JPS6156920B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a method for preventing eavesdropping due to demodulation of adjacent channels in a multi-channel communication method in a CATV system, with respect to an eavesdropping prevention device using gray sync or a similar method. It is related to the method.

In the CATV system, processing to prevent eavesdropping, that is, scrambling processing, is applied to channels that provide programs that are difficult or undesirable to viewers other than specific viewers, such as programs for special fields. To prevent eavesdropping, a gray sync method or a similar method is often applied, in which the synchronization level of the high-frequency television composite signal is lower than the peak value of the video signal. There is. CATV channels are allocated to channels of general television receivers (every 6MHz), and the output of the terminal device (frequency converter) is usually received on one channel of the general television receiver. It is.
Therefore, although the terminal device suppresses the adjacent channel output, the television receiver can actually receive the adjacent channel.

On the other hand, due to various problems in terminal devices and CATV broadcasting stations, processing is sometimes performed to bring the horizontal synchronization signal into phase by performing so-called GEN LOOK on all channels or channels including adjacent channels. this
Apply Gray Sync or a similar method to prevent eavesdropping on channels that have undergone GEN LOCK processing or channels where adjacent synchronization signals are forced to have the same phase, and apply it to multiple channels including adjacent channels. If this is done, even if the terminal is tuned to one channel, the demodulator to prevent eavesdropping will demodulate adjacent channels at the same time. Therefore, by receiving a specific channel, for example, a channel adjacent to a pay program, it is possible to spy on that specific pay channel for free. The present invention aims to provide a system that can prevent such spying.

An example of the implementation of the present invention will be described below with reference to the drawings.

This embodiment is a system in which a frequency twice as high as that of the horizontal synchronization signal is used as a modulation signal to prevent eavesdropping. 1st
In the figure, 1 is a synchronizing signal generator, and 2 is a phase shifter (90° phase shift with respect to the horizontal synchronizing signal here).
1 shows the output waveform of the synchronizing signal generator 1, and b shows the output waveform of the phase shifter 2, showing the phase relationship of the synchronizing signals. The outputs of the synchronizing signal generator 1 and the phase shifter 2 are added to the video signal generator ₃₁ ...3n, and the output channels of the frequency converter 6 are GEN LOCKed every other channel so that they are adjacent to each other.
is being applied. Considering the phase of vertical synchronization here, the frequency is extremely low compared to the horizontal synchronization signal, and the phase difference between adjacent channels is also small, so it can be considered that GEN LOCK is applied to all channels. 4 _{1 .} . . 4n is a modulator that modulates the output of the video modulator and the audio signal into an IF signal. To prevent eavesdropping, this composite signal is processed by anti-eavesdropping modulators 5 ₁ to 5n to scramble the composite signal. This device detects the horizontal synchronization signal from the composite signal, and applies AM modulation so that the synchronization signal is suppressed using a signal with twice the frequency synchronized with the horizontal synchronization signal. The waveform will be as shown in . Frequency converter 6
₁ to 6n, the frequencies are distributed so that the converters 6m and 6m+1 are adjacent to each other. These outputs are added by a carrier wave signal adder 7 and distributed to each terminal 9 by a transmission line 8. 10 is a frequency conversion unit for inputting to the television receiver 14, 11 is a demodulation unit for eavesdropping prevention processing such as a multiplier, and 12 is an output signal and a KEY signal for demodulating the eavesdropping prevention processing. Separated KEY with separation circuit
The signal is processed by the KEY signal demodulation circuit 13 and applied to the eavesdropping prevention processing demodulation section 11, where it is demodulated into a scrambled signal waveform.

That is, the eavesdropping prevention processing demodulation unit 11 performs demodulation so as to suppress the eavesdropping prevention modulation of the signal waveform shown in FIG. 2c. The waveform is shown in FIG. 2e. now,
If the phases of the synchronization signals of adjacent channels are equal, the adjacent channels will be demodulated in the same way, but if the synchronization signals are 90
゜Since the phase is shifted, the phase of the modulation to prevent eavesdropping is
This results in a difference of 180°, and the waveform of the adjacent channel shown in FIG. 2(d) has a deeper modulation component to prevent eavesdropping, as shown in FIG. 2(f). Therefore,
With a television receiver, you cannot watch adjacent channels even if they are received.

Similar eavesdropping is possible for two or more separate channels, but there is no problem because the carrier signal level can be sufficiently suppressed using a relatively inexpensive filter or the like.

As is clear from the above, according to the present invention, even if a desired channel is demodulated for adjacent channels subjected to eavesdropping prevention processing, the adjacent channels are not demodulated and the eavesdropping prevention is completed. Since the eavesdropping prevention processing is performed on the broadcasting station side rather than on each terminal device, the overall cost is low, and since the processing is relatively simple, several types of horizontal synchronization signals with different phases are prepared. By doing so, it is possible to completely prevent eavesdropping on adjacent channels.Since it is a phase shift process, it is possible to completely prevent eavesdropping on adjacent channels. Excellent effects can be achieved, such as being possible with RF signals.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing one embodiment of the present invention, and FIGS. 2 a to 2 f are output waveform diagrams of the main parts of FIG. 1. 1... Synchronization signal generator, 2... Phase shifter, 3 ₁ ~
3n... Video signal generator, 4 ₁ to 4n... Modulator, 5 ₁ to 5n... Eavesdropping prevention modulator, 6... Frequency converter, 7... Carrier wave signal adder, 8... Transmission line, 9...Each terminal device, 10...Frequency converter, 11... Eavesdropping prevention processing demodulator, 12...Separation circuit, 13...KEY signal demodulation circuit, 14...Television receiver.

Claims (1)

[Claims] 1 In a multi-channel television communication system, the phase of the horizontal synchronization signal between adjacent channels is made different on the transmitting side, and amplitude modulation is applied to suppress the synchronization signal using a grey-sync or similar system to prevent eavesdropping. A method for preventing eavesdropping in a CATV system, characterized in that the receiving side performs demodulation to suppress the modulated amount.