GB2337185A

GB2337185A - Return path connection matrix for a multiplexed cable communications system

Info

Publication number: GB2337185A
Application number: GB9909366A
Authority: GB
Inventors: Geoffrey Bray
Original assignee: TRATEC U K Ltd
Current assignee: TRATEC U K Ltd
Priority date: 1998-04-24
Filing date: 1999-04-26
Publication date: 1999-11-10
Anticipated expiration: 2019-04-26
Also published as: GB2337185B; GB9909366D0

Abstract

A return path connection matrix for a multiplexed cable communications system comprises a plurality of optical receivers each coupled to an incoming optical fibre link, an amplifier (1a-1h) for amplifying the received signals from each receiver, a splitter (2a-2h) associated with each receiver for splitting the amplified signals into a plurality of paths, a plurality of signal combiners (3a-3h), each associated with a respective one of the services, and connection means (4) for connecting separate signal paths from each splitter to respective ones of the signal combiners.

Description

2337185 RETURN PATH CONNECTION MATRIX FOR A MULTIPLEXED CABLE

COMMUNICATIONS SYSTEM

Field of the Invention

This invention relates to a return path connection matrix for a multipiexed cable communications system, for example a cable television system.

Background to the Invention

In a cable television network where signals are distributed from an originating source, referred to as the "head-end", by optical fibre links to local nodes which are in turn connected to a plurality of subscribers by coaxial cable operating at radio frequen cies, it is becoming increasingly desirable commercially to offer additional services which require signalling from the subscriber back to the service provider at the head-end. Ex amples of such services are "pay-per-view" television, "tele-shopping", cable telephony and cable computer connections for Internet access and the like. Each such service is returned to a head-end as a separate modulated r.f. carrier mostly using optical fibre links from the nodes. At the head-end, an optical receiver converts the modulated light signal to r.f. signals, which are passed to demodulators for each of the nominated serv ices.

There is a need to provide flexibility in the connections between the receiver and the demodulators to ensure that, as additional services are added, the connections can readily be added without disruption to the existing set-up. There is also a need to balance signal levels to ensure that the correct operating level for each service is main tained.

Summary of the Invention

According to the invention, there is provided a return path connection matrix for a multiplexed cable communications system, comprising a plurality of optical receiv ers each coupled to an incoming optical fibre link, an amplifier for amplifying the re ceived signals from each receiver, a splitter associated with each receiver for splitting the amplified signals into a plurality of paths, a plurality of signal combiners, each associ ated with a respective one of the services, and connection means for connecting sepa rate signal paths from each splitter to respective ones of the signal combiners.

The amplifier and the receiver are conveniently combined into one removable module.

The splitters conveniently split the signals into a number of signal paths equal to the number of separate service signals to be processed, although it is possible that not all signal paths will be active immediately; some may be left to permit additional services to be connected later. it will also be appreciated that if the number of services later exceeds the number of paths on each splitter, the signal on any given path may be further divided in the same manner to increase signal availability for the additional services.

The connection means may comprise a plurality of separate patch cables con- nectable by plug and socket connections to the respective splitters and combiners, whereby the service connections may be readily adapted to meet different service requirements. Alternatively, the connection means may comprise software-contro lied electronic routing and switching hardware, requiring no hard-wired patching cables, but instead permitting the connections to be established using a control computer. Such an is arrangement would facilitate reconfiguration where large numbers of nodes are connected to the head-end.

Each amplifier associated with a receiver is preferably provided with de4ct,-)r means for detecting the noise level in the received signal and control means for svoL-;-ing off the amplifier if the noise level exceeds a predetermined value for a predeter- mined period of time. More preferably, the control means comprise timer means for switching the amplifier on again after a predetermined time-out period. If the noise condition still applies, the amplifier will be switched off again, and a further attempt to restart will be made after a further time-out period. The timeout period may be set at any desired value, but a suitable period has been found to be 10 seconds.

The detector means may be arranged to detect noise in a single frequency band, selected as representative of the full working spectrum. However, it is preferred to sample a number of different frequency bands and to combine the results from these in assessing the noise level. The detector may comprise, for example, a ceramic filter to filter out a 100 kHz sample width at the desired frequency. In the case of the single frequency band, the sample may be at approximately 6 MHz, for example, but other frequencies may be used instead. The voltage of the signal is measured and compared with a reference voltage. If the sample exceeds the reference voltage. the r.f. output may be switched by a relay to a 75 ohm termination.

It will be appreciated that other detector and switching arrangements may be employed in place of the arrangement described.

A second-stage amplifier may be provided on the out put of the combiners for each service. The outputs of the amplifiers associated with the receivers for the connections to the different nodes may be balanced by adjustment of the amplifier gain so that they are substantially equal, and the second-stage amplifiers are preferably set so that there is unity gain from the output of the first amplifier to the output of the second- stage amplifier. Means may also be provided for correcting the amplitude response of the amplifiers to compensate for cable attenuation characteristics. The second stage amplifiers can also be set to provide an output signal at the correct level for a particular service demodulator.

Switching means may be provided for interrupting the line between the amplifier and the splitters to permit, for example, adjustment of the amplifier without affecting the downstream apparatus. To this end, the disconnected line leading to the downstream apparatus is terminated appropriately, for example into 75 ohms to avoid any signal imbalance. The switching means may be remotely operable.

The outputs from the combiners are all the essentially the same signal, being a combination of all the service signals from all the nodes. Each such output is then passed to a service demodulator which detects and demodulates the desired signal from the respective r.f. carrier.

The matrix of the invention is simple to set up and provides flexibility to accommodate growth and the addition of further services.

