GB2127257A - 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 superhet converters to convert channels supplied at V.H.F. to a predetermined U.H.F. channel, using a frequency selectable 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.

Description

SPECIFICATION Distribution apparatus The present invention relates to distribution apparatus and particularly to selective signal distribution apparatus forselecting 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 ofthe various channels supplied on the subscriber's cable is done by selection equipment in the subscriber's residence.

Furthermore, if anytarriffs are to be charged by the cable companyforthe reception of particular channels, this must be achieved by encoding those channels on the cable so that only a subscriber with the rightform 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 by the subscriber using a controller in the subscriber's residence which signals along the subscriber's cable to the switching arrangement at the star-point to make the desired selection ofthe available channels.

In one aspect, the present invention provides a selective signal distribution apparatuswhich may be useful as the switching arrangement atthe distribution star-point of a star-type cabled distribution system. However the selective signal distribution apparatus ofthe present invention is not exclusively for this purpose.

According to one aspect ofthe present invention, there is provided 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 carrierfrequencies, 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 superhetfrequencyconverterto convert a selected one ofthef.d.m. signal carrier frequencies to a predetermined different carrierfrequency, and band-pass filter means to pass only the signal band about said predetermined different carrier frequency for distribution 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 carrier frequency is one of a set of such carrier frequencies at a predetermined nominal frequency spacing, and supply means are provided to supply said plurality of f.d.m. signals all at respective carrierfrequencies 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-lock the carrierfre quencies of said plurality of f.d.m. signals to the referencefrequencyandthe programmable local oscillator is also arranged to phase-lockthe local oscillatorfrequencies to the reference frequency. The advantage of this arrangement will become apparent later herein.

It may be appreciated that the 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 differentcarrierfrequencymay be the vision carrier frequency of a preselected standard U.H.F. television channel. Then the distribution apparatus may be used, for example in the distribution star-point of a cable system, to select one of a number of television signals at differentV.H.F. frequencies on a single cable forsupplyata 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-pointa plurality oftelevision channels including a number of channels supplied asf.d.m. VHF signals, a respective subscriber cable connecting each subscri berto thestar-point, and selective distribution means atthe star-point including,foreach of at least a proportion of the subscribers, at least one superhet frequency converter simultaneously receiving all channels ofsaid 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 superhet frequency converter to convert a selected one of said number of channels to a predetermined U.H.F. channel and band-pass filter means to pass only the T.V. channel converted to said predetermined U.H.F. channel to the subscriber cable of the 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 carrier frequencies of said number of channels are all arranged to be multiples of the 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 of this arrangement arises because the local oscillatorfrequency required to convert one of the V.H.F.vision carrierfrequencies to a standard U.H.F. vision carrier frequency is, when in the U.H.F. television band, exactly at the vision carrier frequency of one ofthe standard U.H.F. channels. Thi advantage will be explained more fully 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 nominalU.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. In this way, all the frequencies generated in the distribution system are phase-locked to one another.

Preferably, each oftheV.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 oscillatorfrequency above the intended U.H.F.

channel frequency, spectrum inversion of the V.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 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 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 number of V.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 star system. 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 manner as 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 primarily tree 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 atany onetimewith only selected additional channels.

Normally, at least the major runs ofthetrunkcables are arranged to carry the 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. channelsfor supply by said continuous distribution means simul taneouslyalong the subscriber cables of said first group of subscribers. It would be appreciated that the losses along a distribution cable at U.H.F. are con siderably 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. signals then supplied to a plurality of star-points.

Preferably, the V.H.F. vision carrier frequencies 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 convert the basic channels to respective standard U.H.R channels.

Conveniently, the V.H.F. vision carrierfrequencies of the 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 for the converters local oscillatorfrequencies also phase-locked to the reference frequency.

