsystem.It can be seen that switching or selective distribution apparatus is required at the star-point only to serve those subscribers ofthe second group. The subscribers of the first group are supplied with all services to which they are entitled in the same manners a tree and branch system. This hybrid arrangement provides considerable advantages in the introduction or setting up of a cable system, in that initially a primarilytree and branch system can be set up with subscribers receiving all basic group channels on their cables. Progressively, then, the switched star system can be introduced to serve subscribers of the second group who can be supplied at any one time with only selected additional channels.

Normally, at ieastthe major runs of the trunk cables are arranged to carrythe basic group of channels also asf.d.m. V.H.F. signals. However, the basic group of channels are then converted to U.H.F. channels for supplybysaid continuous distribution meanssimul- taneouslyalong the subscriber cables of said first group of subscribers. Itwould be appreciated that the losses along a distribution cable at U.H.F. are considerably greaterthan at V.H.F. so that it is desirable to feed all signals at V.H.F. over at least the major runs of the trunk cables.

The distribution system may include distribution conversion points at each of which said V.H.F. to U.H.F. conversion is performed and the U.H.F. signais then supplied to a plurality of star-point.

Preferably,theV.H.F.vision carrierfrequencies of the basic group of channels are also multiples ofthe nominal spacing ofthe standard U.H.F. channels and the distribution conversion points are arranged to convertthe basic channels to respective standard U.H.F. channels.

Conveniently, the V.H.F. vision carrierfrequencies ofthe basic group of channels are also phase-locked to the reference frequency and there are provided superhet converters to convert the basic channels to the standard U.H.F. channels and local oscillator generators to provide forthe converters local oscillatorfrequencies also phase-locked to the reference frequency.

It may be convenientforsaid basic group of channels to be supplied also to the or each star-point additionally as f.d.m. V.H.F. signals for selection by said selective distribution means.

In another aspect of the present invention, there is provided a wide band cabledT.V. distribution system comprising a head station, at least one distribution star-point, trunk cables connecting the head station to the star-pointto carry to the star-point a plurality of television channels comprising a predetermined basic group of channels and further additional channels, a respective subscriber cable connecting each subscri bertothestar-point, continuous distribution means at the star-pointto supply said predetermined basic group only of television channels simultaneously on to the respective subscriber cable of each of at least a first group of subscribers who receive only said basic group ofchannels and selective distribution means at the star-point responsive to selection control signals from a controller operable bya subscriber, being one of a second group of subscribers, to supply, on to the respective subscriber cable of the selecting subscriber, at least one selected television channel being selected from channels supplied to the star-point including said additional channels.

In still a further aspect ofthe present invention, there is provided a selective signal distribution apparatus for selecting at least one of a plurality of frequency division multiplexed signals for distribution to a user, the plurality off.d.m. signals being modulated on respective predetermined carrier fre- quencies, the apparatus comprising first conversion meansto convert all thef.d.m. signalsto a common Intermediate Frequency on separate respective signal paths, switch means responsive to selection control signals to selectthe IF signal from at least one of said paths, and second conversion meansto convert the selected IF signal to a predetermined different carrier frequency for distribution to the user.

The present invention further envisages a wide band cabled T.V. distribution system comprising a head station, at least one distribution star-point serving a number of subscribers, trunk cables connecting the head station to the star-pointto carry to the star-pointa plurality oftelevision channels including a number of channels supplied as f.d.m.V.H.F. signals, e respective subscriber cable connecting each su bscri bertothe star-point, and selective distribution means including first conversion means to convert said number ofchannels to a common Intermediate Frequency on separate respective signal paths, switch means responsive to selection control signals from the respective controller of each of at least a proportion of the subscribers to select the IF signal from at least one of said paths for distribution to the respective selecting subscriber, and second conversion means for each of the selecting subscribers to convert the selected IF signal forthe respective subscriberto a predetermined U.H.F.channelforsupplytothe subscriber cable ofthe respective subscriber.

