JP2002016898A - Two-way catv system, transmission line apparatus, and center device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To sufficiently lower the noise level of incoming signals received by means of a center device without stopping the transmission of the incoming signal from a route on which a noise generating source exists when the noise level of the incoming signals becomes higher in a two-way CATV system. SOLUTION: When the noise level of the incoming signals received by means of the center device 2 exceeds a preset upper limit value of noise, the transmission line apparatus (bidirectional amplifier, diverging device, etc.), positioned in the proximity of the noise generating source on the upstream side is specified (S110-S140) and the damping amount to the incoming signals at the specified transmission line equipment is increased in steps until the noise level of the incoming signals received by means of the center device 2 becomes lower than the upper limit value of noise (S150-S200). Consequently, when abnormal noise is produced, the noise is prevented from exerting an adverse effect upon the whole system without stopping the transmission of the incoming signals from a cable MODEM positioned on the downstream side of the noise generating source to the center device 2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bidirectional CATV system in which a plurality of transmission path devices are connected in a transmission path from a center device to a subscriber terminal device, and a bidirectional CATV system.
The present invention relates to a transmission line device and a center device used in a system.

[0002]

2. Description of the Related Art Conventionally, in a bidirectional CATV system, transmission line devices (for example, both terminals) provided on a transmission line from a center device to a subscriber terminal device (for example, a cable modem or a set top box (STB)). For example, a status amplifier for monitoring a transmission signal level, a power supply voltage level, and the like.

[0003] In the bidirectional CATV system, a transmission line device is provided with a gate switch for opening and closing an up signal passage path, and the gate switch is operated using a communication function with a center device of a status monitor. Control that suppresses ingress noise of an upstream signal by separating a transmission path that does not use the upstream signal is known.

Further, as disclosed in, for example, Japanese Patent Application Laid-Open No. 6-245212, when the noise level of an upstream signal exceeds an allowable range, a gate switch of a bidirectional amplifier located upstream of a noise source is used. Is also known to prevent the adverse effects due to abnormal noise from affecting the entire system by operating the system and disconnecting the noise source from the system.

[0005]

When the gate switch is opened because the noise level of the upstream signal exceeds the allowable range, a worker is dispatched to the site to investigate the cause of the noise generation, and to identify the cause. Restoration work such as closing the gate switch after removal is performed.

For this reason, once the gate switch is opened, the subscriber terminal connected downstream therefrom takes a relatively long time until the above-mentioned restoration work is completed.
There has been a problem that services using uplink signals, such as connection to the Internet via a center device, cannot be received.

The present invention has been made to solve the above problems.
In a bidirectional CATV system, when the noise level of the uplink signal increases, the noise level of the uplink signal received by the center device is sufficiently reduced without stopping the transmission of the uplink signal from the path where the noise source exists. The purpose is to be able to.

[0008]

In the bidirectional CATV system according to the first aspect of the present invention, the noise level of the uplink signal received by the center device exceeds a preset noise upper limit value. If so, a search is made for a noise source, the transmission line device located immediately upstream of the noise source is specified, and the attenuation of the upstream signal in the transmission line device (hereinafter referred to as “specific transmission line device”). The amount is increasing.

Therefore, according to the present invention, if the amount of attenuation of the upstream signal in the specific transmission path device is sufficiently increased until the noise level of the upstream signal received by the center device is within an allowable range, the noise can be reduced. Not only can the noise based on the source be prevented from adversely affecting other equipment, but the noise level of the upstream signal can always be kept below a certain level as a whole of the system. Can be improved in reliability.

At this time, the signal level of the upstream signal transmitted from the subscriber terminal device (such as a cable modem or a set-top box) located downstream of the specific transmission line device is:
Although the signal is attenuated along with the noise level in the specific transmission path device, the communication can be continued although the communication quality is usually lowered due to the system margin.

According to a second aspect of the present invention, an uplink signal transmitted from the subscriber terminal apparatus is transmitted to the center device such that the signal level of the received uplink signal is within a predetermined allowable range. Is automatically adjusted, the noise generated at the noise source is reduced by the automatic adjustment, and the signal of the upstream signal from the subscriber terminal located downstream of the specific transmission line device is adjusted. Only the level will increase. That is, the transmission quality (SN ratio or CN ratio) of the uplink signal output from the specific transmission path device is improved as compared with before the attenuation amount of the uplink signal is increased. However, communication with the center device can be ensured without significantly lowering communication quality, and a service using an uplink signal can be reliably continued.

Here, the transmission path equipment includes a relay amplifier and a branch amplifier for amplifying, branching, and merging an upstream signal and a downstream signal, a branching device called tap-off, and an information distribution board at a subscriber's house. Any device may be used as long as it is a device connected in the transmission path from the center device to the subscriber terminal device, such as a booster installed in the terminal.