It may be convenient for said basicgroupof 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 cabled T.V. distribution system comprising a head station, at least one distribution star-point, trunk cables connecting the head station to the star-point to 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 subscriberto the star-point, continuous distribution means at the star-point to supply said predetermined basic group only oftelevision channels simultaneously on to the respective subscriber cable of each of at least a first group of subscribers who receive only said basic group of channels and selective distribution means at the star-point responsive to selection control signals from a controller operable by a 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 of f.d.m. signals being modulated on respective predetermined carrierfrequencies, the apparatus comprising first conversion means to convert all the f.d.m. signals to a common Intermediate Frequency on separate respective signals paths, switch means responsive to selection control signals to select the IF signal from at least one of said paths, and second conversion means to 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 con nectingthe headstationtothestar-pointto carrytothe star-point a plurality of television channels including a number of channels supplied as f.d.m.V.H.F. signals, a respective subscriber cable connecting each su bscribertothestar-point, and selective distribution means including first conversion means to convert said number of channelsto 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 ofthe subscribers to select the IF signal from at least one ofsaid 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. channel for supply to the subscriber cable ofthe respective subscriber.

The present invention stillfurtherenvisagesfre- quency conversion apparatus for converting the carrierfrequencies of a plurality off.d.m. signals to a set 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 togetherwith a reference frequency and to phase-lock the carrier frequencies of said f.d.m.

signals to the reference frequency, with both said carrierfrequencies and the reference frequency being respective multiples of said nominal frequency spacing, a respectivesuperhetfrequencyconverterfor each ofthef.d.m. signals to be converted and a local oscillator generator arranged to generate and supply to the converters local oscillator frequencies also phase-locked 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 a block schematic diagram illustrating conversion apparatus art a conversion sitefor converting channels of the basic group from 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 application to subscriber cables; Figure 4 illustrates schematically the installation at a subscriber; Figure 5 is a table of the V.H.F. frequency plan for the various V.H.F. channels supplied over the main trunk cables;; Figure 6 is a block schematic diagram of an alternative form of conversion apparatus art a conversion point and operating with three main trunk cables and Figure 7 is a block schematic diagram illustrating an alternative form of distribution star-point operating from threetrunk 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 trunkcables 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 signals transmitted along the main trunks 11 and 12 are transmitted as frequency division multiplexed V.H.F. signals. With an upper frequency limitof400MHzforsignals in these main trunkcables 11 and 12, a maximum reach forthetrunkcables may be obtained of between 8 and 10 km using twenty repeaters spaced at 24dB loss at 400MHz. However with a slight increase in upper frequency limit of up to 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 at which some of the V.H.F. television channels are converted to standard U.H.F. channels for onward transmission on sub-trunks 15 and 16 etc. The sub-trunks 15 and 16 feed 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 13, 14tothevarious distribution star-points 17,18 are kept significantly shorter to avoid excessive loss at U.H.F. The television channels which are converted atthe 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 at V.H.F. on sub-trunks 15 and 16, for additional supply on the subscriber's cables to selecting subscribers.

Figure 2 illustrates an arrangement for converting the basic group ofchannelsfrom V.H.F. to U.H.F.

Spurs 20 and 21 are taken from the main trunk cables 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 spur21 directly to a further amplifier/driver 26 to drivethe second sub-trunk 16 supplying the signals still at V.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 oftheV.H.F. signals from the main trunk 11 via an amplifier 31 and a multi-way splitter 32 to each of a bank of V.H.F. channel pass filters 33. The filters 33 are each tuned to pass one ofthe various V.H.F. channels of at least a basic group of the 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 of the demultiplexed V.H.F.

signals ofthe basic group are then supplied to a respective V.H.F./U.H.F. converter 35 which comprises a superhetconverterfed with a local oscillator frequency from a multiple frequency output local oscillator 36.

Aswill become apparent, the signals supplied along the main trunks 11 and 12 also include at least one nilottone to which the V.H.F. carrier frequencies on the main trunks are synchronised or phase-locked.