The present invention still further envisages fre- quency conversion apparatus for converting the carrierfrequencies ofa plurality off.d.m. signals to a set of predetermined different carrier frequencies at a predetermined nominal frequency spacing, the appar atuscomprising supply means to supply the plurality of f.d.m. signals together with a reference frequency and to phase-lockthe carrierfrequencies of said f.d.m.

signals to the reference frequency, with both said carrier frequencies and the reference frequency being respective multiples of said nominal frequency spacing, a respective superhetfrequency converter for each ofthef.d.m. signals to be converted and a local oscillator generator arranged to generate and supply to converters local oscillatorfrequencies also phaselocked to the reference frequency.

Examples of the present invention will now be described with reference to the accompanying drawings in which: Figure 1 illustrates a cabled T.V. distribution system which embodies features of the present invention; Figure 2 is block schematic diagram illustrating conversion apparatus at a conversion site for convert ingchannelsofthebasiogroupfrom V.H.F.to U.H.F.; Figure 3 is a block schematic diagram of a distribution star-point with a switching arrangement for selective switching of channels for appliction to subscriber cables; Figure 4 illustrates schematically the installation at a subscriber; Figure 5 is a table ofthe V.H.F. frequency plan forthe various V.H.F. channels supplied overthe main trunk cables;; Figure 6 is a block schematic diagram of an alternative form of conversion apparatus at a conversion pointand operating with three main trunk cables and Figure 7 is a block schematic diagram illustrating an alternative form of distribution star-point operating from three trunk cables and incorporating a different embodiment of switching arrangement.

Referring to Figure 1, the overall arrangement of a cabled distribution system is illustrated, having a head station 10 at which the various television and other signals are generated and supplied on to main trunk cables 11 and 12. Two such trunk cables 11 and 12 are illustrated in Figure 1, although more parallel trunk cables may be provided if a greater number of services or greater band width is required.

All television signalstransmitted along the main trunks 11 and 12 are transferred as frequency division multiplexed V.H.F. signals. With an upper frequency limitof400MHzforsignals in these maintrunkcables 11 and 12, a maximum reach forthe trunk cables may be obtained of between 8 and 10 km using twenty repeaters spaced at 24 dB loss at 400MHz. However with a slight increase in upper frequency limitofupto say 450MHz, a useful reach may still be obtained.

At spaced locations along the main trunk cables 11 and 12, there are provided conversion stations 13,14 atwhich someoftheV.H.F.television channels are converted to standard U.H.F. channels for onward transmission on sub-trunks 15 and 16etc.The sub-trunks 15 and 16feed all the signals from the main trunks 11 and 12, with some signals converted to U.H.F., to successive distribution star-points 17, 18.

the length of cable runs 15, 16from the conversion stations 13, 14to the various distributution star-points 17, 18 are kept significantly shorter to avoid excessive loss at U.H.F. The television channels which are converted at the conversion points 13 and 14to U.H.F.

constitute a basic group of channels which are distributed directly bythe star-point 17 and 18 straight on to subscriber cables 19 which feed at leastthis basic group of U.H.F. channels directly to each of at least a first group of subscribers. The star-point 17 and 18 also include switching arrangements enabling a selection to be made of the additional channels fed to the star-points atV.H.F. on sub-trunks 15 and 16, for additional supply on the subscriber's cables to selecting subscribers.

Figure 2 illustrates an arrangementfor converting the basic group of channelsfrom V.H.F. to U.H.F.

Spurs 20 and 21 aretakenfromthemaintrunkcables 11 and 12 by splitters 22 and 23 usually immediately following repeating amplifiers 24 and 25. In the illustrated arrangement, the V.H.F. signals from main trunk 12 are fed bythe spur 21 directly to a further amplifier/driver26 to drive the second sub-trunk 16 supplying the signals still atV.H.F. to one or more star-points. Similarly, the spur 20 is itself divided by a second splitter 27 and one feed 28 taken directly via a amplifier-driver 29 and diplexer 30 to drive the first sub-trunk 15. Thus, in this arrangement, all the V.H.F.

signals applied on the main trunks 11 and 12 are also applied to the sub-trunks 15 and 16 without conversion, However, the splitter 27 supplies a second feed ofthe V.H.F. signalsfromthe maintrunk 11 via an amplifier31 and a multi-way splitter 32 to each ofa bank of V.H.F. channel pass filters 33. The filters 33 are each tuned to pass one ofthevariousV.H.F. channels of at least a basic group ofthe channels supplied on the main trunk 11 thus, the outputs on lines 34from the filters 33 comprise the respective V.H.F. signals demultiplexed. Each ofthe demultiplexed V.H.F.

signals of the basic group are then supplied to a respective V.H.F./U.H.F. converter35 which comprises a superhetconverterfed with a local oscillator frequency from a multiple frequency output local oscillator 36.

