JPS5820190B2 - Descrambling device for video signal transmission - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a scramble device for restoring a video signal destroyed by a scramble device.

In order to supply a specific video signal only to a specific subscriber among the subscribers of the CATV system, consideration must be given to making the video signal unplayable by subscribers other than the specific subscriber. Is it necessary to keep it?

Therefore, on the video signal transmitting side, a specific video signal is destroyed by a scrambling device and transmitted, while a specific subscriber's side uses a descrambling device to restore the destroyed video signal to its normal form so that it can be played back. The means to do so are used.

As a conventional technology of this type, there is a so-called AM gray sync method.

In other words, in this AM gray sync method, on the transmitting side, the RF
The relationship between the horizontal synchronization signal of the video signal and the modulation level of the video signal in the step is reversed so that the modulation depth of the horizontal synchronization signal is deeper than before, and the modulation depth of the video signal is shallower than before. 1 synchronized with the horizontal synchronization signal
A sine wave (encoded signal) of 5.75 KHz (or an integer multiple thereof) is created, and the video signal is AM-modulated with this sine wave to perform scrambling. To return this to its original state, on the receiving side, Descrambling is performed by creating a sine wave (decode signal) of 15.75 KHz (or an integral multiple thereof) whose phase is inverted with respect to the encoded signal, and demodulating the scrambled video signal using this sine wave.

However, in this case, there is a drawback that the descrambled RF video signal does not completely return to the signal before being scrambled, and Ringer remains.

For example, if the RF video signal is A cos ωt and the encoded signal is B sin θt, then the scrambled signal is (A+Bs1nθt) cos ωt = A(1-hn
sin θt) cos ωt plus m−λ.

When this is scrambled with a decoded signal whose phase is inverted with respect to the encoded signal, the signal is A (1+m sinθt ) (1-m5inθt
) cos ωt = A (1-m2sin2θt)
cos ωj, and m for the original signal A cosωt
” It can be seen that the term 5in2θt remains.

This component is determined by the square of the modulation degree m, but from the standpoint of the original purpose of the scrambler, it is preferable that the modulation degree m be large.

However, as the degree of modulation m increases, the ripple component generated in the descrambled signal increases, giving rise to bright and dark stripes in the vertical direction in the reproduced image quality, making the image very difficult to see.

This invention was proposed in order to eliminate the inconveniences caused by the conventional methods described above, and it is possible to completely restore the original signal before scrambling without generating ripple components in the scrambled RF video signal. The present invention aims to provide a screen scrambling device that provides the following advantages.

The ★ side of this invention will be explained below based on the drawings.

In FIG. 1, 1 surrounded by a broken line indicates a transmitting side (scrambler), and 2 surrounded by a broken line indicates a receiving side (descrambler).

First, on the transmitting side 1, a part of the RF video signal i is extracted by the convex cross section 5 and applied to the horizontal synchronization separation circuit 7.

15, 75 KHz by this horizontal synchronization separation circuit 7
A horizontal synchronizing signal component is extracted, and the extracted synchronizing signal component is applied to a phase/amplitude adjustment circuit 10 via an amplifier 8 and a filter 9 that passes 15.75 KHz.

In this circuit 10, a sine wave signal whose phase is adjusted so that the phase becomes nπ (where n is a natural number) at the synchronization signal is obtained.

The state of the signal waveform at this time is shown in FIGS. 2a and 2b.

In other words, a shows a normal RF video signal including a horizontal synchronization signal, and b shows a sine wave whose phase is adjusted to nπ (where n is a natural number) in the synchronization signal part included in the RF video signal. , that is, an encoded signal.

The thus generated sine wave signal is applied to the drive circuit 11, where it undergoes impedance conversion, and then applied to the logarithmic amplitude modulation circuit 6, which is composed of elements with exponential characteristics. The signal is scrambled.

At this time, the RF audio signal is also scrambled in parallel with the RF video signal.

FIGS. 2c and 2d show the signal waveforms at this time.

That is, C shows the state in which the sine wave shown above is passed through an exponential function circuit, and d shows the state of the RF video signal amplitude modulated by the signal shown in C.

The scrambled signal as described above is transmitted to the receiving side via a relay transmission line 12 such as a coaxial cable.
As can be understood from the waveform in Figure 2 d, this scrambled video signal has a very deep modulation degree of the horizontal synchronization signal present in the video signal, and even if it is reproduced as is on a receiver, This results in a state in which horizontal synchronization cannot be achieved at all, and a sufficient scrambling effect can be obtained.

