GB2167276A - Cable distribution system - Google Patents

Abstract

A plurality of subscriber stations each with a plurality of outlets at which television signals broadcast over the system are to be made available, are connected to a plurality of switchpoints each connected to a respective group of subscriber stations. Each subscriber station is connected to its respective switchpoint by at least one optic fibre 16, and a plurality of optic fibre trunk cables 8-12 interconnecting the switchpoints and a plurality of television signal sources 1-5. PCM television signals are applied to each trunk fibre, each sample of each PCM television signal on a trunk fibre being allocated a respective time slot. Each switchpoint comprises a selector 13, 17 in respect of each outlet of each subscriber station in its respective group, the selectors being controllable from the respective subscriber station outlet to select a desired television signal by selecting samples of that television signal appearing in appropriate time slots. The switchpoint also comprises means 18 for allocating selected television signals to respective time slots on the fibre optic cable 16 leading to the subscriber unit, and each subscriber unit outlet comprises means for selecting the signal samples appearing in the appropriate time slots and for decoding the received signals to produce the desired television signal. The PCM coded television signals are distributed over the entire system so as to be generally available at each switchpoint but the final destination of the bit streams delivered to the switchpoints is determined by the recipients of those bit streams rather than from the head end of the system. <IMAGE>

Description

SPECIFICATION Communication system The present invention relates to a communication system and in particular to a communication system in which television signals are distributed to subscribers via fibre optic cables.

A large number of proposals have been made in the past for distributing television signals to subscribers from a central source of television signals. Older systems comprise conductive networks but more recently systems relying upon optic fibres have been proposed and in some cases installed.

It is well known to employ PCM signalling techniques in telecommunication systems. In PCM systems the signals are periodically sampled and digital data representative of the sampled signals are transmitted serially. At the receiving end of the system it is necessary to reconstitute the initial signal by appropriate decoding of the PCM signal transmitted over the system.

In prior art PCM systems the bit streams transmitted always contain address data which defines the intended destination of the data represented by the bit streams. This "labelling" of the various elements of the data being transmitted is not a problem when the intended destination of the data can be predetermined. In the case of television signal distribution systems however different requirements apply as it is necessary to make available over the system a wide range of television signals and to enable subscribers to control the selection of television signals to be delivered to their own receivers. It is presumably for this reason that the PCM techniques so well known in other fields of the telecommunications have not been applied to general cable broadcasting systems.

It is an object of the present invention to provide a communication system relying upon PCM signalling techniques for distributing signals from a number of signal sources to a substantial number of subscribers.

According to the present invention there is provided a communication system comprising a plurality of subscriber stations each with a plurality of outlets at which television signals broadcast over the system are to be made available, a plurality of switchpoints each connected to a respective group of subscriber stations, each subscriber station being connected to its respective switchpoint by at least a single optic fibre, and a plurality of optic fibre trunk cables interconnecting the switchpoints and a plurality of television signal sources, wherein a plurality of PCM television signals are applied to each trunk fibre, each sample of each PCM television signal on a trunk fibre being allocated a respective time slot in a repeating sequence of time slots, each switchpoint comprises a selector in respect of each outlet of each subscriber station in its respective group, the selectors being controllable from the respective subscriber station outlet to select a desired television signal by selecting samples of that television signal appearing in appropriate time slots, the switchpoint comprising means for allocating selected television signals to respective time slots in a repeating sequence of time slots on the fibre optic cable leading to the subscriber unit, and each subscriber unit outlet comprising means for selecting the signal samples appearing in the appropriate time slots and for decoding the received signals to produce the desired television signal.

Thus, in the above system PCM coded television signals are distributed over the entire system so as to be generally available at each switchpoint but the final destination of the bit streams delivered to the switchpoints is determined by the recipients of those bit streams rather than from the head end of the system.

The use of optic fibres rather than conductive cables reduces the required volume of equipment at the switchpoint for a given number of subscribers and therefore a larger number of subscriber units can be supplied from a reasonably sized switchpoint. The use of optic fibres also provides the advantage that the maximum switchpoint to subscriber unit distance can be greater using optic fibres than when using conductive cables.