As will become a ppa rent, the signals su pplied along the main trunks 11 and 12 also include at least one pilottonetowhichtheV.H.F. carrierfrequencies on the main trunks are synchronised or phase-locked.

One ofthefilters 33 (thefilter37) is tuned to select the pilotfrequencyforsupply to the local osciliator36. The local oscillator 36 is arranged to produce on lines 38, 39 etc, local oscillator signals at frequencies phaselocked to the pilotfrequencyand selected to convert the respective V.H.F. signals supplied to converter 35 to predetermined different U.H.F. channels. Itwill be appreciated that since both the V.H.F. signals on line 34 and the local oscillator signals from the oscillator 36 are phase-locked to the pilot frequency, the U.H.F.

channels are also synchronised and phase-locked.

The various U.H.F. channels from converters 35 are then supplied to a combiner40 forfeeding on a single line 41 as frequency division multiplexed U.H.F.

signals via an amplifier/driver 42 and diplexer 39 on to thefirstsub-trunk 15. thus, sub-trunk 15 carries both the basic group ofthe channels at U.H.F. and the original V.H.F. channels from main trunk 11 to the various star-points.

Referring now to Figure 3, the signals on sub-trunks 15 and 16 are spurred off the trunks by splitters 50 and 51 usually located immediately after repeater ampli- fiers 52 and 53.

U.H.F. channels from sub-trunk 15 are supplied through a U.H.F.N.H.F. splitterfilter 54, an amplifier 55, diplexer 56to a star splitter 57 which supplies U.H.F. channels directlyto the subscribercables 58 of each of a first group of subscribers entitled to receive onlythe basic group of channels.

Itwill be appreciated thatthe signals supplied on sub-trunk 15, from main trunk 11, also include the FM radio signals in a band Il V.H.F. All the V.H.F. signals from sub-trunk 15 are fed from splitter 54to a further splitter 59 and thence to a band-pass filter amplifier 60 arrangedto pass only the band II FM radiofrequencies.these band It signals arefedthroughafurther splitter 51 to the diplexer 56 where they are combined with the U.H.F. television channels for applying through the star splitter 57 to the subscriber cables 58.

The V.H.F. television signals from sub-trunk 15 are fed via an amplifier 62 and a splitter 63 to a star splitter 64.Atthe same time, the V.H.F. signals from sub-trunk 16arefedviaamplifier65tostarsplitter66.

In the present arrangement, there is provided, for each ofthe subscribers able to select channels from the additional set of channels (the premium channels), a pairofV.H.F. to U.H.F. converters 67 and 68. the converter 67 includes an output switch 69 able to select eitherthe V.H.F. signals from sub-trunk 15 via one ofthe outputs of star splitter 64, orthe V.H.F.

signals of sub-trunk 16 via one ofthe outputs of stair splitter 66. Each converter 67,68 receives a reference frequencyfrom a reference generator 71 The refer encefrequencyfromthe reference generator71 is phase-locked to the pilotfrequency supplies to the reference generator on line 72 from splitter 63.

Each converter 67 and 68 includes a local oscillator which is arranged to generate a local oscillator frequency phase-locked to the reference frequency from generator71 but at a frequency dependant on selection control signals on line 73 from a data control unit74. The data control unit 74 is arranged to produce control signal on line 73 in responseto data signals received on line 75 and transmitted along the subscri ber ca ble 76 fro m a controller at the su bscri ber. The control signals on line 73 enable the subscriber not onlyto control the local oscillatorfrequency in each of converters 67 and 68 hut also to control the operation ofthe selection switches 69 and 70.The control signals on line 73 are responsiveto channel selection by the subscriberto produce local oscillatorfrequencies in the converter 67 and 68 which convert the selected incoming V.H.F. signal to one of two predetermined U.H.F. channels. Only U.H.F. signals in the predetermined U.H.F. channel are passed by the U.H.F.

channel band-passfilter provided at the output of each of the converters 67 and 68. In this way, the local oscillatorfrequency is set so that only one ofthe incoming V.H.F. signals is converted to the predetermined U.H.F. channel and can pass the band-pass filter. The frequencies to which non-selected incoming V.H.F. signals are converted fall outsidethe pass-band ofthe U.H.F. band-pass filter.

Inthisway,the controller can select any two ofthe incoming V.H.F. signals for conversion to the predetermined two U.H.F. channels and supply via a combiner 77 on to the subscriber cable 76.

In addition to the selected channels at U.H.F., the combiner 77 also combines on to the subscriber cable 76 band II V.H.F. radio signals from a starsplitter78.

An additional converter 80 may be provided controllable by control signals on line 73 from the subscribers' controllerto select one of a predetermined sub-group ofthe incoming V.H.F. signals for conversion to a preset channel in the range chosen for thefirstlntermediate Frequencyforreception of DBS signals (probably between 900MHz and 1500 MHz).

Again this channel is combined at combiner77 onto the subscriber cable 76. This sub-group of V.H.F.

incoming signals mayconstitutewide band television channels of a sort differentfrom the usual colour television signals. The normal colourtelevision signals envisaged in one embodiment ofthis invention are PAL system I which have a bandwidth of 8MHz.

The sub-group of V.H.F. signals may be wide band signals having channels suitable for example for the 27MHz FM signals from a DBS (Direct Broadcast Satellite) system. In the embodiment of Figure 3, provision is made to ensure that only the sub-group of wide band V.H.F. signals are provided to the converter 80 for selection and conversion to the predetermined DBS (IF) channel.

In addition to FM radio and television signals, the cabled distribution system can be used fortwo-way data communication between the subscriber and the head station. Such data communication may enable the subscriberto set up online communication with remote computers and data bases.

The data is conveniently signalled on one ofthe trunk cables, sub-trunk cables and each ofthe subscriber cables in a frequency band below the lowest frequencies used forV.H.F. television channels. A modem 90 demodulates data spurred off sub-trunk 15 at splitter 50 forfeeding by two-way bus 91 and data controller 74 on to the subscriber cable 76 at combiner 92. Return data from the subscriber follows the same route to the modem 90 and is applied to the sub-trunk 15 ata driver93. Data by-pass circuits are provided between the sub-trunk 15 and main trunk 11 at each of the conversion stations.

Figure 4 illustrates schematically the apparatus installed atthe subscriber. The subscriber cable 76 feeds respective T.V. and FM radio sockets 95 and 96 via band-pass filters 97 and 98. A key-pad 99 is provided enabling a subscriber to selecttelevision signals from those available at the star-point. The key-pad 99 may include an infra-red linkwith a fixed installation 100 providing data communication with the subscribers' cable 76 via a band-pass filter 101.

The key-pad 99 causes appropriate control signals to be sent down the subscriber cable 76 for reception at the star-point and appropriate control ofthe converters 67,68,80 and switches 69 and 70 to select desired television signals. The selected television signals are supplied along the subscriber 76 at two predetermined U.H.F. channel frequencies, so that selection of a different television signal does not require retuning the television receiver.

The performance of a long cascade of repeater amplifiers such asthose along thetrunks and sub- trunks of the above described distribution system, with a large number of T.V. channels at different frequencies can be seriously affected by the choice of carrierfrequencies and in general the best performance is achieved when all the vision carrierfrequencies are harmonically related, ie, they are all exact multiples of a suitably chosen number. The conventional PAL system I television signal is arranged with the sound carrier space 6MHz above the vision carrier.

However, in the above described arrangement, the frequency conversion from V.H.F. to U.H.F. involves inversion ofthespectrum oftheT.V. signaissothat, in order to generate standard type U.H. F. signals, the V.H.F. signals are provided spectrum inverted with the sound carrier 6MHz below the vision carrier.

It is desirablethatthe minimum spacing between adjacent vision carrier signals on the trunk cables is between 15 and 16MHz. Furthermore, in the available V.H.F.spectrum (50 to 400MHz) for the V.H.F. sig nals, there are a number of prohibited frequencies at or near to which carrier signals or high energy side bands are not permitted by the regulatory authorities.

Bearing the above criteria in mind, an apparently good compromise can be achieved by the use of 7.8MHz as the base numberforan harmonically related carrier system using a minimum of15.6MHzspacing between adjacent vision carriers. However, such a scheme has an inherent requirementforthe V.H.F. to U.H.F. converters to provide strong suppression ofthe local oscillator signal appearing at the converter output.

This can be seen inthefollowing example. Supposing that an output U.H.F. channel from a star-point on to a subscriber cable is chosen to be the standard U.H.F. channel 21 (471.25MHz vision carrier). A permissble V.H. F. vision carrier based on harmonics of7.8MHzwould be 296.4MHz. In orderto convert 296.4MHzto471.25MHz,a local oscillatorof 767.65MHz is required.

This local oscillator frequency is 0.4MHz in to the vision side band of U.H.F. channel 58 (767.25MHz vision).ln orderforthe local oscillator signal notto produce visible interference on a channel 58 signal, the local oscillator signal must be suppressed to some 60dB below the level of the wanted channel 58 signal.

Onewaytoalleviatethisproblemwould beto arrangeforthetwo U.H.F. channels provided bythe cable distribution system on the subscribers' cable to beat non-standard frequencies. For example, if one channel was at standard channel 21 as above, the second channel could be at 767.65MHz vision carrier.

The local oscillator signal from conversion ofthefirst channel would then zero-beat with the vision carrier of the second channel and as such need onlyto be suppressed by about 30dB relative to the wanted signal in the second channel.

However, for other combinations of channels a much greater off-set may be required of one channel from the nearest standard. For example the local oscillator frequency required to convertV.H.F.

148.2MHz upto U.H.F. 471.25MHz is 619.45MHzwhich is 4.2MHz away from the standard vision carrier frequencyforchannel 39.

These various off-sets lead to great difficulty in arranging the system U.H.F. channels provided to the subscribers so that they fit neatly between locally broadcast U.H.F. signals.

Accordingly, a preferred V.H.F. frequency plan uses as a base number the nominal spacing of the standard U.H.F. channels which is 8MHzforthe United Kingdom. With such an arrangement, the local oscillator frequencies required to convert a V.H.F. frequency which is a multiple of 8MHzto a standard U.H.F.

channel is itself always atthe vision carrier frequency of another U.H.F. channel and will accordingly zerobeat with that other channel requiring only 30dB suppression.

FigureS shows various V.H.F. frequency plans based on 8MHz HRC. The prohibited frequencies in the V.H.F. region are indicated and plan 1 lists the vision and sound carrierfrequencies which could provide the greatest number of V.H.F. television signals with the minimu m carrier spacing of 16M Hz. As will be seen, up to twenty-one V.H.F. channels can be accommodated if the top end ofthe band is extended to 450MHz.

The usual pilot carriers are also produced at multiples of 8MHz, for example at 40MHz and 456MHz. The head station 10 is arranged to phase-lock the vision carriers of all the V.H.F. channels transmitted on to the trunk cables to the pilot frequency. Furthermore, the local oscillators and reference generators at both the conversion stations and the distribution star-points as described above, are also arranged to generate local oscillatorfrequencies which are phase-locked to the pilot. In this way all frequencies produced in the system a re m are mutuallysynchronised and phase-locked so that all interfering signals zero-beatwith one another.

Plan 2 illustrated in Figure 5 employs the same vision carrierfrequencies for the first six channels but then selects frequencies at an increased channel spacing to provideforwider bands such as may be desirableforDBStransmission.Plan3issimilarto plan 1 exceptthatthefirstnineV.H.F.channelsarethe same as plan 1.

Itwill be noted thatall the V.H.F. channelssupplied to the conversion station (see for example Figure 2) may be phase-locked and in accordance with the frequency plan of Figure 5, including those converted to U.H.F. at the conversion station. Further,the local oscillators at the conversion station also produce L.O.

freqencies which are phase-locked and selected to generate U. H.F. channels at the standard channel frequencies. the U.H.F. signals generated at the conversion point are thus not harmonics, but are incrementally related in frequency having spacings which are exact multiples of 8M Hz. This arrangement greatly eases problems of intermodulation during subsequent wide band amplification ofthe U.H.F.

signals at repeater stations. the incremental relationship nulls all odd order intermodulation products.

The even order products are not important since the used band does not extend beyond a single octave.

Figure 6 illustrates an alternative form of conversion station. In Figure 6there are illustrated three, rather than justtwo, main trunk cables 111,112 and 113. At the conversion station, the V.H.F. signals from trunks 112 and 113 are split off and supplied directlyfor driving sub-trunks 114 and 115. However, the V.H.F.

signals from trunk 111 aresplit off and supplied to a bank of V.H.F. band-passfilters 116 at which the various channels and components are split up into different signal paths. The band II FM radio signals are separated out and supplied directly via driver/ampli fier 1 17to a combiner 118 onto sub-trunk 119. Each of the V.H.F. television channels, however, are supplied to respective converters 120 which are each supplied with a local oscillatorfrequency on a line 121 from a local oscillator reference generator 122. The local oscillatorfrequenciesfrom the generator 122 are all synchronised and phase-locked to the pilot frequency fed to the generator 122 on line 123 from the filters 116.

The generator 122 is arranged to generate local oscillatorfrequencies which convert each ofthe received incoming V.H.F. channels to the same common intermediate frequency. These common IF signals on respective signal lines 124 are fed from respective first converters 120 to respective second converters 125. The local oscillator reference generator 122 is also arranged to produce further local oscillator frequencies supplied to the second converters 125 to convert the Signals to respective different U.H.F. channels. The outputs of the second converters 125 are fed via respective driver/amplifiers 126 to the combiner 11 8for combining on the sub-trunk 119.

Figure7 illustrates an alternative form of distribution star-point incorporating a different selective distribution orswitching apparatus. the U.H.F. signals on sub-trunk 119 arefedvia splitters 130 and 131 and combiner 132 directly on to subscriber cable 133 so that all subscribers receive at leastthe basic channels which are converted to U.H.F. at the conversion stations.

The additional channels are supplied from subtrunks 114 and 1 15 to a bank 134 of band-pass filters where the various V.H.F. channels are split up into different paths. Once again the different V.H.F. channels are fed to respective first converters 135 by which converters all the V.H.F. channels are converted to the same intermediate frequency. The local oscillator signals supplied to the converters 135 are generated by local oscillator reference generator 136which is as before synchronised and phase-locked to the pilot signal from the sub-trunks. The various V.H.F. channels now converted to the same intermediate frequency are supplied on respective signal lines 137, 138, 139 etc, to a switch matrix 140.Switch matrix 140 is controlled by switching control signals from a control unit 141 to switch any two ofthe incoming different IF channels to a pair of output lines 142 for each subscriber. that is to say, there is a pair of output lines 142 for each of the subscribers which can select additional channels and the switch matrix 140 enabies simultaneous selection of up to two such channels to be made for each subscriber, from the available incoming channels.

The selected channels still at the same common intermediate frequency are fed on lines 142 to a pair of second converters 143 and 144which converts the selected IF channels to two different predetermined U.H.F.channelsforcombining atcombinerl32onto the subsrciber cable 133. the local oscillator signalsfor the second converters 143 and 144are again supplied by the local oscillator reference generator 136 and are phase-locked to the pilot signal.

The switch control unit 141 provides appropriate control signalsto the switch matrix 140 in accordance with data signals received overthe subscriber cable 133 from the subscribers' controller. In this way each subscriber is able to control the switch matrix to supply him with a desired pair simultaneously of the various available additional channels.

Because a switch matrix 140 is arranged to switch all the various additional channels at a common intermediate frequency, the design of the switch matrix 140 is considerably simplified.

Although two different basic forms of cable distribution system have been described in the above, itwill be appreciated that other variations are possible. For example, different forms of switching or selective distribution arrangement are possible from those described above. Furthermore, the above described forms of switching or selective distribution arrangement may be used in distribution systems otherthan the hybrid arrangement described.

Claims (26)

1. Selective signal distribution apparatus for selecting at least one of a plurality of frequency division multiplexed signals for distribution to a user, the plurality of f.d.m. signals being modulated on respective predetermined carrier frequencies, the apparatus comprising first conversion means to convert all the f.d.m. signals to a common Intermediate Frequency on separate respective signal paths, switch means responsive to selection control signals to selectthe IF signal from at least one of said paths, and second conversion means to convertthe selected IF signal to a predetermined different carrierfrequen cyfordistrihution to the user.

2. A distribution system as claimed in claim 1 arranged wherebythe pluraiity of f.d.m. signals are composite television signals including vision signals modulated on respective vision carrierfrequencies in theV.H.F. band and the predetermined different carrierfrequency is the vision carrierfrequency of a pre-selected standard U.H.F. television channel.

3. A wide band cabled T.V. distribution system comprising a head station, at least one distribution star-pointserving a number of subscribers, a trunk cable connecting the head station to the star-point to carryto the star-point a plurality oftelevision channels including a number of channels supplied asf.d.m.

V.H.F. signals, a respective subscriber cable connecting each subscriber to the star-point, and selective distribution means at the star-point including first conversion means to convert said number of channels to a common Intermediate Frequency on separate respective signal paths, switch means responsive to selection control signals from a respective controller of each of at least a proportion ofthe subscribers to select the I signal from at least one of said paths for distribution to the respective selecting subscriber, and second conversion means for each ofthe selecting subscribers to convert the selected IF signal for the respective subscriber to a predetermined U.H.F.

channelforsupplytothesubscribercableofthe respective subscriber.

4. A distribution system as claimed in claim 3 wherein the plurality of T.V. channels carried to the star-point comprise a predetermined basic group of channels and further additional channels, and there is at the star-point continuous distribution means to supply said predetermined basic group only of channels simultaneously on to the respective subscriber cables of each of at least a first group of the subscribers who receive only said basic group of channels, said proportion of the subscribers constituting a second group and said numberof V.H.F.

channels including said further additional channels.

5. A distribution system as claimed in claim 4 wherein at least the major runs of the trunk cable are arranged to carry' the television channels as frequency division multiplexed V.H.F. signals.

6. A distribution system as claimed in claim 5, wherein said basic group of channels are converted to U.H.F. signalsforsupply by said continuous distribution means along the subscriber cables of said first group of subscribers.

7. A distribution system as claimed in claim 6 and including distribution conversion points at each of which said V.H.F. to U.H.F. conversion is performed and the U.H.F. signals supplied to a plurality of star-points.

8. A distribution system as claimed in claim 7 wherein thef.d.m. V.H.F. signals supplied to the star-point are at synchronised frequencies, the first conversion means is arranged to produce synchro nised Ifsignalsfor all said further additional channels, and said second converters are arranged to convert the respective selected signals to a common U.H.F.

channel for all selecting subscribers.

9. A distribution system as claimed in any of claims3to 8wherein thef.d.m. V.H.F. signals are frequency spaced so as to reduce intermodulation distortion, the respective carriers being harmonically interrelated, and the conversion to U.H.F. employs synchronised local oscillators arranged such that conversion of an unwanted channel at the image frequency of a wanted channel produces an IF signal atexactlythesame IFas the wanted channel.

10. Awide band cabled T.V. distribution system comprising a head station, at least one distribution star-point serving a number of subscribers, a trunk cable connecting the head station to the star-point to carrytothestar-pointa plurality of television channels including a number of channels supplied asf.d.m.

V.H.F. signals, a respective subscriber cable connecting each subscriberto the star-point, and selective distribution means atthe star-point including, for each of at least a proportion ofthe subscribers, at least one superhetfrequency converter simultaneously receiving all channels of said number, a programmable local oscillator responsive to selection control signals from a controller operable by the subscriberto generate a respective local oscillatorfrequency supplied to the superhetfrequency converter to convert a selected one ofsaid number of channels to a predetermined U.H.F. channel and band-passfiltermeansto pass only the T.V. channel converted to said predetermined U.H.F. channel to the subscriber cable ofthe respective subscriber, wherein said predetermined U.H.F.

channel is a standard U.H.F. channel having a standard U.H.F. vision carrierfrequency, and the V.H.F. vision carrier frequencies of said number of channels are all arranged to be multiples ofthe nominal spacing of the standaed U.H.F. channels.

11. A distribution system as claimed in claim 10 wherein the head station is arranged to supply a reference frequency on the trunk cables to the star-point at a multiple of said nominal U.H.F. channel spacing and to phase-locktheV.H.F.vision carrier frequencies of said number of channels to said reference frequency and the programmable local oscillators are arranged to generate said local oscillatorfrequencies also phase-locked to the reference frequency.