One of the filters 33 (the filter 37) is tuned to select the pilot frequency for supply to the local oscillator36. The local oscillator36 is arranged to produce on lines 38, 39 etc, local oscillator signals at frequencies phase lockedtothe pilotfrequency and selected to convert the respective V.H.F. signals supplied to converter 35 to predetermined different U.H.F. channels. It will 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 pilotfrequency, 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 combiner40forfeeding on a single line 41 as frequency division multiplexed U.H.F.

signals via an amplifier/driver 42 and diplexer 30 on to the first sub-trunk 15. Thus, sub-trunk 15 carries both the basic group ofthe channels at U.H.F. andthe 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 amplifiers 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 56 to a star splitter 57 which supplies U.H.F. channels directlyto the subscriber cables 58 of each of a first group of subscribers entitled to receive only the basic group of channels.

It will be appreciated thatthe signals supplied on sub-trunk 15, from main trunk 11, also include the FM radio signals in band II 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 arranged to pass only the band II FM radio frequencies. These band II signals are fed through a further 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 splitter63 to a star splitter 64. Atthe same time, the V.H.F. signals from sub-trunk 16 are fed via amplifier 65 to star splitter 66.

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 pair of V.H.F. to U.H.F. converters 67 and 68. The converter 67 includes an input switch 69 able to select eithertheV.H.F. signals from sub-trunk 15 via one of the outputs of star splitter 64, orthe V.H.F. signals of sub-trunk 16 via one ofthe outputs of star splitter 66.

Each converter 67,68 receives a reference frequency from a reference generator 71. The reference frequency from the reference generator 71 is phase-locked to the pilot frequency supplied to the reference generatoron line72from 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 generator 71 but at a frequency dependant on selection control signals on line 73 from a data control unit 74. The data control unit 74 is arranged to produce control signals on line 73 in response to data signals received on line 75 and transmitted along the subscriber cable 76 from a controller at the subscriber. The control signals on line 73 enable the subscriber not only to control the local oscillatorfrequency in each of converters 67 and 68 but also to control the operation ofthe selection switches 69 and 70.The control signals on line 73 are responsive to channel selection by the subscriber to produce local oscillator frequencies 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 buy a U.H.F. channel band-pass filter provided at the output of each ofthe converters 67 and 68. In this way, the local oscillator frequency is set so that only one of the incoming V.H.F.

signals is converted to the predetermined U.H.F.

channel and can passtheband-passfilter.The frequencies to which non-selected incoming V.H.F.

signals are converted fall outside the pass-band of the U.H.F. band-pass filter.

In this way, 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 star splitter 78.

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 the first Intermediate Frequency for reception of DBS signals (probably between 900MHz and 1500 MHz).

Again this channel is combined at combiner 77 on to the subscriber cable 76. This sub-group of V.H.F.

incoming signals may constitute wide band television channels of a sort different from the usual colour television signals. The normal colourtelevision signals envisaged in one embodiment ofthis invention are PAL system I which have a band width 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 onlythe 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 for two-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 of the trunk cables, sub-trunk cables and each of the subscriber cables in a frequency band below the lowestfrequencies usedforV.H.F.televisionchan- nels. A modem 90 demodulates data spurred off sub-trunk 15 at splitter 50 for feeding 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 at a driver 93. 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 instaliation 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 attwo 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 as those along the trunks and subtrunks 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 carrier frequencies and in general the best performance is achieved when all the vision carrier frequencies 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 ofthe spectrum of the T.V. signals so that, in orderto 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.

Itis desirable that the minimum spacing between adjacent vision carrier signals on thetrunkcables is between 15 and 16MHz. Furthermore, in the available V.H.F. spectrum (50to 400MHz) for the V.H.F. signals, there are a number of prohibited frequencies at or near to which carrier signals or high energy side bands are not permitted by the regu latory 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 of 15.6MHz spacing 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 beseen in thefollowing 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 .25M Hz vision carrier). A permissable V.H.F. vision carrier based on harmonics of 7.8MHzwould be 296.4MHz. In orderto convert 296.4MHzto 471.25MHz, a local oscillator of 767.65MHz is required.

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

One way to alleviate this problem would be to arrange forthe two U.H.F. channels provided by the cable distribution system on the subscribers' cable to be at 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 of the first 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 convert V.H.F.

