JP2002171496A - Catv system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To build up a CATV system that can continue provision of service to each subscriber's home even when a head end receives no radio wave. SOLUTION: CATV centers 10-40 of CATV systems 1-4 adjacent to each other are interconnected by optical fibers 81-95. For example, changeover switches 51a-51f of a line changeover device 50 in the 1st CATV system 1 are usually thrown to the positions shown in Figure and terminals make two- way communication with the head end 12. When the head end 12 cannot receive a radio wave, the line changeover device 50 detects loss of the radio wave to throw the changeover switches 51a-51f to the other positions so as to allow the terminals to continuously make 2-way communication with a head end of other adjacent CATV system. Thus, the CATV system 1 can continue the provision of service to subscribers, and should all of the head ends 12, 22, 33 lose the radio wave, the CATV systems 1-3 can receive an outgoing signal from a facility 40 against radio wave disturbance.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a CATV system configured to transmit various downlink signals such as a television broadcast signal from a head end to a plurality of terminal devices via a transmission line.

[0002]

2. Description of the Related Art A CATV system for transmitting various downstream signals such as a television broadcast signal and a data signal via a transmission line from a head end of a CATV center to a plurality of subscriber terminal devices has been known. As a transmission line, which is a signal transmission medium, a coaxial cable has been mainly used conventionally.

In a coaxial cable, transmission loss tends to increase as the frequency of a signal to be transmitted increases,
In recent years, optical fiber cables have been used in many cases because repeaters such as amplifiers need to be installed every several hundred meters to amplify the attenuated signal. In particular, instead of using all the transmission lines as optical fibers, transmission from the head end to the vicinity of the subscriber's house is performed by optical fibers, and the optical signal is converted to an electrical signal by an optical-electrical conversion device. The so-called HF, which transmits to the terminal device at the customer's home by the same coaxial cable as before
The C (Hibrid Fiber Coax) transmission system is the mainstream of the optical fiber transmission system. In each subscriber's house, a transmission system using an optical fiber can be realized without installing a new device such as an optical / electrical conversion device. It can be built more economically.

In such a CATV system, radio waves such as terrestrial television broadcasts and satellite broadcasts are received at a head end, and the received various broadcast radio waves are converted into a frequency suitable for transmission by a transmission line. It is transmitted as a down signal on the transmission line. In addition to such ordinary television broadcasts, various data broadcasts and independent broadcasts independently provided by a CATV service provider in the service area are also transmitted as downlink signals from the head end to the transmission line, and each subscriber is subscribed. Provided to the home of the elderly.

[0005] Therefore, if any abnormality occurs in the head end or the transmission line and the downstream signal is not transmitted to the terminal device in each subscriber's home, naturally each subscriber cannot watch the television broadcast, and of course, the television broadcast cannot be viewed. Other services (data broadcasting, independent broadcasting, etc.) cannot be received.

In particular, when an optical fiber is used as a transmission line, the physical strength of the optical fiber itself is weaker than that of a coaxial cable. Therefore, when an abnormal situation such as a disaster occurs, the optical fiber is more likely to be disconnected than the coaxial cable. When a disconnection occurs, it is often difficult to identify the location of the disconnection and it takes time to recover from the failure.

Therefore, when an optical fiber is used as a transmission line, it is conceivable to install two systems (main line and sub-line) of the optical fiber in preparation for a trouble such as disconnection of the optical fiber, which has already been put to practical use. By duplicating the optical transmission line in this way, even if one line is disconnected, switching to the other line allows signal transmission between the head end and the terminal device to continue, thus providing services to each subscriber. In the meantime, without affecting subscribers,
Repair work for broken lines can be performed. It is to be noted that such a duplex transmission line can be applied not only to optical fiber but also to transmission using a coaxial cable.

[0008]

However, in the method of duplicating transmission lines as described above, signal transmission to each subscriber's home can be continued as long as at least one of the two lines is normal. However, if any abnormality occurs in the head end itself, which is the signal transmission source, and the wave stops (the downstream signal cannot be transmitted on the transmission line),
Each subscriber will not be able to watch TV broadcasts, and this will have a significant effect especially when emergency information is needed, such as during a natural disaster.

That is, the method of duplexing the transmission line as described above can achieve its original purpose on the premise that the head end is normal, and how the transmission line is duplicated or more multiplexed. However, if the headend stops, no signal can be transmitted to the subscriber side.

To deal with such abnormalities of the head end, for example, the head end is duplicated in the same manner as the transmission line is duplicated (a spare head end is provided in addition to the normally used head end). Is considered,
Technically possible. In this way, even if one headend stops, signals can be continuously transmitted from the other headend, and the reliability of the entire CATV system can be further improved.

[0011] However, the method of providing a plurality of headends as described above requires not only the expensive headend itself and a large economic burden, but also an increase in power consumption and an increase in the burden on maintenance of the headend equipment. Alternatively, there are various problems such as the necessity of securing an installation location (expansion or new installation) in accordance with the expansion of the head end, so that it is technically possible but not practical.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and has as its object to construct a CATV system that can continue to provide services to each subscriber's home even when the headend stops.

[0013]

According to the first aspect of the present invention, there is provided a CATV system for transmitting a downlink signal from a head end to a plurality of terminal devices via a transmission line. A signal switching means is provided on a path connecting the head end and the transmission line.

The signal switching means monitors a down signal output from the head end, and when the down signal is abnormal, the signal is switched from the head end of another adjacent CATV system to the system in place of the down signal. This is to send a down signal sent via a transmission line onto a transmission line.

That is, in the CATV system of the present invention, various downlink signals (hereinafter also referred to as "normal downlink signals") such as a television broadcast signal and a data signal transmitted from the head end are transmitted to the subscriber side via a transmission line. In addition to the signal transmission path to be transmitted to the terminal device, a downstream signal (hereinafter, also referred to as “adjacent downstream signal”) transmitted from the head end of another adjacent CATV system is also input via the inter-system transmission path,
It is configured to be able to transmit to a terminal device via a transmission line.

Normally, the normal down signal is transmitted to the terminal device. At this time, the state of the normal down signal is monitored by the signal switching means. If the downlink signal is abnormal or not transmitted at all and there is a possibility that the terminal device cannot receive the downlink signal normally, the signal switching means transmits the adjacent downlink signal instead of the normal downlink signal to the terminal via the transmission line. It is transmitted to the device.

Therefore, according to the CATV system of the first aspect, even if the downlink signal cannot be transmitted normally to the terminal device, the downlink signal transmitted from the head end of another adjacent CATV system is input. By transmitting to the terminal device, it is possible to continuously provide each subscriber with a service (however, limited to a service provided as a downlink signal by another adjacent CATV system), and the reliability of the CATV system can be maintained. The performance is further improved.

The transmission path between the systems is CATV.
A coaxial cable generally used as a transmission line in the system may be used, or an optical fiber may be used, or the system may be configured to transmit wirelessly.
The specific medium is not particularly limited as long as the downlink signal from the head end of the V system can be reliably transmitted to the terminal device instead of the regular downlink signal.