Preferably the PCM television signals applied to the trunk optic fibres are initially synchronised and re-synchronised at the switchpoints. Re-synchronisation may be achieved using shift registers and short cable sections after detection of the signls received at the switchpoints.

An embodiment of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic illustration of the equipment provided at a switchpoint in respect of a single subscriber unit feeding three subscriber outlets; Figure 2 illustrates equipment located at a subscriber station receiving signals produced by the switchpoint equipment illustrated in Fig. 1; and Figure 3 illustrates sample separating switches of an alternative arrangement for providing appropriately timed outputs to three outlets at a single subscriber unit.

Before referring in detail to the accompanying drawings, a brief description of the general system structure into which the switchpoint equipment of Fig. 1 is integrated will be provided.

The described system is intended to distribute 30 channels of television signals with up to 10 bf these channels in DBS (direct broadcast from satellite) MAC format and the remainder in PAL I format, 20 FM sound channels, most in stereo and downstream data to the subscriber unit.

Each subscriber units outlet of which there may be up to three per subscriber unit is able to receive from the system switchpoints any one selected television channel whether of DBS or PAL I format, the FM sound channels and 64 kbit/s of data. In addition the subscriber has the capacity to transmit to his respective switchpoint an indication of the channels which he wishes to view, 64 kbit/s of data and an upstream television channel. Two-way connections at 2 MHz can also be made available for selected subscriber units. These various channels can be multiplied into high speed bit streams as described below.

The network structure assumes a total separation of upstream and downstream signal paths using a multi-start branched primary downstream network made up of fibre optic trunk cables connecting signal sources to switchpoints, dedicated secondary network upstream and downstream connections in the form of single optic fibre extending between a subscriber unit and the respective switchpoint, and an upstream primary network from the switchpoints which upstream primary network may use the format of the telecommunications authorities such as British Telecom in the United Kingdom.

Good quality PAL I PCM television can be obtained using an 8-bit sample at 3 times the colour subcarrier frequency, 13.3MHz. Thus samples are taken every 75nS requiring 106.4 Mbit/S. It may be necessary on occasion to transmit such signals over standard 140 Mbit/S lines provided by the telecommunication authorities and this represents extra capacity which may be utilized by inserting parity bits, synchronising codes, high quality television sound and auxiliary data channels. The current modulation rate of lasers is approaching 1 Gbit/S with a 0.5nS rise time being typical and thus three or four bit streams each derived from a respective television signal can be multiplexed on one fibre.

The signal format used for DBS transmissions assumes direct reception and therefore does not have to fit the standards of the telecommunications authorities. Its sampling rate is 1.5 times that of PAL I approximately and it is easier to decode the digital parts of MAC signals if the clock is derived as an exact multiple of 20.25 MHZ.

3 PAL 1 or 2 DBS channels can be supported by 3 X 140 Mbit/S with a basic synchronising frame of 6 samples which is equivalent to 450nS. 4 PAL I channels or 2 DBS channels with 140 Mbit/S capacity in 4X 18 nS blocks for other services can be supported by 4X 140 Mbit/S in a frame of 8 samples which is equivalent to 600nS.

It would be theoretically possible to encode the FM band in PCM but as the FM band will ultimately be from 88 to 108 MHz this would require 1.79 Gbit/S which is not practical.

However, the individual FM channels are brought down to intermediate frequency at the signal sources and each of these signals could be further brought down into the 150 to 300kHz band before encoding. To encode 20 such channels in 8 bit PCM at say 750kHz sampling frequency would require 120 Mbit/S and could therefore be integrated with PCM television signal channels which require a similar rate. The theoretical signal to noise ratio of the recovered FM signals is sufficient to give a near perfect stereo audio output.

The separate encoding of the FM channels also allows a channel selection scheme to be used which is similar to that used for television signal selection and avoids problems with FM receivers of poor radio frequency selectivity and linearity. The provision of 64 Kbit/S of data to each subscriber unit downstream may be accommodated by using the 4.7 microsecond blanking period which is present in each line of a conventional television signal and which is currently not used. Providing that line synchronisation is imposed on the different television signals at the head end and delays occuring during transmission via the trunk optic fibres are equalised at the switchpoints only one channel needs to carry normal line synchronising pulses. The remaining channels could carry data signals.Each switch point can supply 600 single channel outlets to respective subscriber units and this can be achieved using a double address system for each sector of the system. Thus, one television channel can be used to carry data to each switchpoint and one line used to direct data to one subscriber served by that switchpoint providing 30X600= 18,000 single channel data outlets per sector. Each subscriber would receive 25 groups of data lasting 4 microseconds (out of the 4.7 microseconds available) each at 640 Mbit/S, i.e. 64 Kbit/S. With regard to the 2Mbit/S downstream data distribution, this accords well with the system clock rate which is tied to the DBS data rate of 20.25MHz.