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

Thesevariousoff-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 8MHz forthe 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 isitselfalwaysatthevisioncarrierfrequency of another U.H.F. channel and will accordingly zero beatwiththatotherchannelrequiring only 30d B suppression.

Figure 5 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 minimum carrier spacing of 16MHz. As will be seen, uptotwenty-oneV.H.F. channels can beaccommo- dated if the top end of the band is extended to 450MHz.

The usual pilot carriers are also produced at multiples of8MHz, for example at 40MHz and 456MHz. The head station 10 is arrangedto phase-lockthevision carriers of all the V.H.F. channels transmitted on to the trunk cablestothepilotfrequency. Furthermore,thelocal oscillators and reference generators at both the conversion stations and the distribution star-points as described above, are also arranged to generate local oscillator frequencies which are phase-locked to the pilot. In this way all frequencies produced in the system are mutually synch ronised and phase-locked so that all interfering signals zero-beat with one another.

Plan 2 illustrated in Figure5 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 desirable for DBS transmission. Plan 3 is similar to plan 1 exceptthatthe first nine V.H.F. channels are the same as plan 1.

It will be noted that ali the V.H.F. channels supplied 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.

frequencies 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 problemsofintermodulation during subsequent wide band amplification of the U.H.F.

signals at repeater stations. The incremental relationship nulls all odd orderintermodulation 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 6 there are illustrated three, rather thanjusttwo, maintrunkcables 11 1,112 and 113. At the conversion station,theV.H.F. signalsfromtrunks 112 and 1 are split offand supplied directlyfor driving sub-trunks 114 and 115. However, the V.H.F.

signals from trunk 111 are split offand supplied to a bank of V.H.F. band-pass filters 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/amplifier 117 to 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 oscillatorfrequencyon 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 oscillatorfrequencieswhich 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 oscillatorfrequencies supplied to the second converters 125 to convert the lFsignalsto respective different U.H.F. channels. The outputs ofthe second converters 125 are fed via respective driver/amplifiers 126 to the combiner 118 for combining on the sub-trunk 119.

Figure 7 illustrates an alternative form of distribution star-point incorporating a different selective distribution or switching apparatus. The U.H.F. signals on sub-trunk 119 are fed via 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 sub trunks 114and 115to a bank 134Of band-passfilters where the various V.H.F. channels are split up into different paths. Once again the differentV.H.F. chan- nels 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 136 which is as before sychronised and phase-locked to the pilot signal from the sub-trunks. The various V.H.F. chan nelsnowconvertedtothesame intermediate frequen cy 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 of the 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 ofthe subscribers which can select additional channelsandthe switch matrix 140 enables simultaneous selection of up to two such channels to be made for each subscriber, from the available incoming channels.

The selected channels still atthesame common intermediate frequency are fed on lines 142 to a pair of second converters 143 and 144 which converts the selected IF channels to two different predetermined U.H.F. channels for combining at combiner 132 on to the subscriber cable 133. The local oscillator signals for the second converters 143 and 144 are 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 signals to the switch matrix 140 in accordance with data signals received over the subscriber cable 133 from the subscribers' controller. In this way each subscriberis ableto control the switch matrixto supply him with a desired pairsimultaneouslyofthe 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, it will be appreciated that other variations are possible. For example, different forms of switching or selective distribution arrangementarepossiblefromthose described above. Furthermore, the above described forms of switching or selective distribution arrangement may be used in distribution systems other than 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 carrierfrequencies, the apparatus comprising at least one superhetfrequency converter simultaneously receiving all the plurality of f.d.m. signals, a programmable local oscillator responsiveto selection control signals to generate a respective local oscillatorfrequency supplied to the superhet frequency converter to convert a selected one ofthef.d.m. signal carrier frequencies to a predetermined different carrier frequency, and band passfiltermeanstopassonlythesignal band about said predetermined different carrier frequencyfor distribution to the user.

