JP3138037B2 - Ghost removal device for CATV using optical transmission line - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a C-type optical fiber cable.
Regarding the ghost removal device of the ATV system, when a video signal from the head end is received by the TV receiver of the terminal of the CATV system, the time difference from the TV broadcast wave that jumps directly into the TV receiver and appears before the original video. The present invention eliminates the pre-ghosting caused by the above, and is particularly suitable for a building co-listening system such as a hotel, an office building, a condominium, etc. in which a large number of TV receivers are collectively installed.

[0002]

2. Description of the Related Art In a CATV system, a TV from a broadcasting station is used.
In addition to retransmission of radio waves received at the headend and transmission from the headend to the terminal using the coaxial cable of the CATV system, various data such as independent broadcasts
Data signal is also transmitted.

However, in an area where a potential electric field (airwave from a TV station in the air) is strong, a broadcast wave from the TV station is transmitted to a CA.
When retransmitting in a TV system, a TV broadcast wave existing as a latent electric field may jump directly into a TV receiver. In this case, the signal transmitted through the coaxial cable of the CATV system has a lower transmission speed than the signal propagating in the air due to the transmission characteristics of the cable, so that the coaxial cable can be used for the terminal TV receiver. Rather than the transmitted retransmission signal,
A diving wave in which a latent electric field directly enters the TV set arrives at the TV set earlier. For this reason, the image due to the jump wave appears as a ghost on the screen of the TV receiver, and the ghost image is shifted to the left of the original image by the retransmission signal as shown in FIG. Appears.

In order to solve this problem, it is conceivable to increase the output level of only the signal of the retransmission channel at the head end and send it out. In this case, the high level retransmission signal is output to all the amplifiers in the system. , The signal is easily distorted, and the distortion of the entire CATV system is worsened. Therefore, conventionally, the retransmission signal also propagates at the same low level as other signals, and the anti-dive device is attached to the TV receiver of each subscriber terminal.

[0005]

However, when a large number of receivers are installed, such as in a building co-listening system of a hotel or office building, etc., it is difficult to mount the dive arrestor on all of the TV receivers. This is not very cost-effective. In addition, there was another drawback that the expected effect was not exhibited in a strong electric field area.

SUMMARY OF THE INVENTION It is an object of the present invention to reduce a front ghost caused by a latent electric field entering a TV receiver.
It is to provide a ghost removing device for an ATV.

[0007]

The horizontal scanning time of an ordinary TV receiver 4 is about 63.5 μs, and the effective horizontal scanning time that appears on the screen is about 45 to 50 μs. When a broadcast wave of the TV station 15 directly enters the receiver 4 in addition to the retransmission broadcast of the TV station 15 transmitted from the head end 3 by the coaxial cable 1, a front ghost as shown in FIG. To be observed. For example, at this time, the effective horizontal scanning time of the TV receiver 4 is 50 μs, and the width of the screen is W (c
m), assuming that the interval between the image from the coaxial cable 1 and the previous ghost due to the jump wave is lg (cm), the video signal transmitted by the CATV coaxial cable 1 is t (μs) with respect to the radio broadcast of the TV station 15 (= 50 × (lg / W))
It will enter the receiver 4 with a time delay.

Accordingly, in the present invention, the video signal transmitted from the head end 3 by the coaxial cable 1 is intentionally further delayed than the original delay, and the retransmitted signal of the coaxial cable 1 The signal enters the receiver 4 just one cycle later than the direct dive signal (FIG. 2B).
A real image delayed by one cycle is overlaid on the front ghost, and the front ghost is erased. Strictly speaking, the real image from the coaxial cable 1 is displayed with a shift of one scanning line below the previous ghost, but since the interval between the scanning lines of the image receiver 4 is extremely small, it is almost impossible for the eyes. I can't confirm.

As means for intentionally delaying the video signal from the coaxial cable 1 by an arbitrary time, a part of the coaxial cable 1 is replaced with an optical transmission line 2 having a certain length, and the inside of the optical transmission line 2 is changed. A signal propagation time T ₀ required for an optical signal to propagate is used. The signal propagation time T ₀ of the optical fiber used as the optical transmission line 2 is given by the following equation. T ₀ = (N / C) × 1 N is the refractive index of the core of the optical fiber, C is the speed of light, and 1 is the length of the optical fiber. From this, if the length (l) of the optical transmission path is changed, it is possible to delay the broadcast signal from the coaxial cable 1 for an arbitrary time.

Therefore, a CAT utilizing the optical transmission line of the present invention
The V-ghost removing device replaces a part of the coaxial cable 1 of the CATV system with an optical transmission line 2 using an optical cable, and adjusts the length of the optical transmission line 2 to convert a video signal from the head end 3 into a CATV system. Is transmitted to the TV receiver 4 after being delayed by a time difference from a previous ghost due to a diving wave appearing on the TV receiver 4 of the terminal.

[0011]

In the ghost eliminator for CATV using the optical transmission line of the present invention, as shown in FIG. 1, an optical transmission line 2 of an optical cable is provided in the middle of a coaxial cable 1 in a CATV system. When the length of the video signal is changed, the signal transmission time of the optical transmission line 2 changes, so that the arrival time of the video signal sent from the head end 3 to the TV receiver 4 can be changed arbitrarily. In the case where the front ghost (dotted line) and the real image (solid line) as shown in a) are displayed on the TV receiver 4, if the length of the optical transmission path 2 is adjusted, the real image displayed on the TV receiver 4 is further delayed. Can be overlaid on the front ghost (Fig. 2 (b)),
As a result, the front ghost becomes invisible.

[0012]

FIG. 1 shows an embodiment of a ghost removing device for CATV using an optical transmission line according to the present invention. 15 shown in FIG.
Is a TV station that broadcasts video using radio waves, and 1 is a CATV
A coaxial cable 3 serving as a relay transmission line in the system; a head end 3 for receiving a broadcast from the TV station 15 and retransmitting the broadcast to the receiver 4 of each subscriber via the coaxial cable 1;
Reference numeral 10 denotes a main line amplifier, 11 denotes a main line distribution amplifier, 12 denotes a main line branch amplifier, and 13 denotes a distribution amplifier, which are the same as those constituting a conventional CATV system.

This embodiment is an example of application to a building co-listening system. As shown in FIG. 1, a TV broadcast from a head end 3 is relayed by a relay transmission network of a CATV system and is transmitted from one of distribution amplifiers 13. It is distributed to the building co-listening system by a coaxial cable 1. In this building co-listening system, the broadcast wave from the TV station 15 generates a front ghost in the TV receiver 4, and the delay time from the front ghost of the real image obtained as a result of the measurement is 25 μs. Therefore, the horizontal scanning time of the TV receiver 4 is 63.5 μs, and the delay time 25
If the real image is delayed by the time (38.5 μs) obtained by subtracting μs, the real image just one cycle later overlaps the previous ghost, and TV
It is displayed on the receiver 4.

Therefore, in the present invention, the E / O converter 8, the optical cable which is the optical transmission line 2, and the O / E converter 9 are provided in a part of the coaxial cable 1 of the CATV system connecting the building co-listening system and the head end 3. Optical transmission device 20 composed of
Attached. This optical cable has a core with a refractive index of 1.5, and its length is about 7.7 km according to the signal propagation formula of the optical cable.

[0015]

The following effects can be obtained by using the CATV ghost removal apparatus using the optical transmission line of the present invention. . Since the real image is superimposed on the front ghost to make the front ghost invisible, the effect is not impaired even in a strong electric field region. . When a similar front ghost is seen in many TV receivers 4 in the building cooperative listening system, it is not necessary to attach an anti-dive device to each of the TV receivers 4, and the building cooperative listening system is not required. Since it is only necessary to install the optical transmission device 20 on the coaxial cable 1 which is a service line for distributing a broadcast, the construction is simple and the cost for ghost countermeasures is reduced.

[Brief description of the drawings]

FIG. 1 is a schematic view showing an embodiment of a ghost removing device for CATV using an optical transmission line according to the present invention.

FIG. 2 (a) is an explanatory view showing an example of a front ghost generated in a receiver, and FIG. 2 (b) is a CATV ghost removal apparatus using the optical transmission line of the present invention in the receiver having a front ghost in FIG. 2 (a). Explanatory drawing of the image at the time of attaching.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Coaxial cable 2 Optical transmission line 3 Headend 4 TV receiver

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Claims (1)

(57) [Claims] 1. A part of a coaxial cable 1 of a CATV system is replaced by an optical transmission line 2 using an optical cable, and a video signal from a head end 3 is adjusted by adjusting the length of the optical transmission line 2 so that a terminal of the CATV system can be used. TV receiver 4
A ghost eliminator for CATV using an optical transmission line, wherein the ghost elimination device uses an optical transmission line, delayed by a time difference from a previous ghost caused by a diving wave appearing in the TV signal.