JPS61166236A - Communication system - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to communication systems, and more particularly to communication systems in which television signals are distributed to subscribers via fiber optic cables.

PRIOR ART AND PROBLEM SOLVED BY THE INVENTION There have been numerous proposals in the past for distributing television signals from their central source to subscribers.

Older systems have conductive networks,
New systems utilizing optical fibers have been proposed and some have been assembled.

The use of PCM signaling technology in telecommunications systems is well known. In a PCM system, a signal is sampled periodically and digital data representing the sampled signal is transmitted continuously. At the receiving end of the system, by appropriate decoding of the PCM signals transmitted to the system,
It is necessary to reconstruct the initial signal.

In prior art PCM systems, the transmitted pit stream always includes address data that defines the intended destination of the data represented by the pit stream. "Tagging" of the various elements of the data being transmitted is not a problem when the intended destination of the data can be predetermined.

However, in the case of television signal distribution systems,
It is necessary to make a wide range of television signals available to the system, and it is necessary to allow subscribers to control the selection of television signals that are transmitted to their own receivers. Different requirements apply.

PCM technology, which is well known in other fields of telecommunications, has not been applied to general cable broadcasting systems because
Maybe it's for this reason.

It is an object of the present invention to provide a communication system that utilizes PCM signaling technology to distribute signals from multiple signal sources to a substantially large number of subscribers.

[Means for solving problems]

In the present invention, the system comprises a plurality of subscriber stations each having a plurality of outlets from which the television signals broadcast are taken, a plurality of switch points each connected to a respective group of said subscriber stations. wherein each subscriber station is connected via at least one optical fiber to its respective switch point, and a plurality of optical fibers interconnecting the switch point and the plurality of television signal sources. a communication system comprising a fiber optic relay cable, wherein a plurality of PCM television signals are applied to each of the fiber optic relay cables; Each sample is assigned to a respective time slot in a repeating sequence of time slots, and each switch point comprises a selector for each outlet of each subscriber station in a respective group, the selector selecting the appropriate time slot. controllable from each subscriber station outlet to select a desired television signal by selecting a sample of the television signal appearing on the optical fiber, the switch point means for assigning a selected television signal to each time slot in the repeating sequence of time slots, and each subscriber station outlet comprises means for selecting a signal sample appearing in the appropriate time slot; and A communication system is provided comprising means for decoding a received signal to create the desired television signal.

Therefore, in the above system, the PCM encoded television signal is distributed throughout the system at each switch point (hereinafter referred to as "switching center") so that it can be widely used, but the pit stream transmitted to the switching center is The final destination is determined by the recipients of those bitstreams rather than by the head end of the system.

The use of optical fiber rather than conductive cables reduces the required volume of equipment in the exchange for a given number of subscribers, so that a large number of subscriber stations can be serviced by a regular sized exchange. obtain.

The use of optical fibers also offers the advantage that the maximum distance between the switching center and the subscriber station can be greater than when using conductive cables.

Preferably, the PCM television signals applied to the trunk optical fibers are initially synchronized and then resynchronized at the switching center. Re-synchronization is accomplished using a shift register and a short cable section after detection of the signal received at the exchange.

〔Example〕

Before describing details of embodiments of the present invention with reference to the accompanying drawings, a brief description of the structure of a general system in which the switching center equipment of FIG. 1 is integrated will be provided.

In the above system, up to 10 channels are in DBS (direct broadcast from satellite) MAC format, and the rest are in P
It is intended to distribute to subscriber stations 30 channels of television signals in AL I format, 20 channels of FM sound, mostly stereo, and downstream data.

Each subscriber station outlet, of up to three per subscriber station box, carries the selected television channel in either DBS format or PAL I format;
An FM sound channel and 64 kbit/s data can be received from the system switching center. Additionally, the subscriber has the ability to transmit to the respective exchange an indication of the channels he wishes to watch, 64 kilotips/second data and upstream television channels.

A two-way connection at 2 MHz may also be utilized for selected subscriber stations. These various channels are
As described below, it can be multiplexed into a high-speed bitstream.

A multi-start branched primary downstream network consisting of fiber optic trunk cables connecting the signal source to the switching center, in the form of a single optical fiber extending between the subscriber station and each switching center. Dedicated Secondary Network Upstream and Downstream Connections and British Telecom
Using an upstream primary network from the switching center that can use the format of a telecommunications authority such as Telecom, the network structure provides total separation of the artspstream signal path and the downstream signal path. .

Good quality PAL I PCM television is obtained by using 8-bit samples at 13.31'lH2, which is three times the color subcarrier frequency. Therefore,
The sample required 106.4 Mbit/s and 7
Samples are taken every 5 nS. Occasionally, such signals are
In addition, the additional capacity required for transmission refers to the extra capacity that can be used by inserting parity bits, synchronization codes, high quality television sound, and extra data channels. The modulation rate of current lasers is approaching 1 Gbit/s with a typical rise time of 0.5 nS, thus a single in-focus stream of 3 or 4, each derived from a respective television signal. can be multiplexed into multiple fibers.

The signal format used for DBS transmission is intended for direct reception and therefore does not need to conform to the standards of telecommunications authorities. Its normalized speed is approximately 1.5 times that of PAL I, and it is easier to decode the digital part of the MMC signal if the clock is an exact multiple of 20.25 MHz.

Three PAL I channels or two DBS channels can be supported by 3.times.140 Mbit/S with a basic synchronization frame of 6 samples, which is equivalent to 450 nS. 140 in 4X18nS block for 4 PAL I channels or other services
Two DBS channels with a capacity of Mbit/s can be supported by 4×140 Mbit/s in a frame of 8 samples, which is equivalent to 600 nS.

Although it is theoretically possible to encode the FM band to PC1'l, the FM band is basically 88 to 108 MIIz.
Therefore, this is an impractical 1.79 Gbit/
Requires seconds. However, the individual FM channels are downgraded to an intermediate frequency at the signal source, and each of these signals can be downgraded further to the 150-300 kHz band prior to encoding. 20 such channels in 8-bit PCM with a sampling frequency of 750kllz
encoding requires 120 Mbit/s and can therefore be integrated with PCM television signal channels that require similar speeds.

The theoretical signal-to-noise ratio of the recovered FM signal is sufficient to provide nearly perfect stereo audible output.

Separate coding of FM channels also allows channel selection schemes similar to those used for television signal selection to be used, avoiding the poor radio frequency selectivity and linearity that FM receivers have. Eliminate problems. Providing 64 kbit/s of data to each subscriber station is accomplished by using the currently unused 4.7 microsecond blanking period present on each line of conventional television signals. obtain. If the line is synchronized, then.

If the delays imposed on the different television signals at the end and during their transmission through the trunk optical fiber are equalized at the exchange, only one channel is needed to carry the regular line synchronization pulse. It's just that. The remaining channels can carry data signals. Each exchange can provide 600 single channel outlets to each subscriber station, which is accomplished using a dual addressing system for each sector of the system. Therefore,
One television channel can be used to carry data to each exchange and one line used to route the data to one subscriber, with one line per sector. It is serviced by a switching center providing 30 x 60', 000 single channel data outlets. Each subscriber has 4
Receive 25 groups of data, or 64 kilobits of data, lasting microseconds. For distribution of 2 Mbit/s downstream data, this is 20.2
A good match with the system clock rate coupled to the 5 MHz DBS data rate: one 140 Mbit/s channel provides up to 70 2 Mbit/s channels for downstream data.

In the system described above, the 64 Kbit/s and 2 Mbit/s data facilities are fiber launch (LA
(unch) amplifiers are classified into groups, so that each data insertion device
Access 30 exchanges with a total capacity of 18,000 single channel outlets. More or less data capacity can be provisioned by appropriately modifying the groups within the central structure of the system.

Now, referring to FIG. 1, the configuration of equipment provided in an exchange is illustrated.