Next, in the transmission line device according to the third aspect, the signal attenuating means changes the amount of attenuation with respect to the upstream signal to the center device according to the attenuation amount adjustment signal superimposed on the downstream signal from the center device. Further, in the center device according to the sixth aspect, the noise level detecting means detects noise in a frequency band used for transmitting the uplink signal, and the noise level exceeds a preset noise upper limit value. In this case, the search means searches for a noise source, specifies a transmission line device located immediately upstream of the noise source, and the noise adjustment means determines that the noise level detected by the level detection means is a noise upper limit. An attenuation adjustment signal for adjusting the attenuation of the upstream signal in the transmission path equipment specified by the search means is generated so as to be equal to or less than the value, and the attenuation adjustment signal is superimposed on the downstream signal and transmitted. .

Therefore, when the transmission line device according to claim 3 and the center device according to claim 6 are used, if the noise level of the uplink signal received by the center device exceeds a preset noise upper limit value, A transmission line device located immediately upstream of the generation source can be specified, and the amount of attenuation with respect to an upstream signal in the transmission line device can be increased. That is, the transmission line device according to the third aspect and the center device according to the sixth aspect can be suitably used when configuring the bidirectional CATV system according to the first aspect.

Further, in the center device according to the present invention, the signal level detecting means detects the signal level of the received uplink signal and adjusts the level so that the signal level is within a predetermined allowable range. The means generates a level adjustment signal for adjusting the transmission level of the uplink signal in the subscriber terminal device, and superimposes the level adjustment signal on the downlink signal and transmits the signal. That is, the center device according to claim 7 can be suitably used when configuring the bidirectional CATV system according to claim 2.

According to a fourth aspect of the present invention, in the transmission line device, the distribution means distributes the downstream signal from the center device into a plurality of signals, and the switching means switches the switching signal superimposed on the downstream signal from the center device. According to the above, the passing and blocking of the downstream signal may be controlled for each distribution path for transmitting the signal distributed by the distribution means.

When the transmission line device of the present invention having such a configuration is applied to a branching device for branching a signal on a transmission line to a signal line (branch route) drawn into each subscriber's house, so-called tap-off, etc. Since a downlink signal is not transmitted from an unused branch terminal, unauthorized use of the CATV service via the unused branch terminal can be prevented. At the same time, it is possible to prevent ingress noise entering from unused branch terminals, and to prevent degradation of the uplink signal quality.

However, the switch means for handling the high-frequency signal flowing through the transmission line of the CATV system cannot completely prevent the signal from passing when the switch is off. However, if the isolation of the switch means is to be sufficiently increased, the switch means becomes very expensive.

Therefore, when the above-mentioned switch means is provided in the transmission line equipment, the noise superimposing means works in conjunction with the switch means, and the switch means passes the down signal. At the time of setting to block, it is desirable that white noise is superimposed on the downstream side of the switch means.

As a result, the leak signal that has passed through the switch means in the off state is buried in white noise, making it impossible to identify the leak signal, and the above-mentioned unauthorized use can be reliably prevented.

[0021]

Embodiments of the present invention will be described below with reference to the drawings. [First Embodiment] FIG. 1 is a schematic configuration diagram of a CATV system according to an embodiment to which the present invention is applied. In the CATV system according to the present embodiment, a transmission signal in a predetermined transmission frequency band (for example, a 70 MHz to 550 MHz band) is used as a downlink signal from the center device 2 to the terminal, and the signal is transmitted from the terminal to the center device 2. As an upstream signal,
A transmission frequency band (for example, 10
(MHz to 55 MHz band).

As shown in FIG. 1, the CAT of the first embodiment
In the V system, 4 (4) from the center device 2 to the terminal terminal to which the subscriber terminal device of the system is connected.
a, 4b, 4c) are the trunk lines 4 connected to the center device 2.
a, a large number of branch lines branched from the trunk line 4a (hereinafter referred to as "branch lines").
4b), a lower branch line 4c further branched from each branch line 4b, etc., and are connected in a so-called tree shape.

The transmission lines 4 are provided with bidirectional amplifiers (branch amplifiers, relay amplifiers) for amplifying transmission signals flowing through the transmission lines 4 bidirectionally and branching and merging the transmission signals as necessary. 6.) There are provided a large number of branching devices 8 as so-called tap-offs, which branch a transmission signal flowing through the transmission line 4 and draw it into a subscriber's house. Note that the bidirectional amplifier 6 and the branching device 8 correspond to transmission line devices in the present invention.

Here, FIG. 2A shows that the bidirectional amplifier 6
3B is a block diagram illustrating a configuration of a main part of the branching device 8. FIG. As shown in FIG. 2A, the bidirectional amplifier 6 converts the downstream signal input from the upstream terminal Tu located on the center device 2 side into a high-pass filter (HPF) 1.
0, and amplified to a predetermined level by the amplifier circuit 12, and then output from the downstream terminal Td to the terminal via the HPF 14, and an upstream path input from the downstream terminal Td located at the terminal. There is provided an upstream path for taking in a signal through a low-pass filter (LPF) 15, amplifying the signal to a predetermined level by an amplifier circuit 13, and outputting the signal from the upstream terminal Tu to the center device 2 via the LPF 11.

The bidirectional amplifier 6 has a gate switch 1 as a signal attenuating means having a function as a variable attenuator between the amplifying circuit 13 and the LPF 11 in the upstream path.
6 and an upstream terminal Tu and an HPF 10,
The bidirectional amplifier 6 performs bidirectional communication with the center device 2 through a branch circuit 17 provided between the LPF 11 and notifies the center device 2 of the state of each section of the bidirectional amplifier 6 and receives a control signal from the center device 2. A status monitor 18 is provided as signal extraction means for generating a switching signal for the gate switch 16 in accordance with the control signal.

In FIG. 2A, the transmission signal is branched and divided.
A configuration for merging and a configuration for the status monitor 18 to detect the state of each unit are omitted. However, the merging of the upstream signals is performed on the downstream side of the gate switch 16. In addition, the gate switch 16 is provided in FIG.
As shown in (a), there are provided selectors 30 (30a, 30b) capable of switching five paths according to a switching signal, and each path is short-circuited, opened, and has a different attenuation amount. Two attenuators 31, 32, 33 are connected. That is, the gate switch 16 allows the up signal to pass (short circuit: on), cut off (open: off), or attenuate (−3 dB / −6 dB / −10 in the present embodiment).
(3 levels of dB).

FIG. 3A shows the gate switch 16.
Is schematically represented, and in practice, for example, FIG.
As shown in (b), a diode, a resistor, an inductor,
It can be constituted by a capacitor. That is, by supplying power to any one of the terminals 1 to 4 to make the diode conductive according to the control signal, an attenuation amount corresponding to the attenuator formed by the resistor sandwiched between the conductive diodes can be obtained. However, when power is supplied to the terminal 1, the power supply is turned on, and when power is supplied to the terminal 5, the power supply is turned off.

FIG. 3C shows the frequency characteristics of the bidirectional amplifier 6. On the other hand, as shown in FIG. 1, the branching device 8 includes a branching circuit 8a for branching a part of a transmission signal from the transmission line 4, and one or more branching circuits reaching a terminal terminal for connecting a subscriber terminal. A gate section 8b having a terminal Tb, distributing the downstream signal branched by the branch circuit 8a to each branch terminal Tb, merging the upstream signal input from each branch terminal Tb, and supplying the combined signal to the branch circuit 8a; Become.

The gate unit 8b of the branching device is
As shown in (b), the downstream signal branched from the transmission line 4 by the branch circuit 8a is fetched via the HPF 20 and distributed by the distribution circuit 22, and then each distributed downstream signal is transmitted to the HPF 24. (24a, 24b,...), The downstream group output from each branch terminal Tb to the terminal side, and the upstream signal input from each terminal side branch terminal Tb are transmitted through the LPF 25 (25a, 25b,...), Respectively. And an upstream group that outputs the combined upstream signal to the branch circuit 8a via the LPF 21 after the data is merged into one by the merge circuit 23.

The gate section 8b has a gate switch 26 (26a, 26b) as a signal attenuating means which also has a function as a variable attenuator, between the LPF 25 of each upstream path constituting the upstream group and the merging circuit 23. ,...), And performs bidirectional communication with the center device 2 via a branch circuit 27 provided between the branch circuit 8a and the HPF 20 and the LPF 21. The state of each part of the branch device 8 is determined by the center. A status monitor 28 is provided for notifying the device 2 or receiving a control signal from the center device 2 and generating a switching signal for each gate switch 26 according to the control signal. The gate switch 26 is exactly the same as the gate switch 16 included in the bidirectional amplifier 6 described above, and the description is omitted here.

By the way, a unique address is assigned to the bidirectional amplifier 6 and the branching device 8 in advance, and each of the status monitors 18 and 28 operates in a specific transmission frequency band (70
When the destination address of the transmission data transmitted from the center device 2 indicates its own device using the center device 2 (MHz to 76 MHz), this is fetched and the command content indicated in the command is executed. .

Further, identification information for identifying each gate switch 26 can be added to the transmission data from the center device 2 to the branch device 8, and the status monitor 28 of the branch device 8 (therefore, the center device 2) can be added. Are configured such that each gate switch 26 can be individually controlled.

That is, the bidirectional amplifier 6 amplifies the downstream signal flowing through the transmission line 4 and amplifies the upstream signal flowing through the transmission line 4 by any of pass, cut-off and attenuation (there are three levels). Or have been to do. Further, the branching device 8 distributes the downstream signal branched from the transmission line 4 and supplies it to each branch terminal Tb. For the upstream signal input from each branch terminal Tb, for each branch terminal Tb, After passing, blocking, or attenuating, they are merged and supplied to the transmission line 4.

Next, as shown in FIG. 1, the center device 2 receives a television broadcast signal transmitted from an artificial satellite or a ground station, and reproduces a television signal recorded on a video tape or video disk. Generates a large number of television signals for broadcasting in the system using video equipment, television cameras for independent broadcasting, etc., and converts each television signal to a downlink signal of a transmission frequency corresponding to a preset channel. And a head end (HE) 40 composed of broadcasting equipment for transmitting to each transmission line (also referred to as a node) 4.