A CATV system according to claim 1,
Alternatively, any other CATV system adjacent to the CATV system according to claim 1 may transmit various data broadcasts to the terminal device as downlink signals in addition to terrestrial television broadcasts and satellite broadcasts received at the head end. Channel-type so-called urban CATV system, of course,
CAT installed for measures against radio interference in difficult viewing areas
V system (hereinafter referred to as "radio wave interference facility"), and furthermore, a so-called two-way system capable of transmitting various information and the like from a terminal device as an upstream signal to a head end via a transmission line in addition to a downstream signal. It may be a CATV system,
The invention described in claim 1 can be applied to any CATV system configured to transmit at least a downlink signal.

Here, if the invention described in claim 1 is to be applied to the above-described bidirectional CATV system, an abnormality occurs in the head end of the bidirectional CATV system, and the downlink signal can be normally transmitted to the terminal device. In the case of disappearance, another CATV adjacent by the signal switching means is used.
Although the downlink signal can be obtained from the system, if the uplink signal transmitted from the terminal device is directly input to the headend where the abnormality has occurred, the uplink signal may not be processed normally. Therefore, when the CATV system is a bidirectional CATV system, for example,
It is good to be constituted as described in.

That is, the CATV system according to the second aspect is the same as the CATV system according to the first aspect,
The ATV system is a bidirectional CATV system capable of transmitting an upstream signal from a plurality of terminal devices to a head end via a transmission line, and another adjacent CATV system is also a bidirectional CATV system.

When the down signal (normal down signal) is abnormal, the signal switching means sends an adjacent down signal on the transmission line instead of the normal down signal and transmits the down signal from the terminal device via the transmission line. The transmitted upstream signal is also transmitted on an inter-system transmission line for transmission to the head end of another adjacent CATV system (that is, the head end for transmitting an adjacent downstream signal).

Therefore, according to the CATV system according to the second aspect, in addition to the same operation and effect as the CATV system according to the first aspect, in addition to the upstream signal transmitted from the terminal device, the downstream signal of the CATV system is used. When there is an abnormality (that is, when there is a possibility that the head end cannot process the uplink signal normally), the signal is processed in the head end of another adjacent CATV system, so that the subscriber can continue to enjoy the interactive service. it can.

When information necessary for normally processing an uplink signal from a terminal device exists only in the head end (own head end) connected to the terminal device (for example, when the terminal device joins the terminal device). For example, when the user's e-mail transmission / reception file is stored only in the mail server of the relevant head end), the other adjacent head end can be processed in the same manner as in the case of processing by his own head end. By connecting the headends with each other by a dedicated line for data sharing, various information stored in one's own headend can be shared by another adjacent headend or stored in one's own headend. The information is always stored at the head end of the adjacent CATV system, such that the information is always stored at the other head end. Signal may be to handle all times properly with.

The CATV according to claim 1 or 2
In the signal switching means of the system, as a specific method of monitoring the downstream signal output from the headend, for example, by monitoring the level of the downstream signal of a specific channel,
It is possible to judge that it is abnormal when it becomes lower than a predetermined level, but this kind of down signal is not always output, for example, in the time zone from late night to early morning when there is no TV broadcast etc. May not be output. Therefore, in such a determination method, there is a possibility that an accurate determination cannot be made, such as a case where a down signal is erroneously determined to be abnormal even though the head end is normal.

On the other hand, in a downlink signal including various information,
Some television programs are broadcast 24 hours (that is, always broadcast). Further, for example, a CATV company may output a voluntary broadcast signal such as a program guide which is always broadcast on a specific channel for company advertisement.

Therefore, in the CATV system according to the first or second aspect, the signal switching means is, as described in the third aspect, a specific signal which is always output among the downstream signals output from the head end of the CATV system. It is preferable to determine whether the downlink signal is normal based on the transmission signal. The specific transmission signal here may be any signal that is always output as a downlink signal, such as the above-described 24-hour (always-on) television broadcast signal or independent broadcast signal.

In this case, the determination may be made simply by checking the presence or absence of the specific transmission signal, or by determining whether the level of the specific transmission signal is equal to or higher than a predetermined level. Any method can be adopted as long as the state of the downlink signal can be determined. If it is determined whether the downlink signal is abnormal using the specific transmission signal, the state of the downlink signal can be accurately determined.

The downlink signal usually includes a pilot signal of a fixed frequency used for adjusting the output level of the downlink signal in various transmission repeaters such as an amplifier installed on a transmission line. . This pilot signal is a reference for the operation level of the entire CATV system, is a very stable signal, and is always output while the head end is operating normally.

Therefore, as described in claim 4, in the invention according to claim 3, it is more preferable to use a pilot signal as the specific transmission signal for determining the state of the downlink signal. In this way, the state of the downlink signal can be determined more accurately.

Further, in the inventions according to the first to fourth aspects, as the transmission path between the systems, any transmission medium, whether wired or wireless, can be applied as described above. The use of an optical fiber is more preferable because it has many advantages over other transmission media, such as a small amount of signal attenuation, low noise contamination, and large-capacity transmission.

[0032]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is an explanatory diagram showing a state in which a plurality of CATV systems are mutually connected by optical fibers. As shown in FIG. 1, in the present embodiment,
A first CATV system 1 configured by connecting a first CATV center 10 and a plurality of terminal devices (details not shown) via a transmission line L1, and a second CATV center 20
And a plurality of terminal devices (details not shown) are connected via a transmission line L2.
A third CATV system 3 configured by connecting a third CATV center 30 and a plurality of terminal devices (details not shown) via a transmission line L3; an electromagnetic interference facility 40; (Not shown) and a fourth CATV system 4 configured to be connected via a transmission line L4.
The optical fibers 81 to 95 are interconnected.

In this embodiment, each of the CATV systems 1 to
The areas where 4 is constructed actually exist adjacent to each other. Each of the transmission lines L1 to L3 is a coaxial cable, but may be constituted by an optical fiber or may be constituted by an HFC transmission system as described in the related art.

The first CATV center 10 has a first CATV
A downstream signal such as a television broadcast signal or a data signal is transmitted to a plurality of terminal devices in an area where the V system 1 is constructed (hereinafter, referred to as “area 1”), and an upstream signal transmitted from each terminal device is processed. Mainly, the head end 12, the distributors 13a and 13b, the line switch 50, and the optical receivers (O / E) 14a and 14
b, 17a, 17b, 17c and each optical transmitter (E / O)
15a, 15b, 16a and 16b.

The head end 12 is a terrestrial television broadcast,
It transmits signals received from satellite broadcasting, communication satellites, etc., independent broadcast signals, etc., to the terminal device as downlink signals, and receives uplink signals output from the terminal device side, and receives appropriate signals according to the types of uplink signals. It is equipment for performing various processes.

The headend 12 includes various servers (not shown), a cable modem, a router, and the like, and is connected to an external Internet. A plurality of terminal devices can connect to the Internet via the headend 12. . That is, the first CATV system 1 is configured as a so-called urban-type bidirectional CATV system capable of transmitting and receiving (two-way communication) various types of information between the headend 12 and each terminal device. System 2 and 3rd CATV system 3 are also urban-type interactive C
An ATV system.

The downstream signal transmitted from the head end 12 is distributed by the distributor 13b, and one of the distributed outputs is supplied to two changeover switches 51d, 5d in the line switch 50.
1e and a high-pass filter (hereinafter abbreviated as “HPF”)
It is sent out onto the transmission line L1 via 55. The remaining two distributed outputs of the downstream signals distributed by the distributor 13b are respectively input to the optical transmitters 15a and 15b and converted into optical signals, and are respectively converted to optical signals via optical fibers 81 and 82. 2 CATV system 2 and 3rd CATV system 3
Transmitted to

The first CATV center 10 has head ends 22 and 3 of adjacent CATV systems 2 to 4 respectively.
Downlink signals transmitted from the terminals 2 and 42 are also input. Specifically, in the second CATV center 20, the downstream signal transmitted from the head end 22 and distributed by the distributor 23b is converted into an optical signal by the optical transmitter 25a and transmitted onto the optical fiber 83. . This downstream signal (optical signal)
Is input to the optical receiver 17a in the first CATV center 10, where it is converted into an electric signal,
Is input to

In the third CATV center 30 as well, the downstream signal transmitted from the head end 32 and distributed by the distributor 33b is converted into an optical signal by the optical transmitter 35a and transmitted onto the optical fiber 84. This downstream signal (optical signal) is transmitted to the optical receiver 17b in the first CATV center 10.
Is converted to an electric signal, and then input to the line switch 50.

The same applies to the radio interference facility 40.
The downstream signal transmitted from the head end 42 and distributed by the distributor 43 is converted into an optical signal by the optical transmitter 44a and transmitted onto the optical fiber 85. This downstream signal (optical signal) is transmitted to the optical receiver 17c in the first CATV center 10.
Is converted to an electric signal, and then input to the line switch 50.

Therefore, since the changeover switches 51d, 51e and 51f of the line switch 50 are normally at the positions shown in the figure, the signals are transmitted from the head end 12 in the same area 1 to the terminal devices in the area 1. The down signal is transmitted. By switching the changeover switches 51d, 51e, and 51f, one of the down signals input from the other adjacent CATV systems via the optical fibers 83, 84, and 85 is transmitted. It can also be transmitted to a terminal device (details will be described later).

On the other hand, an upstream signal transmitted from a plurality of terminal devices and input to the first CATV center 10 via the transmission line L 1 is transmitted to a low-pass filter (hereinafter abbreviated as “LPF”) 54 in the line switch 50 and two Selector switch 51
a, 51b, and is input to the distributor 13a.
3a is input to the head end 12.

The upstream signal is usually supplied to the line switch 50.
Since the changeover switches 51a, 51b, and 51c are all in the positions shown in FIG.
2, the changeover switches 51a, 51b, 5
By switching 1c, it is possible to transmit to either the adjacent second CATV system 2 or third CATV system 3.

Specifically, by switching the changeover switch 51a to the optical transmitter 16a side, the upstream signal transmitted from each terminal device is input to the optical transmitter 16a, where it is converted into an optical signal. To the second CATV center 20 via the optical fiber 86 (more specifically, the optical receiver 24
a) transmitted. When the changeover switch 51b is switched to the changeover switch 51c, the upstream signal is input to the optical transmitter 16b via the changeover switch 51b and the changeover switch 51c, and is converted into an optical signal. The signal is transmitted to the third CATV center 30 (more specifically, the optical receiver 34a).

Further, in the first CATV center 10, the upstream signal transmitted from each terminal device in the adjacent second CATV system 2 and third CATV system 3 is also inputted in the head end 12 and can be appropriately processed. . That is, in the second CATV center 20, the changeover switch 61a is set to the optical transmitter 26a opposite to the state shown in the figure.
Is switched to the optical transmitter 26a via the LPF 64, the changeover switch 61b, and the changeover switch 61a, and is converted into an optical signal. First through
The signal is input to the optical receiver 14a of the CATV center 10.
Then, after being converted into an electric signal by the optical receiver 14a,
The signal is input to the head end 12 via the distributor 13a.

The same applies to the third CATV center 30. When the changeover switch 71b is switched to the optical transmitter 36a side (the changeover switch 71a side) opposite to the state shown in the figure, the upstream signal transmitted from the terminal device becomes LPF7
4, via the changeover switch 71b and the changeover switch 71a, is input to the optical transmitter 36a, where it is converted into an optical signal, and the first CATV center 1
0 is input to the optical receiver 14b. And the optical receiver 1
After being converted into an electric signal at 4b, the electric signal is input to the head end 12 via the distributor 13a.

Next, the second CATV center 20
Various downlink signals are transmitted to a plurality of terminal devices (details are not shown) in an area where the CATV system 2 is constructed (hereinafter, referred to as "area 2"), and an uplink signal transmitted from each terminal device is received. The configuration is almost the same as that of the first CATV center 10.

That is, the downstream signal transmitted from the head end 22 is distributed by the distributor 23b, and one of the distributed outputs is connected to two changeover switches 61 in the line switch 60.
The signal is transmitted onto the transmission line L2 via d, 61e and the HPF 65 and transmitted to a plurality of terminal devices. In addition, distributor 23b
The other two of the distributed outputs from are sent to the first CATV center 10 as described above,
The other is converted into an optical signal by the optical transmitter 25b, and then input to the third CATV center 30 via the optical fiber 90 (specifically, to the optical receiver 37b).

The line switch 60 receives not only the down signal from the head end 22 but also the adjacent CATs.
Head end 12, 32, 4 of V system 1, 3, 4
2 is also input. In particular,
A downstream signal (optical signal) transmitted from the first CATV center 10 via the optical fiber 81 is input to the optical receiver 27a, where it is converted into an electric signal, and then input to the line switch 60. In the third CATV center 30 as well, the optical receiver 2 is transmitted from the head end 32 and transmitted via the distributor 33b, the optical transmitter 35b, and the optical fiber 91.
The downstream signal (optical signal) input to the optical receiver 27 b
After being converted into an electric signal at b, it is input to the line switch 60. The same applies to the radio interference facility 40, where the signal is transmitted from the head end 42 to the distributor 43 and the optical transmitter 44.
b, and the downstream signal (optical signal) input via the optical fiber 92 is converted into an electric signal by the optical receiver 27 c and then input to the line switch 60.

Therefore, since the changeover switches 61d, 61e, and 61f of the line switch 60 are normally at the positions shown in the figure, the signals are transmitted to the terminal devices in the area 2 from the head end 22 in the same area 2. The down signal is transmitted. By switching the changeover switches 61d, 61e, and 61f, one of the down signals input from other adjacent CATV systems via the optical fibers 81, 91, and 92 is transmitted. It can also be transmitted to a terminal device.

On the other hand, the signals are transmitted from a plurality of terminal devices in the area 2 and transmitted through the transmission line L2 to the second CATV center 20.
Is input to the LPF 6 in the line switch 60.
4 and two changeover switches 61b, 61a, which are input to the distributor 23a, and are transmitted from the distributor 23a to the head end 2a.
2 is input.

The upstream signal is normally sent to the line switch 60.
Since the changeover switches 61a, 61b, and 61c are all in the positions shown in FIG.
2, the changeover switches 61a, 61b, 6
By switching 1c, it is possible to transmit to either the adjacent first CATV system 1 or third CATV system 3.

Specifically, by switching the changeover switch 61a to the optical transmitter 26a side, the upstream signal transmitted from the terminal device is sent out onto the optical fiber 88 via the optical transmitter 26a, and is sent to the first CATV center. It is transmitted to 10. Also, the changeover switch 61b is changed to the changeover switch 61
When the switch is switched to the c side, the up signal
b, and input to the optical transmitter 26b via the changeover switch 61c, where it is converted into an optical signal.
Is transmitted to the third CATV center 30.

Further, in the second CATV center 20, each upstream signal transmitted from the terminal device in the adjacent first CATV system 1 and third CATV system 3 is also input at the head end 22 and can be appropriately processed. . That is, in the first CATV center 10, the changeover switch 51a is set to the optical transmitter 16a opposite to the illustrated state.
Side, the upstream signal transmitted from each terminal device in the area 1 is transmitted to the second C
The signal is input to the optical receiver 24a of the ATV center 20. After being converted into an electric signal by the optical receiver 24a, the electric signal is input to the head end 22 via the distributor 23a.

The same applies to the third CATV center 30. When the two changeover switches 71a and 71b are both switched to the opposite states from those shown in the figure, the upstream signal transmitted from the terminal device is changed to the LPF 74 and the changeover switch 71.
b, and input to the optical transmitter 36b via the changeover switch 71a, where it is converted into an optical signal,
Is input to the optical receiver 24b of the second CATV center 20 via the. After being converted into an electric signal by the optical receiver 24b, the electric signal is input to the head end 22 via the distributor 23a.

The third CATV center 30 also has a plurality of terminal devices (details not shown) in an area where the third CATV system 3 is constructed (hereinafter referred to as “area 3”).
This is for transmitting various downstream signals to the terminal and receiving and appropriately processing the upstream signals transmitted from each terminal device. Since the configuration is substantially the same as that of each of the CATV centers 10 and 20 described above, The detailed description is omitted, and the outline is briefly described below.

Normally, the down signal sent from the head end 32 and input to the line switch 70 is
1f, 71e, and transmitted via the HPF 75 onto the transmission line L3. Therefore, a plurality of terminal devices in the area 3 are usually transmitted from the head end 32 and transmitted through the transmission line L.
3 is received. Also,
The upstream signal transmitted from each terminal device is also transmitted to the LPF 74, the changeover switch 71b, the changeover switch 71c, and the distributor 3
Via 3a, it is input to the same headend 32 and processed appropriately. In addition, the downstream signal transmitted from the head end 32 is transmitted to the first CAT via the optical fiber 84.
The optical fiber 91 is transmitted to the V center 10 as well.
Is also transmitted to the second CATV center 20 via the.

Each of the adjacent CATV systems 1,
2, 4 are also input, and the changeover switch 7
By switching 1e to the opposite position as shown, the down signal input to the optical receiver 37a via the optical fiber 82 is transmitted to the terminal device.
If 1f is switched to the opposite position as shown, the optical fiber 9
5, the downstream signal input to the optical receiver 37c is transmitted to the terminal device. When both the changeover switch 71d and the changeover switch 71e are switched to the positions opposite to those shown in the figure, the signal is transmitted through the optical fiber 90. Thus, the downstream signal input to the optical receiver 37b is transmitted to the terminal device.

Further, by switching the changeover switches 71 a and 71 b, the upstream signal transmitted from the terminal device is transmitted via the optical fiber 89 to the first CATV center 1.
0, and can also be transmitted to the second CATV center 20 via the optical fiber 94. Further, the optical fiber 8 from the first CATV center 10
7 when the upstream signal is transmitted via
When an upstream signal is transmitted from the CATV center 20 via the optical fiber 93, the upstream signal is converted into an electric signal by the optical receiver 34a or 34b and then transmitted through the distributor 33a. Thus, the data can be received by the head end 32 and appropriately processed.

The fourth CATV system 4 uses the above CAT
Unlike the V systems 1 to 3, they are installed as measures against radio interference in high-rise buildings and mountainous areas. In the present embodiment, the fourth CATV system 4 is constructed using only terrestrial television broadcasts as downstream signals. The data is transmitted to a plurality of terminal devices in an area (hereinafter, referred to as “area 4”). More specifically, the downstream signal transmitted from the head end 42 is distributed into four by the distributor 43, and one of the distributed outputs is transmitted to a plurality of terminal devices in the area 4. Also, one of the other distribution outputs is the optical transmitter 44.
a, is transmitted to the first CATV center 10 via the optical fiber 85, and the other is converted to an optical signal by the optical transmitter 44b.
Is transmitted to the second CATV center 20 via the optical transmitter 95, and the remaining one is converted into an optical signal by the optical transmitter 44c, and then transmitted to the third CATV center 30 via the optical fiber 95.

The details of each of the line switches 50, 60 and 70 are shown in FIG. 2 which will be described later (however, only the line switch 50). However, in FIG. Only the minimum necessary components are extracted and shown as far as the signal paths of the downstream signal and the upstream signal can be understood.

In this embodiment, each head end 1
The frequency band of the downstream signal transmitted by the head end 42 of the radio interference facility 40 is, for example, 70 to 770 MHz.
It is set to 0 to 300 MHz. In each of the CATV systems 1, 2, and 3, the frequency band of the uplink signal transmitted from each terminal device is, for example, 10 to 55 MHz.
z.

Further, on each of the transmission lines L1 to L3 in each of the CATV systems 1 to 3, the main line amplifier TA1 on the transmission line L1, the main line amplifier TA2 on the transmission line L2, and the main line amplifier TA3 on the transmission line L3. In addition to these trunk amplifiers, there are various transmission repeaters necessary for normally transmitting signals to all terminal devices, such as branch amplifiers, distributors, and extension amplifiers (not shown). is set up. The downlink signal and the uplink signal are input to each terminal device or each CATV center in a normal state while being appropriately amplified by these various transmission relay devices.

Furthermore, each head end 12, 22, 3
2 is a dedicated line (not shown) between the headends 12, 22, 32 so that when an upstream signal is transmitted from another adjacent CATV system, the upstream signal can be appropriately processed. And each head end 12,
The data possessed by 22, 22 can be shared with each other. Thereby, for example, each CATV system 1,
In cases 2 and 3, when e-mail transmission / reception data corresponding to a plurality of terminal devices is stored only in the head end of the CATV system, an admission confirmation of e-mail is requested from a terminal device of another adjacent CATV system. When an upstream signal is transmitted, by referring to the mail server in the head end of the adjacent CATV system via a dedicated line between head ends, the presence or absence of new mail and transmission of mail data Can be performed as usual.

That is, in each of the CATV systems 1 to 3 of the present embodiment, usually, a plurality of terminal devices receive the downlink signal transmitted from the head end in the area where the terminal device is located, and The upstream signal transmitted from the terminal device is also received and processed by the head end in the area.

However, in each of the CATV systems 1 to 3, if an error occurs in the head end and the downstream signal stops, as described above, the terminal device continues to download until the abnormality is recovered. In each subscriber's home equipped with a terminal device that cannot receive a signal, it becomes impossible to receive services such as TV broadcast viewing.

Therefore, in the present embodiment, when the headend stops, the line switch in the CATV center provided with the stopped headend is operated (the changeover switch is operated), so that the adjacent switch is operated. Other CATV
The downstream signal from the head end in the system is transmitted to the terminal device, and the upstream signal transmitted from the terminal device is also input to the head end of another adjacent CATV system (the same head end as the transmission source of the downstream signal). So that it can be properly processed.

Depending on the state of each of the line switches 50, 60, and 70, as described above with reference to FIG. 1, an upstream signal or a downstream signal is transmitted between the CATV centers 10 to 40 via the optical fibers 81 to 95. The signal will be transmitted. Hereinafter, the operation of each of the line switches 50, 60, and 70 will be described. Since all the line switches have substantially the same configuration and function, the line switch 50 provided in the first CATV center 10 is taken as an example. This will be described with reference to FIG. FIG. 2 is a block diagram showing a schematic configuration of the line switch 50 in the first CATV center 10.

As shown in FIG. 2, the line switch 50 mainly includes an up switching unit 51, a down switching unit 52, and a control unit 5.
3, an LPF 54, an HPF 55, an indicator light 56, and pilot signal detectors 57a to 57d. The up switching unit 51 is constituted by the three changeover switches 51a, 51b and 51c already described in FIG. 1, and the down changeover unit 52 is also constituted by the three changeover switches 51d, 51e and 51f already explained in FIG. Have been.

The down signal input from the head end 12 via the distributor 13b is input to the changeover switch 51d in the down changeover section 52 via the pilot signal detection section 57a. Similarly, each downstream signal transmitted from each of the adjacent CATV systems 2, 3, and 4 is transmitted from the head end 22 of the second CATV system 2 via the optical fiber 83 and the optical receiver 17a (see FIG. 1). The incoming downlink signal is sent to the downlink switching unit 5 via the pilot signal detection unit 57b.
2 and transmitted from the head end 32 of the third CATV system 3 via the optical fiber 84 and the optical receiver 17b (see FIG. 1), the downstream signal is transmitted via the pilot signal detector 57c. The down signal input to the changeover switch 51f in the down switching unit 52 and transmitted from the head end 42 of the radio wave obstruction facility 40 via the optical fiber 85 and the optical receiver 17c (see FIG. 1) is a pilot signal detection unit 57d. Down switching unit 52 via
Is input to the changeover switch 51f.

Each of the downlink signals includes a pilot signal of a fixed frequency (for example, 451.25 MHz) as one of the downlink signals. This pilot signal is a stable signal used as a reference signal for adjusting the level of a downstream signal in each of the CATV systems 1 to 4. In the present embodiment, as described later, based on the state of the pilot signal, It monitors whether the down signal is normal. Therefore, each pilot signal detector 57a
To 57d have a function of detecting a pilot signal by, for example, branching off a part of the pilot signal included in the downlink signal, and the detected pilot signal is output to the control unit 53. Is done.

Out of the four downstream signals input to the downstream switching unit 52, only one downstream signal selected by each of the changeover switches 51d, 51e and 51f is transmitted from the downstream switching unit 52 to the HPF 55 side. . That is, in the state shown in FIG. 2 (normal state), there is a downlink signal transmitted from the head end 12 via the distributor 13b and the pilot signal detection unit 57a. HPF 55 through the down switching unit 52
Is transmitted to the respective switches 5 at this time.
The states of 1d, 51e, and 51f are also the initial states (initial positions) of the line switch 50.

The HPF 55 is a downstream signal transmitted from the downstream switching unit 52 (70 to 770 MHz in this embodiment).
z, where the downlink signal transmitted from the fourth CATV system 4 passes 70 to 300 MHz), and the uplink signal (10 to 55 MHz in the present embodiment) transmitted from the terminal side via the transmission line L1 is switched down. The cut-off frequency is set to, for example, 70 MHz. In addition, 2C
The HPF 65 of the ATV center 20 and the HPF 75 of the third CATV center 30 are also provided for the same purpose as the HPF 55, and the cut-off frequencies are both HPF 65.
70 MHz, the same as 55.

On the other hand, the upstream signal transmitted from the terminal device is input to the upstream switching unit 51 via the LPF 54,
A distributor 13a in the same first CATV center 10, an adjacent second CATV system 2, or an adjacent third CA
In the TV system 3, the changeover switches 51a, 51
The data is transmitted only to the one selected in b and 51c, and is input to the head end to which the data was transmitted. In the state shown in FIG. 2 (normal state), the signal is transmitted to the distributor 13a via the switches 51b and 51a, and is input to the head end 12 via the distributor 13a. The states of 51d, 51e, and 51f are also the initial state (initial position) of the line switch 50 as in the case of the down switching unit 52.

The LPF 54 is an upstream signal (10 to 5 in this embodiment) transmitted from the terminal side via the transmission line L1.
5 MHz) to the up-switching section 51 side and to prevent the downstream signal transmitted from the down-switching section 52 from flowing to the up-switching section 51 side. The cutoff frequency is set to, for example, 55 MHz. Is set.
The LPF 64 of the second CATV center 20 and the third
The LPF 74 of the CATV center 30 is also provided for the same purpose as the LPF 54, and its cutoff frequency is
Both have the same 55 MHz as the LPF 54.

The indicator light 56 indicates the state of the up-switching section 51 and the down-switching section 52, more specifically, where the down signal is transmitted and the up signal is transmitted. This is for a person or the like to visually judge. The control unit 53 inputs a pilot signal included in each downlink signal, determines whether each downlink signal is normal, and determines whether each of the adjacent CATV systems 2
By inputting the light receiving level of the down signal (optical signal) transmitted from the ４4 to ４4, the state of the down signal can be monitored also from this light receiving level. That is, although not shown in FIG. 1, each optical receiver 17a, 17b, 1
7c is configured to be able to detect the light receiving level of each input downstream signal (optical signal) and output it to the control unit 53.

The control unit 53 outputs an uplink control signal for controlling the uplink switching unit 51 based on the input pilot signal or downlink signal reception level, and also outputs a downlink control signal for controlling the downlink switching unit 52. A control signal is output, and at the same time, a control signal for turning on the indicator light 56 and a process for outputting an abnormality notification when there is an abnormality in the down signal from the head end 12 are executed. Hereinafter, the line switching process executed by the control unit 53 will be described with reference to FIG.
It will be described based on. FIG. 3 is a flowchart showing a line switching process executed by the control unit 53.

When this process is started, first, in step (hereinafter abbreviated as “S”) 310, the up-switching unit 51 and the down-switching unit 52 are initialized. Specifically, the changeover switches 51a, 51b, 51c of the up changeover unit 51 and the changeover switches 51d, 51e, 51 of the down changeover unit 52.
f is switched to the initial position (the state shown in FIG. 1 or FIG. 2), and this switching is performed by the up control signal from the control unit 53 to the up switching unit 51 and the down switching unit 52 by the control unit 53. This is performed by each output of the downlink control signal to. As a result, a downlink signal transmitted from the head end 12 in the same area 1 is transmitted to each terminal device in the area 1, and an uplink signal transmitted from each terminal device is also input to the head end 12. Will be. That is, each terminal device performs two-way communication with the headend in the same area in this way, which is a normal state.

Next, in S320, it is determined whether or not the down signal from the head end 12 is normal. Specifically, the control unit 53
(Ie, the head end 12
Pilot signal output as one of the downlink signals from
For example, when a pilot signal is not detected at all, it may be determined that the entire downlink signal is abnormal, that is, that the headend 12 has stopped, or, for example, a predetermined signal may be determined. The determination level may be set such that when the level of the pilot signal becomes smaller than this determination level, the entire downlink signal may be determined to be abnormal, and whether the downlink signal can be reliably received on the terminal device side is determined. May be selected as appropriate so that can be determined correctly.

If it is determined in S320 that the down signal from the head end 12 is normal, the process proceeds to S330, and the changeover switches 51a, 51b, 51c, 51d, 51e,
It is determined whether or not 51f is at the initial position. At this time,
Since each of the changeover switches has been set to the initial position in S310, an affirmative determination is made here, and the process returns to S320. Then, as long as the head end 12 is normal and the downlink signal is normally transmitted, S320 and S330
Is repeatedly executed.

On the other hand, if a negative determination is made in S320 due to the occurrence of some abnormality in the head end 12 and the down signal is not normally transmitted, the flow proceeds to S350.
It is determined whether or not the downlink signal transmitted from the head end 22 of the adjacent second CATV system 2 is normal. In this determination, similarly to the determination in S320, a pilot signal is detected from the downlink signal input from the head end 22 to the pilot signal detection unit 57b via the optical fiber 83 and the optical receiver 17a and output to the control unit 53. Then, the determination can be made based on this pilot signal.

In addition, as described above, the control unit 53 receives the downstream signal reception level of the optical receiver 17a as input, so that the determination may be made based on the downstream signal reception level. In this case, for example, when the downlink signal light reception level is zero, that is, when no optical signal is transmitted, it may be determined that there is an abnormality, or, for example, a predetermined determination light reception level is set, and the downlink signal light reception level is reduced. When the light receiving level becomes lower than the judgment light receiving level, the whole downstream signal may be judged to be abnormal.

As described above, the determination of the downlink signal from the head end 22 may be made by either the method based on the pilot signal transmitted as one of the downlink signals or the method based on the received level of the downlink signal. Alternatively, when both the pilot signal and the downlink signal reception level are normal, the downlink signal may be determined to be normal, and the downlink signal transmitted from the head end 22 may be determined by the terminal. Various methods may be selected within a range in which it is possible to correctly determine whether or not reception is possible on the device side.

If the downlink signal has been transmitted normally from the head end 22, an affirmative determination is made in S350, and
Proceed to 360. In S360, the up switching unit 51 is switched to a position where only the switch 51a is opposite to the illustrated position (that is, the optical transmitter 16a side), and the down switching unit 52 is switched to the switching position 51d with respect to the illustrated position. Only the position is switched to the opposite position (that is, the pilot signal detector 57b side). This switching is also performed by each output of the uplink control signal from the control unit 53 to the uplink switching unit 51 and the downlink control signal from the control unit 53 to the downlink switching unit 52, as in the case of the initialization in S310.

Thus, each terminal device in the first CATV system 1 has the head end 22 of the adjacent area 2
Is transmitted, and the upstream signal transmitted from each terminal device is also input to the head end 22. Each terminal device in the area 1 is connected to the area 2
The two-way communication can be continuously performed by the head end 22 in the inside.

At S370, a control signal for turning on the indicator light 56 is output to the indicator light 56, and it is determined that the down signal from the head end 12 has become abnormal and the head end of the adjacent second CATV system 2 has become abnormal. A display indicating that the line has been switched to the side 22 is displayed.

Further, the flow proceeds to S380, where an abnormality notification signal is output. This abnormality report is sent to, for example, the CATV center 10.
It may be carried out by calling the room where the manager is located within, or by sending a phone call or e-mail to a portable terminal such as a mobile phone or PHS which the manager always carries. In this case, the content of the report is
It may be sufficient to simply notify that an error has occurred, or to a specific content such as the head end of the adjacent CATV system in which the head end that is currently performing bidirectional communication with the head end in which the error has occurred. You may contact us. Further, for example, an alarm bell or the like may be sounded to notify the surrounding manager or worker of the occurrence of the abnormality audibly.

After the processing of S380 is completed, S3 is again executed.
Returning to step 20, the same processing is performed. That is, while the head end 12 is abnormal and the head end 22 is normal, the processing proceeds from S320 to S350 as described above, and thereafter, the processing of executing S360 to S380 and returning to S320 is repeated. Become.

Here, if the restoration of the head end 12 is completed and the down signal can be transmitted normally again from the head end 12, an affirmative determination is made in S320,
Proceed to 330. At this time, the changeover switches 51a to 51
Since f is not the initial position but a position for transmitting and receiving signals to and from the head end 22, a negative determination is made in S330, and the process proceeds to S340. Then, in S340, the process of returning each of the changeover switches 51a to 51f to the initial position is executed. This processing is similar to the processing performed in S310.

At S340, each changeover switch 51a-5
After 1f is set to the initial position, each terminal device in the area 1 can transmit and receive signals (two-way communication) with the headend 12 in the same area 1 again, and returns to the normal state. You have returned. On the other hand, in the case where bidirectional communication is performed with the head end 22 of the adjacent second CATV system 2 due to an abnormality of the downstream signal from the head end 12, for example, the interruption of the head end 22 or the optical fiber 83 If an error has occurred up to the down signal from the head end 22 due to disconnection or the like, S35
A negative determination is made at 0, and the process proceeds to S390.

In S390, the third C of the adjacent area 3
It is determined whether the down signal transmitted from the head end 32 installed in the ATV center 30 is normal. This determination is made in the same manner as the determination in S350, and the optical receiver 1 is transmitted from the head end 32 via the optical fiber 84.
The determination is made based on the level of the downstream signal (optical signal) input to the receiver 7b, that is, the downstream signal reception level, or the downstream signal converted into an electric signal from the optical receiver 17b and output is output by the pilot signal detector 57c. Through the down switching unit 52
Therefore, the determination may be made based on the pilot signal detected by the pilot signal detection unit 57c, or the determination may be made by looking at both the downlink signal light reception level and the pilot signal.

If the downlink signal has been transmitted normally from the head end 32, an affirmative determination is made in S390, and
Proceed to 400. In S400, the up switching unit 51 is switched from the illustrated position so that only the changeover switch 51b is in the opposite position (that is, the changeover switch 51c side), and the down changeover unit 52 is changed from the illustrated position to the changeover switch 5.
Switching is performed such that only 1e is in the opposite position (that is, the switch 51f side). This switching is also performed in S310 and S36.
As in the case of 0, the control is performed based on each control signal from the control unit 53. As a result, the down signal transmitted from the head end 32 in the adjacent area 3 is transmitted to each terminal device in the area 1, and the up signal transmitted from each terminal device is also input to the head end 32. That is, each terminal device in the area 1 is connected to the headend 3 in the area 3.
2 can continue to perform two-way communication.