2. Distribution apparatusasclaimed in Claim 1 wherein said predetermined differentcarrierfrequency is one of a set of such carrierfrequencies at a predetermined nominal frequency spacing, and wherein supply means are provided to supply said plurality off.d.m. signals all at respective carrier frequencies which are multiples of said nominal spacing.

3. Distribution apparatus as claimed in Claim 2 wherein the supply means is arranged to supply a reference frequency at a multiple of said nominal spacing and to phase-lock the carrier frequencies of said plurality of f.d.m. signals to the reference frequency and the programmable local oscillator is also arranged to phase-lock the local oscillatorfre quenciestothe reference frequency.

4. Distribution apparatus as claimed in any preceding claim arranged whereby the plurality off.d.m.

signals are composite television signals including vision signals modulated on respective vision carrier frequencies in the V.H.F. band and the predetermined different carrierfrequency is the vision carrier fre quencyofa pre-selected standard U.H.F. television channel.

5. Awide band cabled T.V. distribution system comprising a head station, at least one distribution star-point serving a numberof subscribers, a trunk cable connecting the head station to the star-point to carry to the star-point a plurality oftelevision channels including a number of channels supplied as f.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 of said number of channels to a predetermined U.H.F. channel and band-pass filter means to pass onlythetV.channel convertedtosaid predetermined U.H.F. channel to the subscriber cable of the respective subscriber.

6. A distribution system as claimed in Claim 5 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 standard U.H.F. channels.

7. A distribution system as claimed in Claim 6 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-lockthe V.H.F. vision carrier frequencies of said number ofchannelsto said reference frequency and the programmable local oscillators are arranged to generate said local oscillatorfrequencies also phase-locked to the reference frequency.

8. A distribution system as claimed in Claim 5 to 7 wherein each of the V.H.F. channels is spectrum inverted and each ofthe programmable local oscillators is arranged to generate local oscillator frequencies which are above said predetermined U.H.F.

channel frequency.

9. A distribution system as claimed in any of Claims 5to 8 wherein the plurality of T.V. channels carriedto 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 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 number of V.H.F.

channels including said further additional channels.

10. A distribution system as claimed in Claim 9 wherein at least the major runs ofthe trunk cable are arranged to carry the basic group of channels also as f.d.m. V.H.F signals.

11. A distribution system as claimed in Claim 10 wherein the basic group of channels are converted to U.H.F. channels for supply by said continuous distribution means simultaneously along the subscriber cables of said first group of subscribers.

12. A distribution system as claimed in Claim 11, 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.

13. A distribution system as claimed in Claim 12 wherein the V.H.F. vision carrier frequencies of the basic group of channels are also multiples ofthe nominal spacing of the standard U.H.F. channels, and the distribution conversion points are arranged to convert the basic channels to respective standard U.H.F. channels.

14. A distribution system as claimed in Claim 13 as dependent from Claim 7 and wherein 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 channelsto the standard U.H.F.channels and local oscillator generators to provide forthe converters local oscillatorfrequencies also phase-locked to the reference frequency.

15. A distribution system as claimed in any of Claims 9to 14wherein said basic group of channels are also supplied to the or each star-point additionally asf.d.m. V.H.F. signals for selection by said selective distribution means.

16. Awide band cabled T.V. distribution system comprising a head station, at least one distribution star-point, a trunk cable connecting the head station to the star-pointto carrytothe star-point a plurality of television channels comprising a predetermined basic group of channels and further additional channels, a respective subscriber cable connecting means at the star-pointto supply said predetermined basic group only of television channels simultaneously onto 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 by the 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 starpoint including said additional channels.

17. A distribution system as claimed in Claim 16 wherein at least the major runs of the trunk cable are arranged to carrythe television channels as frequency division multiplexed V.H.F. signals.

18. A distribution system as claimed in Claim 17 wherein said basic group of channels are converted to f.d.m. U.H.F. signals for supply by said continuous distribution means simultaneously along the subscribercables of said first group of subscribers.