Next, at the receiving side 2, the scrambled signal arriving via the relay transmission line 12 is processed by the logarithmic amplitude demodulation circuit 13 (
Only the RF audio signal is separated by a distribution circuit 14 via a circuit (same as the circuit 6 described above).

This RF audio signal is amplified by a narrowband amplifier 15,
Further, the RF multiplied signal is detected by the negative detection circuit 16 to obtain a decoded signal having a phase shift of 180° from the sine wave shown in FIG.

(In this case, since the RF audio signal is FM, the signal negative-detected by the detection circuit 16 can be used as is.

) The signal thus obtained is amplified by a 15.75 KHz narrowband amplifier 11 and is used as a demodulation input to the amplitude demodulation circuit 13.

If the amplification degree of the amplifier 11 of the loop consisting of the above-mentioned configuration forming the receiving side 2 is increased, the roof will follow at a very high speed, so a complete AGC circuit will be constructed and scrambled. The incoming signal is fully demodulated by the logarithmic amplitude demodulation circuit 13 by the opposite effect to that in the case of scrambling, and the output signal can be of the same quality as the original signal.

FIG. 3 shows a specific example of the above-mentioned logarithmic amplitude modulation circuit 6 and logarithmic amplitude demodulation circuit 13. Next, FIG. 3 will be explained.

As shown in the figure, this circuit has an RF signal input terminal and an encode or decode signal input terminal on one side, an RF signal output terminal on the other side, and a power supply terminal for the transistor Q.

In the figure, R1 to R3 are resistors, C1 to C6 are capacitors, and L1 is a coil. This coil L1 is tuned to the RF multiplied signal carrier frequency between it and a capacitor C4 connected in parallel. The inductance is selected so that the

Now, in this logarithmic amplitude modulation (demodulation) circuit, the transistor Q has a collector current Ic.

When used in an area where = IX10-9A,
As the input/output characteristics are illustrated in Fig. 4, Ic'=A
-exp(KVBE)(A, constant).

Function Fl (Vna) of the amplification degree of transistor Q at that time
It is.

Here, the DC bias voltage Vo, the encode signal V1s
If inθt is set, gm (VBB) = B-exp (K (V□+V
1 sin θt )) ・・・・−・・−・−・・−・
(2,1 is a function of the degree of amplification when the bias is changed by the encode signal.

Also, if the liver input signal is v, cos (ω is 1ψ), then RF
The output voltage, i.e. the scrambled signal VO, is vo =B-RL-exp (K (V□+V1sin
θt))vlcos (cc>j+ψ)RL: Load resistance...[3].

If this is input as a large RF input to the receiving side and a circuit similar to the scrambler circuit is used as a descrambler, then if the DC bias voltage is Vo' and the decode signal is -V1sinθt, the amplification function is gm (V part) = B・0浬(
K(Vo'-V1sinθt))
・・・・・・・・・・・・・・・(Since it is 4,1, the output voltage vo' is vo'-B2・RL-exp(K(■o′×Vo))・
vlcos (ω = 1ψ)...[5].

Since B2・RE−ωΦ(K(Vo φVo)) in this fifth equation is a constant, vlcos (ω is 1ψ) is obtained as a result, which is the conventional AM gray sync method described above. No ripple components remain like this.

The present invention is characterized in that when the scrambled video signal is descrambled as described above, logarithmic amplitude demodulation is applied to the RF video signal using a decoded signal. during the RF video signal.

It is possible to eliminate the inconvenience of generating residual ripple components and the like, and to minimize deterioration in image quality.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing an embodiment of the present invention, Fig. 2 is a waveform diagram explaining the waveforms at each stage when scrambling an RF video signal, and Fig. 3 is a diagram of a logarithmic amplitude modulation (demodulation) circuit. A wiring diagram showing an example, and FIG. 4 is an input/output characteristic diagram showing the characteristics of an exponential function circuit. 1... Sending side (scrambler), 2...
...Receiving side (descrambling device).

Claims (1)

[Claims] I RF video signal and R destroyed by amplitude modulating the U video signal and RF audio signal using a signal synchronized with the horizontal synchronization signal in the RF video signal as a modulation wave
It is installed on the receiving side that receives the F audio signal, and includes a detection circuit that extracts the modulation component from the destroyed RF audio signal, and a sine wave signal based on the output from this detection circuit as a demodulated wave. and a logarithmic amplitude demodulation circuit that performs 4-logarithmic amplitude demodulation on the RFffl image signal, and the RF video signal destroyed by the logarithmic amplitude demodulation circuit is restored to a normal RF video signal. Scramble device.