GB2191370A - Cable television distribution system - Google Patents

Abstract

In a cable distribution system VHF distribution is followed by up conversion to UHF at a subscribers premises involving multiple frequency conversions to maintain sound-vision carrier spacing whilst minimising intermodulation, and to provide adequate selectivity. The invention proposes distribution with an inverted sound-vision carrier relationship with a single conversion subtraction type frequency changer for each subscriber. The channel spacing may also be-increased (eg 12-16 MHz) and the frequency changer is switchable to convert either half of the distribution frequency band into a single output band. <IMAGE>

Description

SPECIFICATION Cable TV distirbution system In large citywide tree structured cabled TV distribution systems it is usual to distribute all the signals in the band up to about 450MHz. This can accommodate some 30 or more television channels using a frequency plan based on 8 MHz spacings for the vision carriers. At least for channels are usually allocated on 16MHz spacing and used to distribute the locally available terrestrial broadcast signals.

In the subscribers premises there are usually provided two frequency changing devices:- (a) a highlyselective switched single channel unit capable of converting any selected one of the incom ingVHFchannelsand providingasingle UHFoutput channel; and (b) A broadband "block" converter covering the four (or more) 16 MHz spaced channels only. It is arranged to accept the set of 16 MHz spaced channels and simultaneously convertthem to a set of four (or more) channels at UHF.

The outputs of the two converters are combined togetherto provide the subscriberwith a connection on which there are four broadcast services plus one selected channel. This arrangement allows him to operate his television receiver and video cassette recorder, neither of which can adequately operate on 8 MHz channel spacing, in much the same manner as if they were connected to a local aerial in addition to his selected "cable" channel. In such systems the single channel unit (a) usually involves several frequency conversion passing via a low VHF intermediate frequency in orderto obtain sufficient selectivity to provide adequate adjacent channel rejection. The design of the conventional wideband "block-converter" (b) employs a double superheterodyne technique.

To avoid problems of 2nd and 3rd order intermodulation both the conversions are ofthe "subtraction" type achieving output channels with the same relationship of vision and sound carriers as those present atVHFonthe input. (ASingle "subtraction" conversion of course reverses these relationships).

In smaller distribution systems, where the number ofTVchannels required is likely for some considerable time to be restricted to perhaps 15 or 16, and where costs are even more important, there is a need to simplify the arrangement of equipment in the subscribers home. This is particularly true where operator intends to market his programme package on a "one price-all or nothing" basis. It is the purpose of this invention to provide a system which meetsthis need.

According to the present invention there is provided a cable television distribution system in which a plurality of channels are distributed in a first frequency band to each of a plurality of receiving units, each receiving unit having a single stage subtraction type frequency changer operabieto convert at teast a part of the first frequency band, comprising at least somme ofthesaid channels, to a second frequency band.

Thus, to reduce the complexity (and thus cost) of the converter it is proposed to operate the incoming (eg VHF) signals "inverted", ie vision carrier above sound carrier, and employ a single superheterodyne "subtraction" frequency changer. The output signals being then in a form to be received by a conventional television receiver. By this meansthe intermodulation problems inherent in a conventional "addition" single frequencychangerare overcome.

In orderto operate the system with only wideband converters, iewithoutthecostlysinglechannel converters, it is necessary, to overcome the limited selectivity of the TV receivers, to increase the spacing between the channels at VHF to something more than 8MHz. Using a 16 spacing would satisfy this requirement, but taking account ofvariousfactors, selectively, local oscillatorfrequencies, image rejection etc., in the UHF receiver allows a possible spacing of 13.9 MHzto be used. AVHFfrequency plan using thisspacingwillallowtheallocationofsomei6 channels with an upperfrequency limit of about 340 MHz.

Thus in a preferred embodiment ofthe invention the channel spacing ofthe said channels lies in the range 12to 16MHz.

Preferablythefrequency changer ofthe receiving unit is switchable to convert selectively an upper or lower half ofthefirstfrequency band to the second frequency band.

Thus the local oscillator in the converter can be switched between two frequencies such that 16 input channels (say) can be converted in two blocks of 8 to a singlesetof8channelsatUHF.Thusthenumberof tuning positions requird on the subscribers TV receiver is limited to 8 and the bandspace occupied bythe converted channels is reduced from about 230 MHzto about 120 MHz. This latter is extremely important to the system designer who must allocate his frequenciesto avoid interference to terrestrial "off air" reception and interference from the local TV transmit- ters.

One embodiment of the present invention will now be described with reference to the accompanying drawing which shows in diagrammatic form one proposed arrangement with an example of the frequencies involved. Sixteen television channels are generated in the band 120-350 MHz by conventional means, exceptthatthe sound-vision carrier relationship is inverted and the channel spacing is 13.9 MHz. Typical frequencies are shown in the table.

Distribution is via a tree-structure cable distribution system, to a plurality of subscriber interface units one ofwhich is shown in the drawing. The unit operates as foilows: i)Awideband input signal (120-350 MHz) is applied via an equalizer 1 and variable gain control 2 to a low noisewideband preamplifier 3.

ii) The amplified signal is then frequency changed to UHF in a wideband double balanced mixer 4.

iii} Afurther amplifier 5 provides additional gain (at UHF). This ampiifier also tends to isolatethe mixer from any unwanted signals (eg local oscillator) emnating from a connected TV receiverand/orVCR.

iv}The tocal oscillatorforthewideband frequency change is achieved with a frequency synthesiser 6 referenced to a low frequency (8-10 MHz) crystal oscillator 7. The "programme" forthe synthesiser is switched (8) so as to produce the two frequencies required.

Such an arrangement gives a number of benefits to both the system designer and the subscriberto the system: a) The inverted distribution channels/single superheterodyne frequency changer reduces complexity and thus costs.

b) The "block" conversion allows the subscriber to operate hisVCR in a mannersimilarto that which he would use with a conventional aerial.

c) The switched "dual block" frequency change doubles the number of channels available without requiring additional tuning positions on the subscri- bers equipment.

d)Thedual block arrangement simplifies the allocation of usable frequencies at UHF.

e) The fully wideband design ofthe RF path simplifies the manufacture of units for different sets of output frequencies (for different locations with diffe rent off-air broadcast channels). It is only necessary to alterthe "program me" forthesynthesiser.

The simple overall design should result in a relatively small and low cost unit in the subscribers home. This must reduce maintenance costs and the risks associated with possible loss ofthe unit.

The inverted distribution channels provide a small degree ofsecurity against the "illegal" connection of conventional block converters and/or the use of imported TV receivers with VHFtuning capabilities.

Claims (4)

1. A cabletelevision distribution in which a plurality of channels are distributed in a first frequency band to each of a plurality of receiving units, each receiving unit having a single stage subtraction type frequency changer operable to convert at least a part ofthefirst frequency band, comprising at least some of the said channels, to a second frequency band.

2. A distribution system according to claim 1 in which the channel spacing ofthe said channels lies in the range 12to 16 MHz.

3. A distribution system according to claim 1 or 2 in which the frequency changer of the receiving unit is switchable to convertselectively an upper or lower half ofthe firstfrequency band to the second frequency band.

4. Acabletelevision distribution system substantially as herein described with reference to the accompanying drawing.