JPS63133778A - Cable television commonly viewing transmitter with radio interference corrective measure - Google Patents

Abstract

PURPOSE:To solve a radio interference with an inexpensive means by setting a transmission channel frequency to a frequency lower by an adjacent channel trap frequency difference (1.5MHz) from a normal frequency. CONSTITUTION:When a channel beat is generated in an internal receiver according to the jump in of a general broadcast wave from an external part, the difference in the intensity between a jumping channel and the transmission channel is below -20dB. Herein when an external receiver 2 receives 199.25MHz, the internal receiver 1 is finely controlled to transmit the frequency of 197.75MHz. The 197.75MHz is trapped by the external receiver 2, so that the transmission of the internal receiver 1 does not interfere. A general television receiver is finely controlled from +2MHz--2MHz and easily set to a frequency lower by 1.5MHz.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a cable television public viewing transmission device with interference countermeasures. It is common practice in television listening equipment and CATV to convert UHP band television channels to VHF band for transmission. This is because the cable loss is U and the HF band is VH.
Since it is larger than the F band, it is necessary to install an amplifier in the middle to increase the width, and this is to prevent the hassle and cost.

However, when switching to the VI (F band), the same channel bead as that of an external VHF station may occur.Especially recently, the number of broadcasting stations has increased, and there are no empty channels, so the radio waves of powerful local broadcasting stations are directly transmitted to the receiver. This causes interference to the receiver in the jump-in U I-I P section.When receiving radio waves from distant broadcasting stations, the radio waves are weak and are susceptible to interference.

Previously, countermeasures against interference waves coming in from outside had been considered, such as shielding the receiver, but this had not been done in practice. For this reason, facility operators and general viewers were struggling with this problem, but no fundamental measures were taken. SUMMARY OF THE INVENTION Accordingly, the present invention aims to provide a wired television public viewing transmission device with countermeasures against interference, which solves the above-mentioned problems at a very low cost by utilizing the basic characteristics of television receivers. Examples will be described below with reference to the drawings.

If the same channel beat occurs in the internal receiver due to the intrusion of general broadcast waves from the outside, or if the transmission channel frequency is set to a higher frequency than the normal frequency and causes adjacent channel interference, the transmission channel frequency is set to a higher value than the normal frequency by the adjacent channel trap frequency difference (l). - Lower the frequency by 5 MHz). In the frequency characteristic diagram of the receiver (FIG. 1), it is assumed that the normal transmission channel frequency rA, the lower adjacent channel trap frequency fB, the audio trap frequency rC1 and the upper adjacent channel trap frequency fD.

If the adjacent channel trap frequency difference is fE,
rE = rA
f If the same channel beat occurs in the internal receiver I due to the intrusion of general broadcast waves from outside B, if the frequency of the regular transmission channel is 199.25 MHz, 200
．． Transmit using 75MH2. Adjust the channel frequency fine adjustment section of the internal receiver 1. Currently, the regular channel frequency of the incoming radio waves is 199.25M1-IZ, but the transmission channel frequency is 200.75M1-IZ, and there is a difference of 1.5MHz, so it is just the adjacent channel of the TV receiver. It becomes a trap frequency and is trapped and no interference occurs. According to the technical standards of the Cable Television Broadcasting Law Enforcement Regulations, same-channel interference among beat interference is defined as 150 db or less. The difference in strength between the incoming radio wave and the transmission channel is less than -20 db. It is also said that the lower adjacent trap requires a trapping effect of -30 db. This creates a difference of -50db.

Normally, in the intermediate frequency tank circuit of a television receiver, the frequency characteristics of the band are obtained using a staggered type, and a trap circuit is provided. The frequency characteristics are set to 199.
When receiving 25MHz, it transmits at 197.75MHz. The internal receiver 1 is finely adjusted. 197.75MH2
is trapped in the external receiver 2. Fine adjustment of general television receivers is possible from +2 MHz to -2 MHz FI Z. Originally, there is no way to prevent interference from interference on the same channel, but by shifting the frequency of the transmission channel upward or downward within the channel fine adjustment range of the television receiver, there is an adjacent channel trap that must be included as a frequency characteristic of the receiver. The technical idea is to focus on the frequency difference of 1.5M I (Z) and drop the interference wave to this trap frequency.The frequency of the transmission channel can be changed by changing the transmission oscillation crystal, etc. in the transmission device. It is also possible to use a circuit that can obtain a transmission channel frequency above or below 1.5 MHz using a digital circuit, PLL circuit, etc. When the area is specified, the number of channels of the local broadcasting station is known. The crystal may be changed.Also, a frequency change circuit 3 may be used to switch the frequency using a changeover switch 4 so that it can be used in any region.The frequency change circuit 3 includes a crystal circuit 5 for the fundamental frequency, and a mixing circuit 6. .1・5 MHZ generation circuit 7
, the selection circuit 8. Digital circuit and P
It is common practice to generate an arbitrary frequency using an LL circuit, and any method may be used. TV communal listening equipment and CATV
However, since the number of viewers is large, it is not possible to make changes such as changing the TV receiver. Also, it is necessary to ensure that external viewers are not disturbed. TV public viewing transmission section 10 and CATV
A cable is connected to the transmission section 11, but from an economic point of view it is necessary to reduce cable loss and avoid using a relay multiplier. For facilities with a large number of viewers and a wide area, the present invention provides a significant cost reduction. Furthermore, this invention does not disturb external viewers, can be easily implemented at low cost, and is extremely effective.

[Brief explanation of the drawing]

Figure 1 is a frequency characteristic diagram Figure 2 is a block diagram Figure 3 is a perspective view

Claims (1)

[Claims] If the same channel bead occurs in the internal receiver due to the intrusion of general broadcast waves from outside, the transmission channel frequency should be changed from the normal frequency to the adjacent channel trap frequency difference (1
- Set the frequency to a higher frequency (5 MHz) or connect the external receiver to C
In the case of interference caused by leaked radio waves from ATV terminals, etc., the transmission channel frequency is lower than the regular frequency by the difference in adjacent channel trap frequency (1.5 MHZ).Cable TV shared listening transmission with interference prevention measures. Device