One dedicated 140 Mbit/S channel would provide capacity for downstream data for up to 70 2Mbit/S channels.

In the system described above the 64kbit/S and 2Mbit/S data facilities are provided on the basis that fibre launch amplifiers are grouped so that each data insertion unit accesses 30 switchpoints with potential for a total of 18,000 single channel outlets. More or less data capacity can be provided by making appropriate grouping changes in the central structure of the system.

Referring now to Fig. 1 the structure of equipment provided at a switchpoint is illustrated schematically.

Five trunk fibres 1 to 5 deliver PCM television signals to the switchpoint. Detectors 6 convert the optical signals on the trunk fibres to electrical signals. The signals applied to the trunk fibres 1 to 5 are initially in synchronism but require re-synchronisation at the switchpoint and this is achieved by delay equalisers 7.

The outputs of the delay equalisers are applied to respective busbars 8 to 12. Each subscriber unit which generally corresponds to a subscriber's home has the capacity to receive three independent signals which would be applied to separate outlets each serving a respective receiver, video recorder or the like. Thus in respect of each subscriber outlet a switch 13 is provided which enables that outlet to select a particular fibre (1 to 5) from which it wishes to receive a television signal. In the configuration of the switches shown in Fig. 1 the three outlets of the subscriber unit are selecting from left to right fibres 2, 4 and 2.The switches 13 are controlled by signals generated by a clock extract and selection control circuit 14 responsive to control inputs 15 representative of the desired signal selections made by the subscriber and derived from appropriate connections to the switch point from the subscriber units.

The outputs of the switches 13 are applied to respective time slot selection switches 17 that are in turn controlled by the control circuit 14. The switches 17 close to pass signals appearing on a selected fibre 1 to 5 only at a predetermined time allocated to the particular signal which the subscriber selection signals appearing on inputs 15 have identified as the desired signal.

Thus the three outputs appearing at the outputs of the switches 17 are in their original time slots as on the fibre.

The outputs of the switches 17 are applied to delay selection devices 18 which again are controlled by the selector control circuit 14. The delay devices are arranged to ensure that the three signals selected by the switches 17 occupy time slots in sequence corresponding to the channels required at each outlet plate so that the three signals can be applied to the common optic fibre leading to the subscriber unit via an OR gate 19 and electrical to optical coverter diode 20.

In Fig. 1 the switching arrangements have been illustrated as if relay operated. It will be appreciated that in practice the switching rates required demand solid state switching devices and that the relay type illustration included in Fig. 1 is provided simply for explanatory purposes.

The arrangement of Fig. 1 essentially selects three digital bit streams from the signals applied to optic fibres 1 to 5 and then rearranges the digital bit streams so as to leave decoding to equipment provided at the subscriber unit.

Referring now to Fig. 2, the subscriber units equipment is illustrated schematically. The optical signal applied to the subscriber units signal fibre 16 is converted to an electrical signal by a detector 21 and applied in parallel to three switches 22. The switches 22 are controlled by a decoder logic circuit 23 which also provides a clock signal to digital to analogue circuits 24. The outputs of the digital to analogue circuits are applied to respective low pass filters 25 the outputs of which are applied to video outputs 26 and modulators 27 feeding UHF outputs 28.

The control logic is arranged to control the switches 22 so as to close only during time slots allocated to signals which it is desired to deliver to the output 26 or output 28.

Having described in general terms the structure of the equipment required at the switchpoint and at the subscriber unit, the allocation of signals to particular time slots in one possible configuration will be described.