Five relay fibers 1-5 carry the PCM television signals to the exchange. The detector 6 converts the optical signal on the relay fiber into an electrical signal. The signals applied to the relay fibers 1-5 are initially synchronous but require resynchronization at the exchange, which is accomplished by the delay equalizer 7.

The output of the delay equalizer is applied to each bus 8-12.

Each subscriber station, typically corresponding to a subscriber's home, has the ability to accept three independent signals, each applied to a separate outlet serving a receiver, video recorder, or the like. Therefore, for each subscriber's outlet, that outlet is connected to a separate fiber (1
A switch 13 is provided which allows the subscriber to select between . In the switch position shown in FIG. 1, the three outlets of the subscriber station select fibers 2.4 and 2 from left to right. The switch 13 is activated by a signal generated by a selection control circuit 14 corresponding to the clock extraction and control personnel 15 representing the desired signal selection made by the subscriber and resulting from the appropriate connection from the subscriber station to the exchange. controlled.

The output of switch 13 is applied to each time slot selection switch 17 which is controlled in turn by selection control circuit 14. Switch 17 closes only at predetermined times assigned to the individual signals that the subscriber selection signal appearing at input 15 has identified as the desired signal, in order to pass the signal appearing on the selected fiber. Therefore, switch 1
The three outputs appearing at the output end of 7 are in their original time slots as they are on the fiber.

The output of switch 17 is applied to a delay selection device 18 controlled by selection control circuit 14 .

The delay selection device selects the three selected by the switch 17.
are arranged to ensure that the three signals sequentially occupy the corresponding time slots in the required channels at each outlet plate, so that the three signals are connected to the OR gate 19 and Photo conversion diode 20
can be applied to the common optical fiber 16 leading to the subscriber station via.

In FIG. 1, the switching device is illustrated as if a relay were activated.

In practice, the required switching rates require solid state switching devices, and the relay-type illustration included in FIG. 1 is for illustrative purposes only.

The installation of FIG. 1 first selects three digital bit streams from the signals applied to relay fibers 1 to 5, and then converts the digital bit streams in order to leave decoding to equipment located at the subscriber station. Rearrange.

Referring now to FIG. 2, the equipment of a subscriber station is schematically illustrated. Optical signals applied to the subscriber station's signal fiber are converted into electrical signals by a detector 21 and applied in parallel to three switches 22. Switch 22 is controlled by decoder logic circuit 23, which also provides a clock signal to digital-to-analog converter 24. The outputs of the digital-to-analog converters are applied to respective low-pass filters 25, the outputs of which are applied to a video output 26 and a modulator 27 that supplies UHF to a UHF output 28. be done.

The decoder logic circuit 23 includes a video output section 26 or u
The switch 22 is controlled to close only during the time slot assigned to the signal desired to be transmitted to the HF output 28.

Having described in general terms the structure of the equipment required at the switching center and the subscriber station, we will now discuss the assignment of signals to individual time slots in one possible arrangement.

Because of the relationship between the PAL I and DBS signals described above, the two services are integrated only when a common sampling frame is selected in which any combination of three PAL I or DBS signals can be consistently arranged. Ru. PALI every 75nS and DBS every 50nS
Since it must be sampled every S, the shortest common frame is 150 nS. However, to avoid collision, the DBS signal and )'A1. It is necessary to offset the I signal so that the hub transmits three signals of one type, each using its own time slot, except when signals of the other type are not present. . This means that each 12.5 nS of available 150 nS
This means that it needs to be divided into 12 time slots, which is nS.

8 bit samples at 12.5 nS means a bit rate of 640 Mbit/s.

One possible arrangement of slots is shown in Table 1 below, where the slots correspond to 3 PAL I signals or 3 DBS signals.
Odd slots are assigned to DBS samples, and even slots are assigned to PALT samples, except when assigned to signals.