Then, at step S410, the indicator lamp 56 is turned on, and when both the down signals from the head end 12 and the head end 22 are abnormal and the line is switched to the head end 32 side of the adjacent area 3, A display indicating that there is. Further, the process proceeds to S420, outputs an abnormality notification signal similar to S380, returns to S320 again, and performs the same processing thereafter. In other words, while both the down signals from the head end 12 and the head end 22 are abnormal and the down signal from the head end 32 is normal, the process proceeds from S320 to S350 as described above, and proceeds to S390. The process of executing the process of S420 and returning to S320 is repeated.

Here, when the restoration of the head end 12 is completed and the down signal can be transmitted normally again from the head end 12, an affirmative determination is made in S320,
By proceeding to the processing of S330 and subsequent steps, each terminal device in the area 1 is connected to the head end 12 in the same area 1 as usual.
And two-way communication will be performed. Although the head end 12 has not been repaired yet, the repair of the head end 22 or the optical fiber 83 has been completed and the head end 22 has not been repaired.
If the downlink signal from the server becomes normal again, an affirmative determination is made in S350, and the processes in S360 to S390 are executed to perform the bidirectional communication with the head end 22 again in place of the head end 32. Become like

On the other hand, as described above, the head end 12
In the case where bidirectional communication is performed with the head end 32 in the adjacent area 3 due to an abnormality of the down signal from the head end 22, for example, due to the interruption of the head end 32 or the disconnection of the optical fiber 84, etc. If an error has occurred up to the down signal from the head end 32, a negative determination is made in S390, and the process proceeds to S430.

In S 430, the head end 4 installed in the radio interference facility 40 of the adjacent fourth CATV system 4
It is determined whether or not the downlink signal transmitted from the second is normal. This determination is also made in the same manner as in S350 and S390, and the light receiving level of the downstream signal input from the head end 42 to the optical receiver 17c via the optical fiber 85 or the downstream signal is converted into an electric signal. Then, the determination may be made by referring to both the pilot signal detected by the pilot signal detection unit 57d, or both the reception level of the downlink signal and the pilot signal.

If the downlink signal has been transmitted normally from the head end 42, an affirmative determination is made in S430, and
Proceed to 440. In S440, based on the downlink control signal from the control unit 53, the downlink switching unit 52 is set such that only the changeover switch 51e and the changeover switch 51f are at the opposite positions (that is, the pilot signal detection unit 57d side) from the illustrated position. Switch to. At this time, the changeover switch 51
e is the changeover switch 51f by the process of S400.
Is switched to the side. As a result, area 1
The downstream signal transmitted from the head end 42 in the adjacent area 4 is transmitted to each of the terminal devices.

Since the radio interference facility 40 is a facility for transmitting only the downstream signal, the upstream signal is transmitted to the radio interference facility 4.
It cannot be transmitted to zero. For this reason, in addition to the abnormality of the downlink signal from the head end 12, when the downlink signals from the head ends 22 and 32 of the adjacent areas 2 and 3 are both abnormal, the uplink signal from each terminal device in the area 1 Can no longer be transmitted, but the radio interference facility 40
(In this embodiment, terrestrial television broadcasting) can be received, and it is possible to avoid a situation where even television viewing cannot be performed at all.

Then, at step S450, the indicator lamp 56 is turned on, and it is determined that the down signals from the head end 12, the head end 22, and the head end 32 are all abnormal, and that the line is connected to the head end 42 side of the adjacent area 4. Is displayed to indicate that has been switched. Further, S460
Then, an abnormality notification signal similar to that of S380 or S420 is output, and the process returns to S320 again to perform the same processing.
That is, while the down signals from the head ends 12, 22, and 32 are all abnormal and the down signals from the head end 42 are normal, S320 to S35 are performed as described above.
0, the process proceeds to S430 via S390, and thereafter, S440 to S440.
The process of executing the process of S460 and returning to S320 is repeated.

Here, when any one of the head ends is restored and the down signal can be transmitted normally from that head end, one of the processes of S320, S350 and S390 corresponds to the restored head end. A positive determination is made in the processing performed, and the processing subsequent to the positively determined processing is executed. For example, if the head end 22 has been repaired, an affirmative determination is made in S350, and if the head end 12 is further repaired, an affirmative determination is made in S320.

Then, as described above, each head end 1
In the case where the downstream signals from the headends 2, 22, and 32 are all abnormal and the downstream signals are transmitted from the head end 42 of the radio interference facility 40 to each terminal device in the area 1, the downstream signals from the head end 42 are further transmitted. If an error occurs in the steps up to, a negative determination is made in S430, and the process proceeds to S470. Here, each head end 12, 22, 32, 42
Then, a display indicating that any of the downlink signals is abnormal is turned on, and further proceeds to S480 to notify the administrator or the like that such an abnormality has occurred (or that an abnormality has simply occurred).

In this state, each subscriber in area 1 cannot enjoy any CATV service, and any CA
It will wait for the TV system headend to be repaired. In the meantime, in the control unit 53, the control unit 53 executes S320 to S350, S390, S430, S470, and S480.
The process up to is repeated.

Although the line switch 50 has been described in detail in the above description, the structure of the other line switches 60 and 70 is also shown in FIG. 2 except that the initial positions of the respective changeover switches are different. This is almost the same as the line switch 50. That is, in the line switch 60, the changeover switches 61a, 61b, and 61c that constitute the up switching unit, and the changeover switches 61d, 61e, and 61f that constitute the down switching unit.
The initial positions of the changeover switches 71a, 71b, 71c constituting the up-switching unit and the changeover switches 71d, 71e, 71f constituting the down-switching unit in the line switch 70 are all the positions shown in FIG. Becomes

The processes executed by the control units (not shown, but equivalent to the control unit 53 in FIG. 2) of the line switches 60 and 70 have basically the same contents as the processes shown in FIG. Since the processing is executed, the description thereof is omitted here. That is, the second CATV system 2 or the third CA
In the TV system 3, as in the case of the first CATV system 1, the bidirectional communication between the head end and each terminal device is normally performed inside each CATV system, but a downlink signal from the head end is transmitted. When an abnormality occurs, each switch is switched so that bidirectional communication can be performed with a headend in another adjacent area, and an indicator lamp is turned on and an abnormality notification is output.

As described in detail above, in the present embodiment, for example, in the first CATV system 1, each terminal device normally performs bidirectional communication with the head end 12, but the downlink signal from the head end 12 When the abnormality is detected by the control unit 53, the changeover switches 51a to 51f are appropriately switched so that any one of the headends 22, 32, and 42 in another adjacent area is connected. Continue two-way communication (however, headend 4
In the case of 2, one-way communication using only downlink signals can be performed.