Because of the relationship between PAL I and DBS signals referred to above the two services can be integrated only if the common sampling frame is selected into which any combination of three PAL I is DBS signals can be slotted without conflict. As PAL I must sample every 75nS and DBS every 50nS, the shortest common frame is 150nS. To avoid clashes however it is necessary to offset the DBS and PAL I signals so that each uses its own time slots except when a fibre carries three signals of one type and no signals of the other type are present. This leads to the need to subdivide the 150nS available in to 12 time slots each of 12.5nS. An 8-bit sample in 12.5nS implies a bit rate of 640 Mbit/S.

One possible arrangement of slots is set out in the following table, the odd slots being allocated to DBS samples and the even slots to PAL I samples except when slots are to be allocated either to three PAL I signals or three DBS signals: Slot 1 2 3 4 5 6 7 8 9 10 11 12 a lDBS D1 D1 D1 b 2DBS D1 D2 D1 D2 D1 D2 N c 3DBS D1 D3 D2 D1 D3 D2 D1 D3 D2 0 d 1PAL Pi P1 T e 2PAL P1 P2 P1 P2 f 3PAL P1 P2 P3 Pi P2 P3 U g lD+lP D1 P1 D1 P1 D1 S h 2D+1P D1 P1 D2 D1 D2 Pi D1 D2 E i lD+2P D1 P1 P2 D1 P1 D1 P2 D The options a to i in the above table represent the possible combinations of signals transmitted to a subscriber unit from a switchpoint.The signal time slot allocations applied on the five trunk fibres 1 to 5 (Fig. 1) can be arranged as shown in the following table: Fibre 1 dl d2 d3 d4 dl d2 d3 d4 dl d2 d3 d4 Fibre 2 d5 d6 d7 d8 d5 d6 d7 d8 -d5 d6 d7 d8 Fibre 3 d9 dlO - - d9 dlO - - d9 dlO - Fibre 4 pl p2 p3 p4 p5 p6 pl p2 p3 p4 p5 p6 Fibre 5 p7 p8 p9 plO pll p12 p7 p8 p9 plO pll pl2 It will be seen that using only five trunk fibres 10 DBS signals can be easily accommodated but only 12 PAL I signals can be accommodated. Further system capacity could be provided by adding extra trunk fibres. For example a sixth fibre could cater for a further 6 PAL I channels.

To create a selection such as that set out in line i of the first of the two above tables, where the single DBS signal corresponds to d7 of the second table and the two PAL I signals P1 and P2 correspond to signals p4 and pi 1 of the second of the above tables, the switchpoint is required to make the following selections: For d7, select fibre 2, extract the signals appearing in time slots 3, 7 and 11, and delay by four slots.

For p4, select fibre 4, slots 4 and 10, and delay by four slots.

For pull, select fibre 5, slots 5 and 11, and delay by five slots.

It is necessary to provide a processor in the switch point which assesses the channel requirements set up by the subscriber unit and controls the selection of fibres, time slots and delays appropriately. It must then pass to the subscriber unit information enabling the subscriber unit to separate out the three signals provided from the switchpoint and to allocate the outlets to which the three signals are to be applied. Thus for example the subscriber unit must be informed that slots Nos. 7, 11 and 3 relate to a DBS signal to be applied to output No. 1, slots Nos. 8 and 2 relate to a PAL I signal to be applied to outlet No. 2, and slots Nos. 10 and 4 relate to a PAL I signal to be applied to outlet No. 3. The subscribers equipment as illustrated in Fig. 2 can then select and distribute the appropriate signals to the three outlets.

FM radio channels are included in slots 6 and 12. As mentioned above 20 FM channels can be carried in time slots normally occupied by a single PAL I channel. Both slots 6 and 12 are vacant uniess 3 DBS signals are demanded by the outlets at one subscriber unit. This is an unlikely event as it would require three DBS compatible television sets or video recorders in one home and it is therefore possible simply to reduce the FM radio service in this situation.

Alternatively, as there is no requirement for time relationships between adjacent channels the contents of slot 6 could be moved to slot 4 or 8.

At the subscriber unit the start of a decode sequence of 10 to 12 slot frames (1.5 microseconds) would be recognised from encoded synchronising data in line fly-backs and the slots corresponding to the FM channel required gated out to the digital to analogue converter 24. A frequency changer would be required however to translate the selected channel to an unoccupied frequency in the FM band.

The line timebase periods which allow 64kbit/S downstream communication from the head end to the individual subscriber may be used for 'broadcast' data and for text input to the home where it may be directed to videotex converters for television viewing or to printers or computer stores.

At the switchpoint, after being restored to appropriate logic levels and extraction of timebases, the data would be stripped from all signals. That related to the switchpoint would be directed to appropriate outlet feeds by logic and delayed so as to conform with the channel selected by the individual subscriber outlet. The method works only if a PAL I signal is used, because DBS has no vacant fly-back period, but this is no special problem as interactive facilities are not offered on DBS.

The basic structure of the switchpoint described with reference to Fig. 1 is only one of a variety of different structures which could be employed.

For example it would be possible to spatially divide the switchpoint inputs by extracting them once only from their arriving time division multiplexed format on the input fibres 1 to 5. These individual signals may then be retimed by delays to make all samples simultaneous. The simultaneous samples could then be made available with 1, 2 or 3 units of delay so that the circuit which "re-packs" the samples for transmission to the subscriber unit has only to identify the channels required for a given outlet and bring them together. Thus in a split PAL I and DBS system it is possible to create a 54-way bus for 18 PAL signals (and similarly for DBS signals).

This leads to considerable simplification. This is firstly because all the signals once they are launched onto the bus in synchronism stay synchronised and therefore signal distribution circuits are simplified. Buffers could be clocked to improve ragged timing as necessary. Secondly channel selection requires a once only switching of logic or diodes at slow speed so that high speed logic circuits are avoided. The subscriber related equipment basically requires only a 54 bit register, 54 diode switches and an LED driver.

Since the first, second and third outlets in a subscriber units premises would be associated uniquely with 0, 1 or 2 units of delay respectively, the channels could be organised to feed the bus in 3 groups of 18, only one of each group being selected for each outlet. This would facilitate the provision of 1 and 2 outlets if required.

A schematic illustration of an arrangement for separating samples in the manner described above is illustrated in Fig. 3. Six optic fibres 29 to 34 are applied to fibre optic receivers 35 which convert the incoming optical signals to electrical signals. The signals on each of the six fibres 29 to 34 can be of exactly the same format as the signals applied to each of the five optic fibres 1 to 5 (Fig. 1). A series of six sample separating switches 36 distribute the signals received on the respective optic fibre to any one of several groups of three delay lines 37, 38 and 39 etc according to how many channels are carried on each fibre. Each of the delay lines in the set 37, 38 or 39 introduces a time delay which brings the channels into synchronism and adds either 1, 2 or 3 time slots. Signals intended for a first outlet of the subscriber unit are taken from the one slot delayed outputs of which there is now one for each channel. Samples for outlets 2 and 3 are segregated in a similar manner.

Any subscriber outlet may now take three channels for the three outlet plates which it feeds without further retaining by selecting one each of the one, two and three unit delayed versions of the input signals.

Claims (4)

1. A communication system comprising a plurality of subscriber stations each with a plurality of outlets at which television signals broadcast over the system are to be made available, a plurality of switchpoints each connected to a respective group of the subscriber stations, each subscriber station being connected to its respective switchpoint by at least a single optic fibre, and a plurality of optic fibre trunk cabdles interconnecting the switchpoints and a plurality of television signal sources, wherein a plurality of PCM television signals are applied to each trunk fibre, each sample of each PCM television signal on a trunk fibre being allocated a respective time slot in a repeating sequence of time slots, each switchpoint comprises a selector in respect of each outlet of each subscriber station in its respective group, the selectors being controllable from the respective subscriber station outlet to select a desired television signal by selecting samples of that television signal appearing in appropriate time slots, the switchpoint comprises means for allocating selected television signals to respective time slots in a repeating sequence of time slots on the fibre optic cable leading to the subscriber unit, and each subscriber unit output comprising means for selecting the signal samples appearing in the appropriate time slots and for decoding the received signals to produce the desired television signal.

2. A system according to claim 1, wherein the PCM television signals applied to the trunk optic fibres are synchronised, and means are provided to re-synchronise the signals at the switchpoints.

3. A system according to claim 2, wherein the re-synchronising means comprises shift registers and short cable sections.

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