Margin below Table 1 Slots 1 2 3 4 5 6 7 8 9 10
11 12alDBs Di Di D
Ib2DBS DI D2 DI D2 Di
D2c3DBs DI D3 D2 DI D
3 D2 Di D3 D2 not dlPAL Pi
PI e2PAL PI P2 Pi P2 user f3PAL
Pi P2P3
, PI P2P3glD
+IP Di Pi Di PI h2D+IP for DI
Di Pi D2 Di D2 Pi DI
D2i 1D+2P DI PI P2 DI
PI DI P2 Choices in Table 1 above a w i
represents the possible combinations of signals transmitted from the exchange to the subscriber station. The signal time slot assignments applied to the five relay fibers 1-5 (FIG. 1) may be arranged as shown in Table 2 below.

Margin below Table 2 Fiber 1 dl d2 d3 d4
dl d2 d3 d4 di
d2 d3 d4 fiber 2 d5
d6 dl d8 d5 d6
dl d8 d5 d6 dl
dB fiber 3 d9 dlO--d9
dlO--69dlO--Fiber 4 pi
p2 p3 p4 p5 p6
pi p2 p3 p4 p5
p6 fiber 5 p7 p8 p9
plopHp12p? p8 p9
By using p125 tree relay fiber, 10 DBS signals and 12 PAL
It will be appreciated that I signals may be accommodated. Even greater system capacity can be provided by adding additional relay fibers. For example, the sixth fiber has six additional P
An AL I channel can be provided.

A single DBS signal corresponds to dl in Table 2 and has 2' P
The AL I signals P1 and P2 correspond to p4 and pH of Table 2, respectively.To make the selections listed in column i of Table 1, the exchange is required to make the following selections: be done.

For dl, select fiber 2, extract the signals appearing in time slots 3.7 and 11, and delay by 4 slots.

2 in p4 selects fiber 4, slots 4 and 10, and delays by 4 slots.

For pll, select fiber 5, slots 5 and 11, and delay by 5 slots.

assessing the channel requirements set by the subscriber station;
It is necessary to provide the switching center with a processor that properly controls the fibers, time slots and delays. Then,
The processor communicates information to the subscriber station that allows the subscriber station to separate the three signals provided by the exchange and to assign the outlets to which the three signals are applied.

Thus, for example, the subscriber station may associate slots 7.11 and 3 with a DBS signal applied to outlet 1, slots 8 and 2 with a PAL I signal applied to outlet 2; slots 10 and 4 are 3
PALI signal applied to the outlet. A subscriber unit such as that shown in FIG. 2 then selects the appropriate signal and distributes it to the three outlets.

FM radio channels are included in slots 6 and 12. As mentioned above, 20 FM channels are 1 P
It may be transmitted in the time slots normally occupied by the AL I channel. If three DBS signals are not required by the outlets at one subscriber station, both slots 6 and 12 are free. Since it is unlikely that one household would require three DBS compatible television sets or video recorders, it is easily possible to put an FM radio service in this situation. Alternatively, the contents of slot 6 may be moved to slot 4 or 8 since there is no requirement regarding the time relationship between neighboring channels.

At the subscriber station, the beginning of the decoding sequence of 10 to 12 slot frames (1.5 microseconds) is determined by the symbol synchronization data in the retrace line and the digital-to-analog converter 2.
The slot corresponding to the FM channel requested to be output to 4 is approved. A frequency converter is required to convert the selected channel to an unoccupied frequency within the FM band.

The line time base period, which allows 64 kbit/s downstream communication from the headend to individual subscribers, is used for "broadcast" data and text input into the home, where the text input is used for television viewing. videotex converter or printer or computer storage device.

At the exchange, data is extracted from all signals after restoration to the appropriate logic levels and extraction of the time base. Data related to the exchange is fed by logic to the appropriate outlet feeder and delayed to match the channels selected by the individual subscriber outlets.

The method can only be used if PAL I signals are used. The reason is that DBS does not have an empty flyback period. However, this is not a special problem since the DBS is not given the ability to interact.

The basic structure of the exchange described with reference to Figure 1 is as follows:
This is just one of a variety of different configurations that can be used.