In the center device 2, a measuring device (for example, a spectrum analyzer) 44 for analyzing the frequency component of the frequency band of the upstream signal (10 MHz to 55 MHz) is connected to the transmission line 4 according to the selection signal. A node switcher 42 for selecting any one of the nodes N1 to Nn and supplying an upstream signal flowing from the transmission line 4 to the measuring instrument 44 via the selected node Ni (i = 1 to n); Bidirectional amplifier 6 provided on transmission line 4
To transmit a command for controlling the operation state to the transmission line device such as the transmission line device 8 and the branching device 8, an address previously assigned to each of the transmission line devices 6 and 8, a command representing the command content, And a state notification signal transmitted from each of the transmission line devices 6 and 8 to notify an operation state from an upstream signal in a transmission frequency band corresponding to a preset control channel. A monitoring control device 50 that performs control such as taking out and the like, and a control signal generated by the monitoring control device 50 is converted into a downlink signal of a transmission frequency band corresponding to a preset control channel. And a downlink control signal transmitting section 46 for transmitting to the transmission line 4.

The supervisory control device 50 stores path data representing the connection relationship of the transmission line 4 and the arrangement of the transmission line devices 6 and 8, and search history data including the setting states of the gate switches 16 and 26. Using the database 52 and the node switcher 42 and the measuring device 44,
The signal level of the upstream signal and the noise level (or SN
Ratio, CN ratio), a signal measuring section 54 as signal level detecting means and noise level detecting means, and a signal measuring section 5.
Control signal for adjusting the output level of the cable modem connected to the terminal terminal as the subscriber terminal device so that the signal level of the uplink signal measured at 4 is within a predetermined allowable range. A signal level adjusting unit 56 as a level adjusting unit for generating a (level adjusting signal);
A control signal (attenuation amount adjustment signal) for adjusting the amount of attenuation of the uplink signal transmitted by the transmission line device is set so that the noise level measured by the signal measurement unit 54 is equal to or less than a preset noise upper limit value. And a noise level adjusting unit 58 that generates the noise.

Here, the control executed by the noise level adjusting unit 58 will be described with reference to the flowchart shown in FIG.
This process is executed individually and repeatedly for each of the nodes N1 to Nn in use to which the transmission line 4 is connected. As shown in FIG. 4, when this process is started, first, the signal measuring unit 54 measures the noise level of the uplink signal (S110), and the noise level NL obtained by the measurement is set in advance. It is determined whether or not it is larger than the noise upper limit value NLmax (S120). If the noise level is equal to or lower than the noise upper limit value (NL ≦ NLmax), no processing is performed, and this processing is terminated as it is. On the other hand, if the measured noise level of the uplink signal is higher than the preset noise upper limit value (NL) >
NLmax), assuming that it is necessary to search for a noise source, first, search history data for the node Ni to be processed is read from the database 52 (S130), and based on this search history data, A noise source search process for specifying the transmission line devices 6 and 8 located immediately upstream of the source is executed (S140).

With respect to the transmission line devices 6, 8 (hereinafter referred to as "specific transmission line devices") specified by the noise source search processing, the setting of the gate switches 16, 26 is set to the maximum attenuation (in this embodiment,- 10 dB) (S150), and if it is the maximum attenuation, the attenuation cannot be further increased. Therefore, the gate switches 16 and 26 are turned off for the specific transmission line device. An attenuation adjustment signal, which is a control signal for instructing the setting, is output (S160).

On the other hand, the gate switch 1 of the specific transmission line device
If the setting of 6, 6 is not the maximum attenuation, the attenuation of the gate switches 16, 26 is set to 1 for the specific transmission line device.
An attenuation amount adjustment signal instructing to increase (increase) the level is output (S170), and the controlled devices 6, 8 having received the attenuation amount adjustment signal set the gate switches 16, 26 as instructed. After waiting for a necessary time, the signal measuring section 54 measures the noise level (S18).
0).

Subsequently, it is determined whether or not the measured noise level NL is larger than the noise upper limit NLmax (S19).
0), if it is larger than the noise upper limit (NL> NLmax),
Assuming that the noise level could not be sufficiently reduced, S1
Returning to S50, the processes of S150 to S190 are repeated.

On the other hand, if the noise level measured in S180 has changed to the noise upper limit or less (NL ≦ NLmax), or in S160, the setting of the gate switches 16 and 26 of the specific transmission line device is changed. If the setting is OFF, the identification information of the specific transmission path device specified in S140 and the setting states of the gate switches 16 and 26 set in S160 or S170 are read in S130. The database 52 is added and updated to the search history data.
, And a display or notification indicating that fact is made (S200), and this process ends.

That is, by executing the noise level adjustment processing, the transmission line devices 6 and 8 located immediately upstream of the noise source are specified, and the specific transmission line devices 6 and 8 receive by the center device 2. Until the noise level of the uplink signal becomes equal to or lower than the noise upper limit value, the attenuation of the uplink signal increases.