Brief Description of the Drawings

In the drawings, which illustrate an exemplary embodiment of the invention:

Figure 1 is a block diagram of the matrix; and Figure 2 is a block diagram of an input amplifier forming part of the matrix illustrated in Figure 1 Detailed Description of the Illustrated Embodiments

Referring to Figure 1, the matrix as illustrated serves 8 input nodes and 7 serv ices, with an additional output spare to accommodate expansion. It will be appreciated, however, that the numbers of nodes and services are not limited to the arrangement illustrated; there may be more or fewer of each.

Each optical fibre from a respective node (not illustrated) connects to a respec tive receiver whose output is in turn connected to an adjustable amplifier 1 a to 1 h. de scribed hereinafter with reference to Figure 2. The output from each amplifier 1 is passed to a respective splitter 2a to 2h, which in the illustrated embodiment is an eight way splitter. Eight combiners 3a to 3h are provided in the matrix as illustrated, one for each service, including one, 3e, as a spare for future expansion. Patch cables 4 ar-c.

used to make the required connections from the outputs 5 of the splitters -1. --.) - the inputs 6 of the combiners 3a to 3h in such a manner that each output 5 for any given splitter 2 is connected to a respective one of the combiners 3 (in the case of the spare combiner 3e the connections need not be made until the additional service is to be included, and so the extra combiner can also be omitted until required). Thus, the output of each combiner 3 includes signals from all of the nodes.

The output from each combiner 3 is then passed to a respective adjustable am plifier 7, and from there to a demodulator for the appropriate service acting selectively on the particular carrier frequency for that service. The amplifiers 1 are adjusted so that their outputs are all substantially equal in level, while the second- stage amplifiers 7 are set so as to compensate for any loss incurred in passage through the splitter and combiner and its associated cabling, i.e. the gain at the output of each amplifier 7 is set to be unity with respect to the output from the respective input amplifier 1.

Adding an extra service to the matrix is simply a matter of making the patch ca ble connections from each splitter 2 to the new service combiner 3.

Referring now to Figure 2, each input amplifier 2 has a r.f. input 8 with a gain select switching arrangement 9 that permits a pre-amplification stage 10 to be switched in if high gain is selected. The main amplification stage 11 follows an interstage tilt stage 12, for equalisation of cable losses up to 6d13 at 200 MHz and to improve further the amplifier's distortion characteristics, and a gain adjust stagel 3. A 200 MHz lowpass filter 14 offers a rejection typically of 35c113 for unwanted and spurious signal compo nents. Following this stage, a portion of the output signal is fed to a detector/switching circuit 15 and to an output test port 16. The circuit 15 includes a pre- amplification stage 17, a 6MHz ceramic filter 18 which selects a 10OkHz sample, and a comparator 19 which compares the sample voltage to a reference voltage and signals to a relay 20 via a 1 Os delay stage 21. The relay 20 switches the output of the amplifier from the r.f.

output 22 to a 75 ohm termination 23 either in response to a signal from the compara tor, passed via the delay stage 21, or if the amplifier is not powered on. If the noise level does not drop below the threshold during the delay period, the output is switched.

and is restored only when the noise level drops again below the threshold. If the noise level does drop during the delay period, the output signal is not switched. A switch 24 is provided to disable this function if it is not required.

When the switching has occurred, an error signal may be generated to be re ported to a control station computer. Error signals may also be generated by failure of the amplifier or of a power supply, for example.

While the invention has been described with reference to the return of signals from the hubs to the head-end, a similar arrangement could be used for forward path signalling away from the head-end, for example at hubs or nodes.

Claims

1 A return path connection matrix for a multiplexed cable communications system, comprising a plurality of optical receivers each coupled to an incoming optical fibre link, an amplifier for amplifying the received signals from each receiver, a splitter associated with each receiver for splitting the amplified signals into a plurality of paths, a plurality of signal combiners, each associated with a respective one of the services, and connection means for connecting separate signal paths from each splitter to respective ones of the signal combiners.

2. A matrix according to Claim 1, wherein the connection means comprise a plurality of separate patch cables connectable by plug and socket connections to the respective splitters and combiners, whereby the service connections may be readily adapted to meet different service requirements.

3. A matrix according to Claim 1, wherein the connection means comprise software-controlled electronic routing and switching hardware, permitting the connections to be established using a control computer.

4. A matrix according to Claim 1, 2 or 3, wherein each amplifier associated with a receiver is provided with detector means for detecting the noise level in the received signal and control means for switching off the amplifier if the noise level exceeds a predetermined value for a predetermined period of time.

5. A matrix according to Claim 4, wherein the control means comprise timer means for switching the amplifier on again after a predetermined time-out period.

6. A matrix according to Claim 5, wherein the period of time is set to1O seconds.

7. A matrix according to Claim 4, 5 or 6, wherein the detector means is 25 arranged to detect noise in a single frequency band, selected as representative of the full working spectrum.

8. A matrix according to Claim 4, 5 or 6, wherein the detector means is arranged to sample a number of different frequency bands and to combine the results from these in assessing the noise level. 30

9. A matrix according to any of Claims 4 to 8, wherein the detector comprises a ceramic filter to filter out a 100 kHz sample width at the desired frequency.

10. A matrix according to any preceding claim, wherein a second-stage amplifier is provided on the out put of the combiners for each service.

11. A matrix according to Claim 10, wherein correcting means are provided for correcting the amplitude response of the amplifiers to compensate for cable at5 tenuation characteristics.

12. A matrix according to any preceding claim, wherein switching means are provided for interrupting the line between the amplifier and the splitters.

13. A matrix according to Claim 12, wherein the switching means are remotely operable.

14. A return path connection matrix for a multiplexed cable communications system, substantially as described with reference to, or as shown in, the drawings.