12. A distribution system as claimed in claim 10 or 11 wherein each of the V.H.F. channels is spectrum inverted and each ofthe programmable local oscillators is arranged to generate local oscillatorfrequencites which are above said predetermined U.H.F.

channel frequency.

13. Adistribution system as claimed in any preceding claim wherein the plurality of T.V. channels carried to the star-point comprise a predetermined basic group of channels and further additional channels, and there is atthe star-point continuous distribution means to supply said predetermined basic group only of channels simultaneously on to the respective subscriber cable of each of at leastafirst group ofthe subscribers who receive only said basic group of channels, said proportion of the subscribers constituting a second group and said number of V.H.F.

channels including said further additional channels.

14. Adistributon system as claimed in claim 13 wherein at leastthe major runs of the trunk cable are arranged to carry the basic group of channels also as f.d.m. V.H.F. signals.

15. A distribution system as claimed in claim 14 wherein the basic group of channels are converted to U.H.F.channelsforsupplybysaidcontinuousdis- tribution means simultaneously along the subscriber cables of said first group of subscribers.

16. Adistribution system as claimed in claim 15, and including distribution conversion points at each of which said V.H.F. to U.H.F. conversion is performed and the U.H.F. signals supplied to a plurality of star-points.

17. A distribution system as claimed in claim 16 wherein the V.H.F. vision carrierfrequencies ofthe basic groupfchannels are also multiples ofthe nominal spacing ofthe standard U.H.F. channels, and the distribution conversion points are arranged to convert the basic channels to respective standard U.H.F. channels.

18. A distribution system as claimed in claim 17 as dependent from claim 2 and wherein the V.H.F. vision carrier frequencies ofthe basic group of channels are also phase-locked to the reference frequency and there are provided superhet converters to convert the basic channels to the standard U.H.F. channels and local oscillator generators to provide forthe converters local oscillatorfrequencies also phase-locked to the reference frequency.

19. Adistribution system as claimed in any of claims 13 to 18 wherein said basic group of channels are also supplied to the or each star-pointadditionally asf.d.m. V.H.F. signals for selection by said selective distribution means.

20. Awide band cabled TV. distribution system comprising a head station, at least one distribution star-point, a trunk cable connecting the head station to the star-pointto carrytothestar-pointa plurality of television channels comprising a predetermined basic group of channels and further additional channels, a respective subscriber cable connecting each subscri berto the star-point, continuous distribution means at the star-pointto supply said predetermined basic group only of television channels simultaneously on to the respective subscriber cable of each of at least a first group of subscribers who receives only said basic group of channels and selective distribution means at the star-point responsive to selection control signals from a controller operable bythe subscriber, being one of a second groupofsubscribers,to supply, on to the respective subscriber cable ofthe selecting subscriber, at least one selected television channel being selected from channels supplied to the starpoint including said additional channels.

21. A distribution system as claimed in claim 20 wherein at least the major runs ofthetrunk cable are arrangedto carry the television channels as frequency division multiplexed V.H.F. signals.

22. A distribution system as claimed in claim 21 wherein said basic group of channels are converted to f.d.m. U.H.F. signals for supply by said continuous distributution means simultaneously along the subscribercables of said first group of subscribers.

23. A distribution system as claimed in claim 22 and including distribution conversion points at each of which said U.H.F. conversion is performed and the U.H.F. signals supplied to a plurality of star-points.

24. Frequency conversion apparatusforconverting the carrierfrequencies of a plurality off.d.m.

signalsto aset of predetermined different carrier frequencies at a predetermined nominal frequency spacing, the apparatus comprising supply means to supply the plurality of f.d.m. signals together with a reference frequency and to phase-lockthe carrier frequencies of said f.d.m. signals to the reference frequency, with both said carrier frequencies and the reference freqlrency being respective multiples of said nominal frequency spacing, a respectivesuperhet frequency converterfor each ofthef.d.m. signals to be converted and a local oscillator generator arranged to generate and supplyto the converters locate oscillator frequencies also phase-lockedtothe referencefrequency.

25. Selective signal distribution apparatus s lb- stantially as herein before described with reference to and as illustrated in Figures 3 or7 ofthe accompany- ing drawings.

26. A wide band cabled T.V. distribution system substantially as hereinbefore described with referenceto and as illustrated in the accompanying drawings.