For example, by extracting the exchanger inputs only once from the arrival time portion of the exchanger inputs that are format-multiplexed and transmitted to the relay fibers 1 to 5, it is possible to spatially separate them. These individual signals are retimed by delays so that all samples are present at the same time. The simultaneously present samples are then made available with 1.2 or 3 delay units, so that the circuitry to "repack" the samples for transmission to the subscriber station is not required for a given outlet. Just check the channels and bring them together. Therefore, the separated P
18 PAs in AL I and DBS systems
It is possible to create buses in 54 directions for the L signal (the same is true for the DBS signal).

This leads to considerable simplification. This is because, firstly, all signals sent to the bus in synchronization remain synchronized, which simplifies the signal distribution circuitry. Buffers are used when necessary to improve timing misalignment. Second, channel selection requires only one switching of logic or diodes at low speeds, thus avoiding high speed logic circuits. The subscriber related equipment consists of one 54-bit register, 54 diode switches and one LE.
Basically, only the D driver is required.

1 in the subscriber station. The second and third outlets are each uniquely associated with 0, 1, or 2 delay units, so that the channels are organized to feed the bus in 3 groups of 18. and only one of each group is selected for each outleft.

This facilitates the provision of one or two outlets, if necessary.

A schematic illustration of an apparatus for separating samples according to the method described above is shown in FIG. 6 optical fibers 29
.about.34 are applied to the fiber optic receiver 35 that converts an input optical signal into an electrical signal. 6 fibers 29
The signals on each of ~34 fibers are connected to 5 relay fibers 1
5 (FIG. 1). A series of six sample separation switches 36 transfer the signals received on each optical fiber to delay lines 37, 38 and 39, the total number of which depends on how many channels are transmitted on each fiber. etc. into one of several groups of three delay lines. Each delay line within each group synchronizes the channels and introduces a time delay that adds 1.2 or 3 time slots. The signal provided to the first outlet of the subscriber station is taken from the one slot delayed output, one for each channel.

Samples for outlets 2 and 3 are separated in a similar manner.

Any subscriber outlet can tap three channels for the three outlet plates by selecting each one of the different types of input signals delayed by 1, 2, and 3 units.

(26) Schematic diagram of equipment provided at a switch point (exchange) for ' subscriber stations; FIG. FIG. 3 is a schematic diagram of an alternative arrangement sample separation switch that provides suitably timed outputs to three outlets at a subscriber station.

1 to 5...Relay fiber, 6...Detector, 7.
・Vase such as delay, 13...Switch, 14...
- Selection control circuit, 16... common optical fiber, 17
...Time slot selection switch, 18...Delay selection device, 22...Switch, 23...Decoder logic circuit, 24...Digital-to-analog converter, 25...Low-pass filter, 26...Video output section, 27...
・Modulator, 28...UHF output section, 29~
34... Optical fiber, 36... Sample separation switch, 37-39... Delay line.

Claims (1)

[Claims] 1. A plurality of subscriber stations each having a plurality of outlets from which television signals broadcast to the system are taken; a plurality of subscriber stations each connected to a respective group of said subscriber stations; a switch point, each said subscriber station being connected to said respective said switch point via at least one optical fiber, and interconnecting said switch point and said plurality of television signal sources; a plurality of optical fiber relay cables, wherein a plurality of PCM television signals are applied to each of the optical fiber relay cables, and each of the PCM television signals on one of the optical fiber relay cables. each sample of is assigned to a respective time slot in a repeating sequence of time slots, each said switch point comprising a selector for each outlet of each said subscriber station in a respective said group, said selector selecting the appropriate time slot. controllable from each said subscriber station outlet to select a desired television signal by selecting a sample of the television signal appearing in a slot, said switch point controlling said light channel to said subscriber station; means for assigning a selected television signal to each time slot in a repeating sequence of time slots on the fiber, and each said subscriber station outlet comprises means for selecting a signal sample appearing in said appropriate time slot; A communication system comprising means for decoding a received signal to create the desired television signal. 2. The PCM television signals applied to the optical fiber relay cable are synchronized, and the switching center is provided with resynchronization means for resynchronizing the PCM television signals. The communication system according to item 1. 3. A communication system according to claim 2, wherein said resynchronization means comprises a shift register and a short cable section.