When the amount of attenuation of the upstream signal in the specific transmission line devices 6 and 8 increases, the signal level of the upstream signal output from the cable modem located downstream thereof greatly decreases along with the noise level. However, in this case, in the center device 2, the signal level adjusting unit 56 adjusts the output level of the uplink signal to the cable modem so that the signal level of the uplink signal received by the center device 2 is within an allowable range. A process as a level adjusting means for sending an instruction for increasing the level is executed. That is, the upstream signal supplied to the center device 2 via the specific transmission path devices 6 and 8 is:
Only the signal level increases, and the transmission quality (SN ratio, CN ratio) is improved as compared to before the attenuation of the upstream signal in the specific transmission path devices 6 and 8 increases.

When there is a display or notification based on S200, the corresponding transmission line device 6, 8
The site work for removing the cause of noise on the downstream side of the above is performed, and after the removal of the cause of noise is confirmed, the gate switches 16 and 26 of the transmission line devices 6 and 8 may be switched on.

Here, the details of the noise source search processing executed in step 140 will be described with reference to the flowchart shown in FIG. As shown in FIG. 5, in the noise source search process, a group to be controlled is selected based on the search history data for the node Ni read in S130 (S310). In the CATV system, the transmission line devices 6 and 8 are grouped into a trunk line 4a, a branch line 4b, and a branch line 4c, and the transmission line devices 6 and 8 on the trunk line 4a are set as the highest-order group. For each transmission line branched from the transmission line devices 6 and 8 belonging to the upper group, a lower group is hierarchically set by the transmission line devices 6 and 8 on the transmission line.

Then, here, after the start of this processing, S3
When 10 is executed for the first time, the highest-order group is selected, and each time the execution of S310 is repeated, a lower-order group is selected. However, when there are a large number of peer groups, the transmission line devices 6 and 8 at the branch source are selected in order from the one located at a lower position in the upper group. If 8 is the same, it is assumed that selection is made in accordance with a preset order.

When a group is selected in this way, the transmission line device 6 or 8 belonging to the group which is located at the most downstream side is selected as a device to be controlled (S320). At this time, the current setting state of the gate switches 16 and 26 is extracted and stored for the selected control target device from the search history data previously read.

Next, a control signal (attenuation adjustment signal) for instructing the controlled devices 6, 8 to turn off the gate switches 16, 26 is output (S330), and the control signal is received. After the controlled devices 6 and 8 wait for the time necessary to turn off the gate switches 16 and 26, the signal measuring unit 54 measures the noise level (S340). Subsequently, a control signal for instructing the control target devices 6, 8 to restore the gate switches 16, 26 to the original setting state stored in S320 is output (S350).

Then, it is determined whether or not the noise level NL measured in S340 is larger than the noise upper limit value NLmax (S360), and if it is larger than the noise upper limit value (NL).
> NLmax), assuming that there is no noise source downstream of the transmission path devices 6 and 8 to be controlled, the process returns to S320, and the control target devices 6 and 8 are newly selected, and S320 to S36
0 is repeated.

On the other hand, if the noise level measured in S340 has changed to the noise upper limit or less (NL ≦ NLmax), it is determined that there is a noise source downstream of the controlled devices 6 and 8, and It is determined whether there is a lower group having the control target devices 6, 8 as a branch source (S37).
0). If there is a lower group, the process returns to S310, a new control target group is selected, and S310 to S310.
The process of S370 is repeated, and if there is no lower group, the control target devices 6, 8 are specified as the transmission line devices 6, 8 located immediately upstream of the noise source, and the search results are stored. (S380), this process ends.

That is, in the noise source search processing, first,
The transmission line device 6 of the main line 4a is selected as a device to be controlled,
If the noise level is improved by turning off the gate switches 16 and 26 of the controlled device, the subsequent search can be limited to each branch line 4b branched from the controlled device. The transmission path devices 6 and 8 located immediately upstream can be efficiently specified.

Next, S110, S180, and S340 are performed.
The noise level measurement process started by the signal measurement unit 54 and executed by the signal measurement unit 54 will be described with reference to the flowchart shown in FIG. As shown in FIG. 6, when this processing is started, first, the measurement mode of the measuring device 44 is set to a normal mode for measuring a frequency component of a signal to be measured at a certain point in time (S410). The measuring device 44 analyzes the frequency component of the frequency band of the upstream signal (10 MHz to 55 MHz), and from the analysis result, a measurement point (measurement frequency) set in a noise upper limit value setting process described later.
Is stored as a measured value (S420).

It is determined whether or not the measurement of the noise level has reached a predetermined number of times C (for example, 10 times) set in advance (S430).
Returning to 0, the measurement of the noise level is repeated. On the other hand, when the measured value of the noise level reaches the predetermined number C, the average NLav of the measured value is calculated according to the equation (1) (S440). It is determined whether the group is being searched (S45).
0).

[0054]

(Equation 1)

It is to be noted that whether or not a group search is being performed is determined by starting the processing from S180 and S340, and determining whether or not the transmission is a specific transmission path device or a device to be controlled at that time. It is determined that a group search is being performed when a plurality of subscriber terminals are connected downstream of the road device. Here, it is assumed that the number N of subscriber terminals is connected downstream of the transmission path device.

If the group search is not being performed, the average NLav of the measured values calculated in S440 is set as the noise level NL as it is (S460). A value obtained by correcting the average NLav by the equation (2) is set as the noise level NL (S4
70), end this processing.

NL = NLav−10 × logN (2) In the equation (2), the units of NL and NLav are [dB].
Equation (2) is equivalent to converting the average NLav of the measured values into a noise level at each terminal.

Next, the above S120, S190, S360
The noise upper limit value update process that is periodically executed by the signal measuring unit 54 to update the noise upper limit value NLmax referred to by will be described with reference to the flowchart shown in FIG. FIG.
As shown in the figure, when this processing is started, first, the measuring device 44
Is set to the peak hold mode for holding the maximum level of the measurement result (S510), and the measuring device 44 supplies the frequency band (10
(MHz to 55 MHz) is repeatedly analyzed (S520). At this time, the maximum value for each frequency component during the measurement period is obtained as a measurement result by the measuring device 44.

Based on the measurement result, a plurality of measurement points are determined from a band having a level lower than a predetermined judgment threshold (S530), and the frequency fm and the measurement value NLd of the measurement point are stored. (S540).
It should be noted that the determination threshold is set to a signal level such that the channel is reliably identified as an empty channel to which no uplink signal is transmitted. The measurement point set in S530 is also used in S420 of the previous noise level measurement processing.

Then, a value obtained by adding a prescribed value (for example, 5 dB) as a margin to the stored measured value NLd is set as the noise upper limit value NLmax (S550), and this processing ends. In this embodiment, S130, S1
40 (S310 to S380) and S200 correspond to the searching means in the present invention, and S150 to S190 correspond to the noise adjusting means.

As described above, the CAT of this embodiment
In the V system, when the noise level of the uplink signal received by the center device 2 exceeds the noise upper limit, the transmission line 4 where the noise source exists is not simply separated from the system, but the noise source is not separated. By increasing the attenuation of the upstream signal in the transmission line devices 6 and 8 located immediately upstream, the noise level of the upstream signal received by the center apparatus 2 is set to be equal to or lower than the noise upper limit value.

Therefore, according to the CATV system of the present embodiment, it is possible not only to prevent the entire system from being affected by such abnormal noise, but also to prevent the abnormal noise from being located downstream of the noise generating source when the abnormal noise occurs. Since the upstream signal from the cable modem to the center device 2 is not interrupted, services using the upstream signal such as a connection service to the Internet via the center device 2 can be provided continuously, and Reliability can be improved.

Moreover, in the CATV system of the present embodiment, the search for the noise source is performed hierarchically in units of groups, so that the transmission line devices 6, 8 located immediately upstream of the noise source can be efficiently specified. Therefore, a decrease in communication quality due to an increase in ingress noise can be quickly eliminated.

Further, according to the CATV system of the present embodiment, when the branching device 8 has a branch terminal Tb to which a drop-in line to a subscriber's house subscribed to only a service by a down signal is connected, the branch terminal Tb is connected to the branch terminal Tb. By setting the corresponding gate switch 26 to the off-state and realizing the characteristic shown by the solid line in FIG. 3 (c), the service using the upstream signal at the above-mentioned subscriber's house (for example, Internet connection)
Can be reliably prevented from being illegally used. [Second Embodiment] Next, a second embodiment will be described.

FIG. 8A is an explanatory diagram showing the internal configuration of the information distribution board 60 installed at the subscriber's house of the bidirectional CATV system. As shown in FIG. 8A, the information distribution board 6
0, a distributor 6 that distributes a downstream signal input through a protector to two systems and mixes upstream signals from both systems.
2, a booster 64 that bidirectionally amplifies a downstream signal and an upstream signal flowing through this transmission line is connected to one of the transmission lines branched by the distributor 62, and the other is provided on the other transmission line. , Cable modem 66 and router 68
Is connected.

As shown in FIG. 8B, the booster 64 has a configuration in which the branch circuit 17 and the status monitor 18 are omitted from the bidirectional amplifier 6 shown in FIG. Gate switch 16 inserted in the path
Are configured to be operated by an external control signal. The same components as those of the bidirectional amplifier 6 are denoted by the same reference numerals in the drawings, and description thereof will be omitted.

Then, the output of the booster 64 is distributed to a plurality (not shown) and then supplied to an information outlet provided in each room. By connecting a television receiver, a CATV telephone, or the like to this information outlet via a set-top box, it becomes possible to view programs and receive CATV telephone services.

The router 68 is connected to a plurality of personal computers (personal computers) PCs and enables communication between the PCs. An arbitrary personal computer PC connected to the router 68 is connected to a CATV via a cable modem 66. It is configured to allow Internet connection via the system.

Further, like the status monitor 18 of the bidirectional amplifier 6, the cable modem 66 has a function of analyzing a control signal from the center device 2, and sends the control signal to the booster 64 via the router 68. (Attenuation amount adjustment signal).

In the bidirectional CATV system of this embodiment in which such an information distribution board 60 is installed at the subscriber's house, the booster 64 is treated as a transmission line device equivalent to the bidirectional amplifier 6, the branching device 8, and the like. As a result, the influence of the noise source can be suppressed to a smaller range, and the reliability of the system can be further improved.

Also, in this case, if the gate switch 16 for switching the passage or blocking of the up signal of the booster 64 is controlled so as to be turned on only during the viewing of the viewer participation program or when using the CATV telephone, Ingress noise can be reduced more effectively. Third Embodiment Next, a third embodiment will be described.

In this embodiment, the gate unit 8b of the branching device
Is only partially different from that of the first embodiment, and therefore the description will focus on the different parts. That is, in the present embodiment, the gate unit 8b of the branching device
As shown in FIG. 9A, in addition to the configuration of the gate unit 8b described in the first embodiment (see FIG. 2B), the HPFs 24 (24a, 24b,
..) And the distribution circuit 22, a gate switch 29 that switches between passing and blocking of a downstream signal passing through each downstream path.
(29a, 29b,...).

Then, the added gate switch 2
Reference numeral 9 denotes a downstream side (branch terminal Tb) of the switch element 70 that switches between passing and blocking of a downstream signal as shown in FIG. 9B.
On the side, a noise generator 72 is connected via an attenuator 74, and a power supply line to the noise generator 72 is provided with a power switch 76 interlocked with the switch element 70.

The status monitor 18 receives a control signal from the center device 2 and, in accordance with the control signal,
It is configured to generate a switching signal for the gate switch 29 as well as the gate switch 16. Then, when the gate switch 29 is set to an on state for passing a down signal, that is, when the switch element 70 is in an on state, the power switch 76 is turned off and the noise generator 72 does not operate. When the switch is set to the off state, ie, when the switch element 70 is in the off state, the power switch 76 is turned on, and the white noise generated by the noise generator 72 is transmitted to the branch terminal Tb.
Side will be supplied. It should be noted that the intensity of the superposed white noise may be the same as that of the downstream signal leaking downstream when the switch element 70 is in the off state.

In the bidirectional CATV system of this embodiment configured as described above, the gate switch 29 on the downstream path
Is set to ON, as shown in FIG. 10 (a), by operating the gate switch 26 on the up route, as shown in FIG. Since similar characteristics can be realized, similar effects can be obtained.

In the bidirectional CATV system according to the present embodiment, when there is an unused branch terminal Tb of the branching device 8, the gate switches 26 and 29 of both the upstream and downstream paths are turned off. By setting (see FIG. 10 (b)), C through the unused branch terminal Tb is set.
Unauthorized use of the ATV service can be prevented.

Moreover, such unused branch terminals Tb
Can be easily realized by remote control from the center device 2. Further, in the present embodiment, even if the isolation of the switch element 70 is insufficient and the downstream signal leaks to the branch terminal Tb, the white noise superimposed by the gate switch 29 causes
Since it is made difficult to extract the downlink signal, the above-described fraud prevention effect can be further ensured.

In other words, when the switch element 70 is off, white noise is superimposed on the downstream side of the switch element 70, so that the switch element 70 constituting the gate switch 29 that switches between passing and blocking of a downstream signal is provided.
It is possible to adopt a device having a not so large isolation, and it is possible to inexpensively configure a device which is extremely effective in preventing the above-mentioned fraud.

In this embodiment, the distribution circuit 22
Represents the distribution means, the gate switch 29 corresponds to the switching means, and the noise generator 72, the attenuator 74, and the power switch 76 correspond to the noise superimposing means. In the present embodiment, the amount of attenuation at the gate switches 16 and 26 is set to ON ($ 0d).
B), -3dB, -6dB, -10dB, off setting (≒
(−30 dB) is set in five steps, but the present invention is not limited to this, and may be set in four or less steps or five or more steps, as long as different amounts of attenuation can be set in each step. May be set to an appropriate value.

Further, in the present embodiment, the attenuation is changed stepwise, but a variable attenuator capable of continuously changing the attenuation may be used. In this case, since the signal is not momentarily interrupted when the attenuation is switched, control at the time of switching can be facilitated. The configuration for changing the amount of attenuation may be any of an electronic type and a mechanical type.

Further, in the present embodiment, a value obtained by adding a prescribed margin to the measured noise level is set as the noise upper limit value.
/ N (determined from the modulation / demodulation method and the operation level) may be set as the noise upper limit value. For example, in the case of the QPSK method, since the C / N only needs to be 18 dB or more, a value that is 18 dB lower than the operation level may be set as the upper limit.

In the present embodiment, the search for the target device is performed by group selection. However, the search is performed by directly selecting each device (terminal) from the downstream side sequentially and individually based on the database. You may.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of a bidirectional CATV system according to an embodiment.

FIG. 2 is a block diagram illustrating a configuration of a transmission line device.

FIG. 3 is an explanatory diagram showing a configuration and an operation of a gate switch.

FIG. 4 is a flowchart showing the contents of a noise level adjustment process.

FIG. 5 is a flowchart showing the contents of a noise source search process.

FIG. 6 is a flowchart showing the contents of a noise level measurement process.

FIG. 7 is a flowchart showing the contents of a noise upper limit value update process.

FIG. 8 is a block diagram illustrating an internal configuration of an information distribution board and a configuration of a booster according to a second embodiment.

FIG. 9 shows a gate section of a branching device according to a third embodiment;
FIG. 3 is a block diagram illustrating a configuration of a gate switch provided in a downstream path.

FIG. 10 is an explanatory diagram illustrating a signal passing characteristic at a branch terminal.

[Explanation of symbols]

2 center device, 4 transmission line, 6 bidirectional amplifier (transmission line device), 8 branch device (transmission line device), 8a, 17,
27: branch circuit, 8b: gate section, 10, 14, 20,
24 ... High-pass filter, 12, 13 ... Amplifier circuit, 1
1, 15, 21, 25 ... low-pass filter, 16, 2
6, 29 gate switch, 18, 28 status monitor, 22 distribution circuit, 23 junction circuit, 30 selector, 31-33, 74 attenuator, 42 node switcher, 44 measuring instrument, 46 downlink Control signal transmission unit, 50 monitoring and control device, 52 database, 54
Signal measuring unit, 56: Signal level adjusting unit, 58: Noise level adjusting unit, 60: Information distribution board, 62: Distributor, 64: Booster, 66: Cable modem, 68: Router, 70
... Switch element, 72 ... Noise generator, 76 ... Power switch, Tu ... Upstream terminal, Td ... Downstream terminal, Tb ... Branch terminal

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Toshihiro Sugiura 80, Nojo, Asada-cho, Nisshin-shi, Aichi F-term (reference) in Maspro Denko Corporation 5C056 FA02 FA03 GA01 GA11 HA12 HA14 HA15 5C061 BB03 BB05 BB15 CC03 5C064 BA01 BB05 BC12 BC13 BC14 BD11 5K046 BB03 DD15 DD18 DD25 5K052 AA02 AA11 BB13 DD07 EE12 FF11 GG12

Claims (7)

[Claims] 1. A bidirectional CAT in which a plurality of transmission path devices are connected in a transmission path from a center device to a subscriber terminal device.
In the V system, when the noise level of the uplink signal received by the center device exceeds a preset noise upper limit, a transmission line device located immediately upstream of the noise source is specified, and the transmission line device A bidirectional CATV system characterized by increasing the amount of attenuation for upstream signals. 2. The signal level of an uplink signal transmitted by the subscriber side terminal device is set such that the signal level of the uplink signal received by the center device is within a predetermined allowable range. 2. The bidirectional CAT according to claim 1, wherein the automatic adjustment is performed according to an instruction from the center device.
V system. 3. A transmission line device connected in a transmission line of a bidirectional CATV system from a center device to a subscriber terminal device, wherein the attenuation adjustment signal is superimposed on a downstream signal from the center device. Transmission line equipment provided with signal attenuating means for changing the amount of attenuation with respect to an upstream signal to the center device according to the following. 4. A distribution means for distributing a downlink signal from the center device to a plurality of signals, and a distribution path for transmitting the signal distributed by the distribution means in accordance with a switching signal superimposed on the downlink signal from the center device. 4. The transmission line device according to claim 3, further comprising: switch means for controlling passage and blocking of a downstream signal for each transmission. 5. A noise superimposing means for superimposing white noise on a downstream side of the switch means when the switch means is set to block the passage of a down signal in cooperation with the switch means. The transmission path device according to claim 4. 6. A center device of a bidirectional CATV system in which a plurality of transmission line devices are connected in a transmission line to a subscriber terminal device, wherein the center device in a frequency band used for transmission of an uplink signal. A noise level detecting means for detecting a noise level; and if the noise level detected by the noise level detecting means exceeds a preset noise upper limit value, a noise source is searched for and a noise source immediately adjacent to the noise source is detected. Search means for specifying a transmission line device located upstream, and a noise level detected by the detection unit is equal to or less than the noise upper limit value, of the uplink signal in the transmission line device specified by the search unit A noise adjustment unit that generates an attenuation adjustment signal for adjusting the attenuation, and superimposes the attenuation adjustment signal on a downstream signal and transmits the resultant signal. 7. A signal level detecting means for detecting a signal level of a received uplink signal, and said subscriber is set so that a signal level detected by said signal level detecting means is within a predetermined allowable range. Level adjusting means for generating a level adjustment signal for adjusting the transmission level of the uplink signal in the side terminal device, and superimposing the level adjustment signal on the downlink signal and transmitting the signal. 6. The center device according to 6.