19. A distribution system as claimed in Claim 18 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.

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

21. A distribution system as claimed in Claim 20 arranged whereby the plurality of F.D.M. signals are composite television signals including vision signals modulated on respective vision carrier frequencies in the V.H.F. band and the predetermined different carrier frequency is the vision carrierfrequency of a pre-selected standard U.H.F. television channel.

22. 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-pointto carry to the star-point a plurality of television 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 signalsfromthe respective controller of each of at least a proportion of the subscribers to selectthe IF 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 convertthe selected IF signal for the respective subscriberto a predetermined U.H.F.

channel for supply to the subscriber cable of the respective subscriber.

23. A distribution system as claimed in Claim 22 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 supplysaid predetermined basicgrouponlyof 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 constitut ing a second group and said number of V.H.F.

channels including said further additional channels.

24. A distribution system as claimed in Claim 23, wherein at leastthe major runs of the trunk cable are arranged to carrythetelevision channels as frequency division multiplexed V.H.F. signals.

25. A distribution system as claimed in Claim 24, wherein said basic group of channels are converted to U.H.F. signals for supply by said continuous distribution means along the subscriber cables of said first group of subscribers.

26. Awide band cabled T.V. distribution system substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

26. A distribution system as claimed in Claim 25 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.

27. A distribution system as claimed in Claim 26 wherein thef.d.m. V.H.F. signals supplied to the sta r-po int a re at synchronised frequencies, the first conversion means is arranged to produce synchronised IF signals for 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.

28. A distribution system as claimed in any of Claims 22 to 27 wherein the f.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 atthe image frequency of a wanted channel produces an IF signal at exactly the same IF as the wanted channel.

29. Frequency conversion appa ratus for convert- ing the carrier frequencies of a plurality of f.d.m.

signals to a set of predetermined different carrier frequencies at a predetermined nominal frequency spacing, the apparatus comprising supply means to supplythe plurality of f.d.m. signals together with a reference frequency and to phase-lock the carrier frequencies 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 superhet frequency converterfor each of the f.d.m. signals to be converted and a local oscillator generator arranged to generate and supply to the converters local oscillator frequencies also phase-locked to the reference frequency.

30. Selective signal distribution apparatus substantially as hereinbefore described with reference to and as illustrated in Figures 3 or7 ofthe accompany- ing drawings.

31. Awide band cabled T.V. distribution system substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

New claims or amendments to claims filed on 17-11-83.

Superseded claims 1-26.

Neworamended claims:-

1. 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 carry to the star-point a plurality of television channels including a number of channels supplied as f.d.m.

V.H.F. signals, a respective subscriber cable connect ing each subscriberto the star-point, and selective distribution means atthe star-point including, for each of at least a proportion of the subscribers, at least one superhetfrequencyconvertersimultaneously receiving all channels of said number, a programmable local oscillator responsibe to selection control signals from a controller operable by the subscriberto generate a respective local oscillatorfrequency supplied to the superhetfrequency converterto convert a selected one of said number of channels to a predetermined U.H.F.channel and band-pass filter means to 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 carrier frequency, and the V.H.F. vision carrier frequencies of said number of channels are all arranged to be multiples of the nominal spacing of the standard U.H.F. channels.

2. A distribution system as claimed in Claim 1 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-lockthe V.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.

3. Adistribution system as claimed in Claim 1 or2 wherein each ofthe 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.

4. A distribution 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 meanstosupplysaid predetermined basicgrouponly of channels simultaneously on to the respective subscriber cable of each of at least a first group of the subscriberswho 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.

5. A distribution system as claimed in Claim 4 wherein at least the major runs ofthe trunk cable are arranged to carrythe basic group of channels also as f.d.m. V.H.F. signals.

6. A distribution system as claimed in Claim 5 wherein the basic group of channels are converted to U.H.F. channels for supply by said continuous distribution means simultaneously 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 andthe U.H.F. signals suppliedto a plurality of star-points.

8. A distribution system as claimed in Claim 7 wherein the V.H.F. vision carrierfrequencies of the basic group ofchannels are also multiples of the nominal spacing of the standard U.H.F. channels, and the distribution conversion points are arranged to convert the basic channels to respective standard U.H.F. channels.

9. A distribution system as claimed in Claim 8 as dependentfrom Claim 2 and wherein the V.H.F. vision carrier frequencies of the basic group of channels are also phase-lockedtothe reference frequency and there are provided superhet converters to convert the basic channelsto the standard U.H.F. channels and local oscillator generators to provide forthe converters local oscillatorfrequencies also phase-locked to the reference frequency.

10. A distribution system as claimed in any of Claims 4to 9 wherein said basic group of channels are also supplied to the or each star-pointadditionally as f.d.m. V.H.F. signals for selection by said selective distribution means.

11. Awide band cabled T.V. distribution system comprising a head station, at least one distribution star-point, a trunk cable connecting the head station to the star-point to 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 berto the star-point, continuous distribution means at thestar-pointto supply said predetermined basic group only of television channels simultaneously onto 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 by the 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.

12. A distribution system as claimed in Claim 11 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.

13. A distribution system as claimed in Claim 12 wherein said basic group of channels are converted to f.d.m. U.H.F. signals for supply by said continuous distribution means simultaneously along the subscriber cables of said first group of subscribers.

14. A distribution system as claimed in Claim 13 and including distribution conversion points at each of which said U.H.F. conversion is performed andthe U.H.F. signals supplied to a plurality of star-points.

15. 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 thef.d.m. signals to a common Intermediate Frequency on separate respective signal paths, switch means responsive to selection control signals to select the IF signal from at least one of said paths, and second conversion means to convertthe selected IF signal to a predetermined different carrier frequency for distribution to the user.

16. A distribution system as claimed in Claim 15 arranged whereby the plurality off.d.m. signals are composite television signals including vision signals modulated on respective vision carrier frequencies in theV.H.F. band and the predetermined different carrierfrequency is the vision carrier frequency of a pre-selected standard U.H.F. television channel.

17. 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-pointto carry to the star-point a plurality oftelevision channels including a number of channels supplied as f.d.m.

V.H.F. signals, a respective subscriber cable connecting each subscriberto the star-point, and selective distribution means atthe 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 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 ofthe selecting subscribers to convertthe selected IF signal for the respective subscriberto a predetermined U.H.F.

channel for supply to the subscriber cable ofthe respective subscriber.

18. A distribution system as claimed in Claim 17 wherein the plurality of T.V. channels carried to the star-pointcomprise 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 ofthe subscribers who receive only said basic group of channels, said proportion of the subscribers constitut ing a second group and said number of V.H.F.

channels including said further additional channels.

19. A distribution system as claimed in Claim 18, wherein at leastthe major runs ofthe trunk cable are arranged to carrythetelevision channels as frequency division multiplexed V.H.F. signals.

20. Adistribution system as claimed in Claim 19, 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.

21. A distribution system as claimed in Claim 20 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.

22. A distribution system as claimed in Claim 21 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 synchronised IF signals for 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.

23. A distribution system as claimed in any of Claims 17 to 22 wherein the f.d.m. V.H.F. signals are frequency spaced so asto reduce intermodulation distortion, the respective carriers being harmonicaily interrelated, and the conversion to U.H.F. employs synchronised local oscillators arranged such that conversion of an unwanted channel attheimage frequency of a wanted channel produces an IF signal at exactly the same IF as the wanted channel.

24. Frequency conversion apparatus for converting the carrier frequencies of a plurality of f.d.m.

signals to a set of predetermined different carrier frequencies at a predetermined nominal frequency spacing, the apparatus comprising supply means to supply the plurality off.d.m. signalstogetherwith 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 frequency being respective multiples of said nominal frequency spacing, a respective superhet frequency converterfor each of the f.d.m. signals to be converted and a local oscillator generator arranged to generate and supply to the converters local oscillator frequencies also phase-locked to the referencefrequency.

25. Selective signal distribution apparatus substantially as hereinbefore described with reference to and as illustrated in Figures 3 or7 of the accompanying drawings.