JPH0552095B2 - - Google Patents

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to an aggregate noise suppression device in a two-way CATV system, which transmits a video signal from a subscriber side to a head end using an uplink of a two-way CATV system. In this case, the aggregate noise transmitted to the head end through the uplink is reduced, the C/N (carrier to noise) at the head end is improved, and the image quality is improved.

(Prior Art) Conventionally, there has been an example of a two-way CATV system as shown in FIG. Figure 4 bidirectional
The CATV system consists of 12 uplink lines. In this system, all upstream signals are transmitted toward the head end as shown in the figure. In this case, the transmission bands of the 12 uplinks are all in the same frequency band (usually 10-50MHz), and the upstream amplifiers of all bidirectional amplifiers in a bidirectional system handle signals in all frequency bands of 10-50MHz. It is designed to be amplified.

(Problems with conventional technology) For this reason, if the upstream frequency band is 10 to 50MHz, not only video signals but also thermal noise generated from upstream amplifiers, noise generated at terminals, etc. mix into the upstream line during transmission. Even if there is external noise, all of it is amplified uniformly. Therefore, in uplinks that do not transmit video signals from terminals, only thermal noise generated from the uplink amplifiers, noise generated at the terminals, and external noise mixed into the uplink are amplified.

Moreover, all of these amplified noises merge (concentrate) into the head end through the main line. This is already known as collective noise in two-way CATV systems. Therefore, even if the noise transmitted from each subscriber side or the noise generated by each amplifier is small, the noise concentrated at the head end becomes considerably large.

Moreover, if video signals are transmitted to the head end only from some subscribers and no video signals are transmitted from other subscribers, only noise will be amplified by the upstream amplifiers of the subscribers to whom no video signals are transmitted. , only this amplified noise is transmitted to the headend. In this case, in addition to the noise on the uplink where the video signal is transmitted, the noise on the uplink where the video signal is not transmitted also joins the head end through the main line, so the head end is compared to the video signal carrier. The disadvantage is that the ratio of noise increases and the C/N (carrier-to-noise) ratio deteriorates, resulting in a significant deterioration of the image quality received at the head end.

To solve the above-mentioned problem, it is possible to cut off the amplifier on the upstream line where the video signal is not transmitted, but doing so would also cut off the pilot signal for AGC and the SV signal for monitoring on the upstream line. , it becomes impossible to adjust the gain or control the amplifier in the uplink.

The purpose of the present invention is to solve the problems of collective noise,
The objective is to improve the image quality received at the head end.

(Means for Solving the Problems) A collective noise suppression device in a two-way CATV system according to the present invention is an uplink noise control device branched from the output side of an amplifier circuit 4 inserted into an uplink line 9 in a two-way amplifier of a two-way CATV system. A first filter 5 that passes only the frequency band of the video signal from among the signals.
and a switching circuit 7 which is connected to the output side of the amplifier circuit 4 and whose switching is controlled by the output of the first filter 5, and which switches and selects the insertion of the amplifier circuit 4 into the downstream uplink 9. The second filter 10 blocks the frequency band of the video signal and passes only the necessary signal band necessary for monitoring, gain control, etc. of the bidirectional amplifier. When there is a video signal in the through circuit 8,
However, when there is no video signal, the second filter 10
is selected.

(Function) In the present invention, there is a first filter 5 that passes only the frequency band of the video signal from among the upstream signals branched from the output side of the amplifier 4, and an output of the first filter 5 that is connected to the output side of the amplifier 4 and that passes only the frequency band of the video signal. A switching circuit 7 is switched and controlled by a through circuit 8 which selectively inserts the amplifier circuit 4 into the uplink line 9 at the subsequent stage and passes the uplink signal, and a through circuit 8 which blocks the frequency band of the video signal and blocks the frequency band of the video signal. It is composed of a second filter 10 that passes only the necessary signal band necessary for bidirectional amplifier monitoring, gain control, etc. When there is a video signal in the upstream signal, the through circuit 8 is used, and when there is no video signal, the through circuit 8 is used. Since the second filter 10 is selected, when an upstream video signal is not transmitted from a subscriber, noise and other unnecessary signals within the video signal band on the uplink of that subscriber are cut out. For this reason,
The deterioration of C/N on the head end side is reduced, and the received image quality on the head end side is improved. Furthermore, necessary signals such as pilot signals, SV signals, and security signals are transmitted to the head end without being cut, making it difficult to monitor the bidirectional amplifier.
Control remains possible as before.

(Example) What is shown in FIG. 1 is an example of an uplink signal transmitted on an uplink of a bidirectional CATV system. It is 10 to 10 of the 10MHz to 50MHz band.
The video signal band is 30MHz, and the necessary signal band from 35MHz to 50MHz is necessary for controlling and monitoring amplifiers such as pilot signals, SV signals, and security signals.

In the present invention, when transmitting a video signal through an uplink, the upstream signal in the entire band of 10MHz to 50MHz shown in FIG. 30M
Only the noise and other unnecessary signals at Hz are cut out, and the necessary signals at 35MHz to 50MHz are amplified by the upstream amplifier 2.

Therefore, in the present invention, a first filter 5 as shown in FIG.
A second filter 10 and a switching circuit 7 are provided.

As the first filter 5, a bandpass filter that passes the frequency band of the video signal (10MHz to 30MHz in FIG. It is designed so that it can be taken out.

The second filter 10 is used to filter necessary signal bands (the first
A high-pass filter is used in which only the band (35 MHz to 50 MHz in the figure) is passed through, and the band below that band (10 MHz to 30 MHz in FIG. 1) is not passed, and it is installed in parallel with the through circuit 8. This through circuit 8 passes the upstream signal as it is.

Note that 3 in FIG. 2 is a branching filter within the bidirectional amplifier 1.

In Figure 2, when a video signal is transmitted to the uplink, the frequency band of the video signal (10MHz to 30M
Hz) passes through the first filter 5, is detected by the detector 6, and is input to the switching circuit 7. When the frequency band of the video signal is detected by the detector 6, the switching circuit 7 is connected to the through circuit 8, and the upstream signal containing the video signal is output to the upstream line 9 as it is.

When the video signal is not detected by the detector 6 in FIG. 2, that is, when the video signal is not transmitted on the uplink, the switching circuit 7 switches to the second filter 10 side because there is no video signal passing through the first filter 5. Change. Therefore, among the upstream signals, the ones that pass through the second filter 10 are the pilot signal, SV signal,
Bandwidth of necessary signals necessary for security signals, amplifier monitoring, gain control, etc. (35MHz to 50MHz)
(10MHz to 30MHz)
does not pass. Therefore, noise and other unnecessary signals included in the range of 10MHz to 30MHz are not amplified by the amplifier 2.

(Effects of the Invention) As described above, the present invention provides that when a video signal is not transmitted from the subscriber side to the uplink 9, the uplink 9
Only the necessary signals necessary for monitoring and controlling the amplifier of This improves the image quality received at the head end. Moreover, monitoring of the bidirectional amplifier, gain control, etc. can be performed without any trouble.

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram showing an example of an upstream signal, Fig. 2 is an explanatory diagram showing an example of the present invention, Fig. 3 is an explanatory diagram showing an example of a switching circuit, and Fig. 4 is an explanatory diagram showing an example of a switching circuit.
FIG. 2 is an explanatory diagram of an uplink of a CATV system. 1 is a bidirectional amplifier, 2 is an upstream amplifier, 3 is a branching filter, 4 is an upstream amplifier section, 5 is a first filter,
6 is a detector, 7 is a switching circuit, 9 is an upstream line, 10 is a second filter, and 11 is a head end.

Claims (1)

[Claims] 1. A first filter 5 that passes only the frequency band of the video signal from among the upstream signals branched from the output side of the amplifier circuit 4 inserted into the uplink line 9 in the two-way amplifier of the two-way CATV system, and the same amplifier circuit 4. a switching circuit 7 which is connected to the output side of the filter and whose switching is controlled by the output of the first filter 5;
The switching circuit 7 selects and selects the insertion of the amplifier circuit 4 into the uplink line 9 at the subsequent stage, and the through circuit 8 passes the upstream signal and blocks the frequency band of the video signal to monitor and control the gain of the bidirectional amplifier. When there is a video signal in the upstream signal, the through circuit 8 is selected, and when there is no video signal, the second filter 10 is selected. A collective noise suppression device in a two-way CATV system, characterized in that: