JPS59161937A - Group noise suppressing method for bidirectional catv system - Google Patents

Abstract

PURPOSE:To suppress effectively noises with increment of the C/N by cutting the noise and undesired signals when no signal is sent from a subscriber. CONSTITUTION:Only a desired signal is transmitted by cutting the noise and undesired signals. For this purpose, a switching circuit 7 is provided to each amplifier 2 of the ascending circuit of a bidirectional amplifier 1. A BPF5 is set at the output side of an amplifying part 4 to transmit the frequency band of a video signal. The video signal is extracted and delivered to an ascending circuit 9 through the through-part and via a wave detector 6. The switch 7 is set at the side of an HPF10 when no video signal is contained in the input signal of the amplifier 4 to transmit only the desired signals including a pilot signal, etc. which are needed for monitor and gain control of the amplifiers. The transmission is not permitted for the signals having bands lower than that of the desired signal. Thus only the desired signal is transmitted to the circuit 9, and therefore the group noises transmitted to an head end 11 can be suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for suppressing collective noise in a two-way CATV system. The image quality is improved by reducing collective noise transmitted from the head end to the head end and increasing the C/N ratio.

In bidirectional CAT and V systems, all the noise transmitted from the subscriber side to the headend side via the uplink and the noise generated by the upstream amplifier are collected on the main line, so the noise transmitted from each subscriber side and the noise transmitted from each subscriber side are Even if the noise generated in the upstream amplifier is small, the noise concentrated at the head end becomes quite large.

In particular, bidirectional amplifiers in bidirectional systems are all designed to amplify signals in the same frequency band, so any signal transmitted from each subscriber side to the main line, not just video signals, can be amplified.
Even noise is amplified uniformly. Therefore, for example, if a video signal is transmitted to the main line from only some of the many subscribers and no video signal is transmitted from other subscribers, only the noise will be amplified by the upstream amplifier that does not transmit the video signal. , because it is transmitted to the headend, the noise from each subscriber side and the noise generated by the winter uplink amplifier are concentrated at the headend, and the ratio of noise becomes higher than the video signal, resulting in poor C/N. The disadvantage is that the image quality becomes smaller and the quality of the received image deteriorates significantly.

In order to eliminate this problem, all amplifiers on back lines where video signals are not transmitted may be cut off. However, if this is done, the pilot signal for AGC and the SV multiplied signal for monitoring will also be cut, making it impossible to perform gain adjustment and control of the amplifier in the uplink. In order to solve all the problems of the present invention, when the video signal is not transmitted from the subscriber, it cuts the noise and other unnecessary signals in the video signal band on the 9 lines, and monitors the uplink amplifier. Only necessary signals such as pilot signals necessary for control, SV double signal security signals, etc. are transmitted to the head end side.

Figure 1 shows an example of an uplink signal transmitted on the upper nine lines of a two-way CATV system.
Of the z-50 MHz band, 10 to 30 MHz is the entire video signal band, and 35 MHz to 50 MHz is the necessary signal band such as k pilot signal and Sv double signal security signal necessary for controlling and monitoring the amplifier.

In the present invention, when transmitting a video signal, the upstream signal shown in Fig. 1 is transmitted, and when not transmitting the entire video signal, the upstream signal is
-30MJ) Noise and other unnecessary signals in the lz band are cut, and only the necessary signals in the 3.5MJ (501'az band) are transmitted to the uplink.

In order to cut noise and unnecessary signals and transmit only the necessary signals, bidirectional amplification + 11 uplink amplifiers (2) are required.
For example, a switching circuit (7) as shown in FIG. 3 may be provided. This switching circuit (7) operates as follows.

A band pass filter (5) is provided on the output side of the amplifying section (4) to pass the frequency band of the video signal (10 MHz to 30 MHz in the example shown in FIG. 1), and the video signal is taken out from the output of the amplifying section (4). It is detected by a total detector (6) and input into a switching circuit (7). This switching circuit (7) is a detector, and when there is an input signal from 6), it passes through 9Ill (8).
The uplink signal containing the video signal is output to the uplink line (9). If the upstream signal input to the amplification section (4) does not include a video signal, that is, if no video signal is transmitted to the uplink, there is no video signal to be extracted to the bandpass filter (5), so switching The circuit (7) is switched to the bypass filter (1o) side. The filter 00) is used for pilot signals, sV double signals, security signals, and the necessary signal band (35MHz to 50MHz in the case of Figure 1) necessary for monitoring the amplifier and controlling the gain.
z) and the following bands (10MH in Figure 1)
Since z~30MI (Z) is not traced, the uplink (9
) only the necessary signals are transmitted.

Note that (3) in FIG. 2 is a branching filter.

As shown above, when video signals are not transmitted from the subscriber side to the uplink, the present invention cuts noise and other unnecessary signals transmitted to the uplink, which is necessary for monitoring and controlling the uplink amplifier. Since only the necessary signals are transmitted, collective noise transmitted to the head end (1 side) can be suppressed, and the uplink amplifier can be monitored, gain controlled, etc., which is very convenient.

[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, and FIG. 3 is an explanatory diagram showing an example of a switching circuit. (1) is a bidirectional amplifier, (21 is an upstream amplifier, (3) is a branching filter, (4) is an upstream amplifier, (5) is a bandpass filter, (6) is a detector, and (7) is a switching circuit, (9) is the uplink, α0) is the bypass filter, (11) is the head end.

Claims (1)

[Claims] In a two-way CATV system, when the video signal is not transmitted from the subscriber side to the headend side, it cuts noise and other unnecessary signals in the uplink video signal band,
A collective noise suppression method in a two-way CATV system in which only necessary signals such as pilot signals, Sv signal security signals, etc. necessary for monitoring and controlling amplifiers of upper nine lines are transmitted to the head end side.