Therefore, according to the present embodiment, even if a downlink signal cannot be transmitted normally from the head end to the terminal side in one CATV system, the signal is transmitted from the head end of another adjacent CATV system instead. The CATV service (limited to the service provided as a downstream signal by another adjacent CATV system) is continuously provided to each subscriber by inputting the downstream signal and transmitting it to the terminal device. Can be provided, and the reliability of the CATV system is further improved.

[0107] Not only the downlink signal but also the uplink signal transmitted from the terminal device may be any one of the headends 12, 22, 32 of the first CATV system 1, the second CATV system 2, and the third CATV system 3. As long as one is normal, it can be processed at its normal headend, so that the subscriber can continue to enjoy interactive services.

Further, since the down signal is transmitted also from the fourth CATV system 4 for radio wave interference countermeasures, even if any of the headends 12, 22, 32 becomes abnormal, the headend 42 is not used. As long as is operating normally, the broadcast provided by the radio interference facility 40 (here, only the terrestrial TV broadcast) can be viewed, so that the minimum TV broadcast viewing can be ensured.

Further, since the CATV centers 10, 20, 30, and 40 in each of the areas 1 to 4 are connected to each other by optical fibers, signal attenuation is smaller than that in the case of using a coaxial cable or wireless communication. Since the volume is small, noise is less likely to be mixed, and large-capacity transmission is possible, signals can be transmitted and received more efficiently and reliably.

Furthermore, since the occurrence of an abnormality in the down signal is notified to the administrator of the CATV center by outputting an abnormality notification or turning on the indicator lamp, the administrator can accurately grasp the details of the abnormality. , It is possible to take prompt measures for the abnormality. In the present embodiment, each CA is compared with a method in which a plurality of headends are provided in the same area to perform backup when a downlink signal is abnormal.
Since it is only necessary to connect the TV centers with an optical fiber, the existing facilities in the CATV center can be used as they are, and the head-end interruption measures can be realized more economically.

Here, the correspondence between the components of the present embodiment and the components of the present invention will be clarified. In the present embodiment, each of the line switches 50, 60, 70 corresponds to a signal switching unit of the present invention, and each of the optical fibers 81 to 95 corresponds to an inter-system transmission line of the present invention.

It should be noted that the embodiments of the present invention are not limited to the above-described embodiments at all, and it goes without saying that various forms can be adopted as long as they fall within the technical scope of the present invention. For example, in the above embodiment, by connecting the three urban CATV systems (each of the CATV systems 1 to 3) and one radio interference facility 40 to each other, it is possible to implement a countermeasure in the event of a down signal abnormality due to a head end stop or the like. But
The number and types of CATV systems connected to each other are not particularly limited.

In the above embodiment, the radio interference facility 4
When the 0 head end 42 stops, the downstream signal is not transmitted to the terminal device in the area 4. Therefore, the downstream signal is also transmitted to the radio disturbance facility 40 from the head end in another adjacent area. It may be configured as follows. Then, even if the head end 42 stops, the television broadcast viewing can be continued by the down signal from the other head end.

Further, in the above embodiment, the head end 1
The determination of the state of the downlink signal from S2 (S320) is performed based on the pilot signal. However, the present invention is not limited to this. For example, the determination may be made based on a television broadcast signal that is constantly broadcast for 24 hours. Any signal may be used as long as it is a signal that is always output and that can correctly determine whether the terminal apparatus can reliably receive the downlink signal. Determination of Downlink Signal from Another Adjacent CATV System (S
350, S390, and S430).
The determination is not limited to the determination based on the pilot signal or the downlink signal reception level.

Further, in the above embodiment, for example, area 1
In the above, when the down signal from the head end 12 in the same area 1 is not transmitted normally, the down signal from the adjacent area is taken in the order of area 2, area 3, and area 4. However, it is needless to say that the order is not limited.

Furthermore, in the above embodiment, each area 1
As the signal transmission medium for interconnecting the optical fibers 4 to 4, an optical fiber having excellent transmission characteristics is adopted. However, the present invention is not limited to the optical fiber. For example, the distance between adjacent CATV centers is short and a coaxial cable is sufficient. In some cases, they may be connected to each other by a coaxial cable. In consideration of various conditions such as the distance between adjacent CATV centers, more efficient
What is necessary is just to select a signal transmission medium that can be constructed economically.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a state in which a plurality of CATV systems are mutually connected by optical fibers according to an embodiment.

FIG. 2 is a block diagram showing a schematic configuration of a line switch in a first CATV center.

FIG. 3 is a flowchart illustrating a line switching process performed by a control unit.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... 1st CATV system, 2 ... 2nd CATV system, 3 ... 3rd CATV system, 4 ... 4th CATV system, 10 ... 1st CATV center, 12, 22, 3
2, 42 ... head end, 13a, 13b, 23a, 2
3b, 33a, 33b, 43 ... distributors, 14a, 14
b, 17a to 17c, 24a, 24b, 27a to 27
c, 34a, 34b, 37a to 37c ... optical receiver, 15
a, 15b, 16a, 16b, 25a, 25b, 26
a, 26b, 35a, 35b, 36a, 36b, 44a
~ 44c: Optical transmitter, 20: 2nd CATV center, 3
0: Third CATV center, 40: Radio interference facility, 5
0, 60, 70: Line switcher, 51: Up switch unit, 51
a to 51f, 61a to 61f, 71a to 71f: switch, 52: down switch, 53: controller, 54, 6
4, 74 ... low-pass filter (LPF), 55, 65,
75: high-pass filter (HPF), 56: indicator light, 5
7a to 57d: pilot signal detection unit, 81 to 95: optical fiber, L1 to L4: transmission line, TA1 to TA3: trunk amplifier

Continued on the front page F term (reference) 5C056 FA02 FA06 HA04 HA20 5C064 BA01 BB05 BB06 BC10 BC16 BD08 5K030 GA12 HB02 HC11 JA10 JL03 JL08 KA05 KA13 LB08 LD07 MA04 MB01 MD02

Claims (5)

[Claims] 1. A CATV system configured to transmit a downlink signal from a head end to a plurality of terminal devices via a transmission line, wherein at least a path connecting the head end and the transmission line is provided. Monitoring the downstream signal output from the end, when the downstream signal is abnormal, instead of the downstream signal,
A CATV system comprising signal switching means for transmitting a down signal transmitted from a head end of another adjacent CATV system via an inter-system transmission line onto the transmission line. 2. The CATV system according to claim 1, wherein the CATV system is a bidirectional CATV system capable of transmitting an upstream signal from the plurality of terminal devices to the headend via the transmission line. The other CATV system is also a bidirectional CATV system, wherein the signal switching means transmits the up signal to the inter-system transmission line when the down signal is abnormal, so that the adjacent other CATV system is transmitted. A CATV system for transmitting to a system head end. 3. The CATV system according to claim 1, wherein the signal switching unit is configured to perform the signal switching based on a specific transmission signal that is constantly output among the downlink signals output from a head end of the CATV system. A CATV system, which determines whether a downlink signal is normal. 4. The CATV system according to claim 3, wherein the specific transmission signal is a pilot signal of a constant frequency output as one of the downlink signals. 5. The CA according to claim 1, wherein the transmission path between the systems is an optical fiber.
TV system.