JPS6253582A - Descrambling method - Google Patents

Abstract

PURPOSE:To simply and easily constitute a circuit for descramble and to improve its accuracy by introducing a scramble signal to a frequency/voltage conversion circuit and comparing its output with a set voltage. CONSTITUTION:An F/V converter 12 generates a voltage signal in proportion to the carrier frequency of an intermediate cycle video carrier 1 after a channel selection. A comparator 14 level-compares the voltage signal outputted from the F/V converter 12 with a reference voltage 13 and generates a synchronizing signal period start point detecting signal, supplying it to a gate signal generating circuit 4. A switch 17 is closed by a gate signal outputted from the circuit 4 in a signal period and couples the output of a local oscillation circuit 16 with an intermediate cycle amplifier circuit (IF amplifier)/frequency conversion circuit 18. The circuit 18, to which an intermediate cycle video signal is introduced, is operated as the IF amplifier in a normal time, and with receiving the output of the local oscillation circuit 16 by the closing of the switch 17, it functions as the frequency conversion circuit, converting the carrier frequency of an input signal to a prescribed frequency.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a scrambled television broadcasting system that provides broadcasting to specific subscribers.

[Conventional technology]

In broadcasting systems such as so-called subscript television that provide television broadcasting for a fee to specific subscribers, a scrambled signal sent from a television station is descrambled on the receiving side and returned to a normal television signal for viewing.

FIG. 2 is a block diagram showing an example of a conventional descrambling method. In the figure, 1 is an intermediate frequency video carrier wave of a scrambled television signal tuned on the receiving side, and is a scrambled signal. This intermediate frequency video carrier wave 1 includes a video carrier wave and an audio carrier wave, and as shown in FIG. 3(a), the amplitude of the video carrier wave is compressed to 1/2 during the video synchronization signal period, and the audio carrier wave is , As shown in FIG. 3 (bl), the amplitude is doubled in the video synchronization detection timing signal period. This timing signal period rises slightly faster than the video synchronization signal period. 2 is the audio carrier wave selection. The circuit separates the audio carrier wave from the intermediate frequency video carrier wave 1 and supplies it to a comparator 3. The comparator 3 detects the timing signal period of the input audio (general transmission wave). 4
is a gate signal generation circuit which receives a timing signal period detection signal from the comparator 3 and generates a gate signal (video synchronization signal level demodulation period detection signal) having a time width corresponding to the video synchronization signal period of the video carrier wave. The signal is supplied to an AMM modulator (amplitude modulator) 5. AM modulation circuit 5 While the gate signal is arriving, the amplitude of the video synchronization signal period of the input video carrier wave is doubled, the synchronization signal level of the video carrier wave is demodulated, and the descrambled intermediate frequency video carrier wave 6 is generated. Send.

[Problem that the invention seeks to solve]

In this way, in the conventional descrambling method, the starting point of the synchronization signal level fli of the video carrier wave is detected using the audio carrier wave rather than the video carrier wave, which makes the circuit for descrambling complicated. In addition, since the audio carrier wave is subject to attenuation and waveform distortion in the transmission path and receiver, there is a problem in that the demodulation accuracy decreases.

This invention was made to solve the above problem.
It is an object of the present invention to provide a descrambling method in which a circuit for descrambling can be constructed more simply and easily, and its accuracy can be improved compared to the conventional method.

[Means to solve the problem]

In order to achieve the above object, the present invention detects the synchronization signal level demodulation period by guiding a scrambled signal to a frequency/voltage conversion circuit and comparing its output with a set voltage, thereby determining the synchronization signal period carrier frequency of the video carrier wave. The configuration is such that demodulation is performed to the carrier frequency of the video period.

[Effect]

In this invention, there is no need to separate and extract the audio transmission wave from the intermediate frequency video carrier wave, so the circuit for descrambling becomes simple, and since the audio carrier wave is not used, demodulation accuracy can be improved.

[Embodiments of the invention]

FIG. 1 shows an embodiment of the present invention, in which the video period carrier frequency (58, 75 MHz) of the intermediate frequency video carrier wave 1 is
), the carrier frequency of the synchronization signal period is either higher (eg, 59.75 MHz) or lower.

Reference numeral 12 denotes an F/V converter (frequency/voltage conversion circuit) to which the selected intermediate frequency video carrier wave 1 is guided and generates a voltage signal proportional to the carrier frequency. 14 is a comparator (comparison circuit) that compares the levels of the voltage signal output by the F/V converter 12 and the reference voltage 13, generates a synchronization signal period start point detection signal, and sends this signal to the gate signal generation circuit 4. supply 16 is a local oscillation circuit. 1
Reference numeral 7 denotes a switch, through which the gate signal (synchronous signal level demodulation period detection signal) output from the gate signal generation circuit 4 is guided, and is closed during the signal period to pass the output of the local oscillation circuit 16 to an intermediate frequency amplifier circuit (IF amplifier)/frequency conversion circuit 18
join to. The IF amplifier/frequency conversion circuit 18 receives an intermediate frequency video signal, normally functions as an IF amplifier, and functions as a frequency conversion circuit when it receives the output (58°75MHz) of the local oscillation circuit 16 by closing the switch 17. to convert the carrier frequency of the input signal to a predetermined frequency (in this embodiment, the carrier frequency of the video period).

In this way, in this embodiment, the demodulation start point of the synchronization signal level is detected using the difference in carrier frequency between the video period and the synchronization signal period in the video carrier wave, so there is no need to use the audio carrier wave as in the conventional method. It disappears.

In the above embodiment, the comparator 14 detects the synchronization signal start point and the gate signal generation circuit 4 provides the synchronization level demodulation start period. It is possible to adopt a configuration that provides a starting period, and in this way, the circuit configuration can be made simpler than in the above embodiment.

〔Effect of the invention〕

As explained above, this invention detects the synchronization signal period start point, which is the reference for the synchronization signal level demodulation start point, from the video carrier wave, so it is possible to improve the demodulation accuracy compared to the case where an audio carrier wave is used. There is an advantage that the scramble signal can be descrambled simply by combining simple circuit elements.

[Brief explanation of the drawing]

FIG. 1 is a block diagram for explaining the invention in detail,
FIG. 2 is a block diagram for explaining a conventional descrambling method, and FIG. 3 is a television RF signal waveform diagram in the conventional example. In the figure, 4-...-gate signal generation circuit, 12-F/
V converter, 14 - comparator, 16 - local oscillation circuit, 17 - switch, 18 - IF amplifier/frequency conversion circuit. In addition, in the figures, the same reference numerals indicate the same or corresponding parts.

Claims (1)

[Claims] When descrambling the scrambled signal in a television broadcasting system that transmits and receives scrambled signals, the scrambled signal is a signal obtained by converting the synchronization signal period of the video carrier wave to a frequency different from the carrier frequency of the video period, and Detecting a synchronization signal level demodulation period by guiding the scrambled signal to a frequency/voltage conversion circuit and comparing its output with a set voltage, and demodulating the synchronization signal period frequency of the video carrier wave to the carrier frequency of the video period. A descrambling method characterized by: