Video Scrambler and Descrambler Apparatus

This invention relates to a scramble apparatus for distorting video signals and to a descrambler apparatus for restoring the same.

In order to feed specific video signals to only specific subscribers among the subscribers in a CATV system, it is necessary to transform the aforementioned video signal to an unreproducible condition for those subscribers other than the specific subscribers. For this purpose, means are utilized in which the specific video signals are distorted by the scrambler apparatus at the site of the transmitter during the transmission of the video signals, while the thus distorted video signals are restored into normal form by descrambler apparatus at the sites of specific subscribers.

It is known to provide scrambling and descrambling by what is called an AM gray sync system. That is, in the AM gray sync system, an arrangement is provided wherein at the transmitter, the relationship of the modulation levels between the horizontal synchronizing signal and the video signal in a RF stage is inverted so that the amplitude of the horizontal synchronizing signal is less than is previously experienced while the amplitude of the video signal is greater than is previously experienced. More specifically, a sine wave (an encode signal) of 15.75 KHz (or integral-fold thereof), which is in synchronism with the horizontal synchronous signal, is formed, by which the video signal undergoes AM modulation to thereby carry out scrambling; and to return this to is initial condition, a sine wave (a decode signal) of 15.75 KHz (or integral-fold thereof), of which the phase is reversed with respect to the firstly mentioned encode signal, is formed at the receiver, by which the scrambled video signal is restored to thereby carry out descrambling. In this case, however, there is a disadvantage such that the descrambled RF video signal is not completely identical to the unscrambled signal and includes a ripple. For example, let A cos .omega.t represent the RF video signal and B sin .theta.t the encode signal, then the scrambled signal is given by

Where m = B/A. This is descrambled by the decode signal, which is reversed in phase with respect to the encode signal, then the resulting signal is given by

indicating that the term of m.sup.2 sin.sup.2 .theta. t remains relative to the original signal A cos.omega.t. This ripple component is determined by the square of the modulation degree m, but preferably the modulation degree m is greater in terms of the object inherent in the scrambler. However, because the ripple component produced in the descrambled signal becomes greater as the modulation degree m increases, the result is that the reproduced image suffers from longitudinal bright and dark stripes, resulting in an extreme difficulty in viewing the image.

In accordance with the present invention, a video signal is scrambled and then may be descrambled without the presence of a ripple component, by AM gray sync modulation with a sine wave signal which may be distorted logarithmically, exponentially or in some other non-linear fashion, such as a non-linear mathematical power term. In one example of a system in accordance with the invention, horizontal synchronizing signals are separated and employed to control the phase of sine waves of like frequency. The video signal is mixed with the sine wave signal in a logarithmic amplifier, or with a sine wave modified in accordance with an exponential function, so that a distorted sine wave is imposed on the video signal such that the synchronizing signal components are immersed in the gray level and the video signal is non-linearly distorted for transmission. The same characteristic may be imparted on the frequency modulated aural component, as an amplitude modulation characteristic, and the aural signal may thereafter be employed as a time reference in descrambling the video signal to its normal state. In descrambling, an amplitude controlled sine wave of inverted phase relative to the scramble signal may be distorted in a similar fashion to the distortion used in scrambling and used for a compensatory modulation.

A better understanding of the invention may be had by reference to the following description, taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram showing a first example of a system in accordance with the present invention;

FIG. 2 is a block diagram showing a second example of a system in accordance with the invention;

FIG. 3 is a graphical representation of the characteristics of an exponential function circuit;

FIG. 4 is a schematic diagram showing one example of a logarithmic amplitude modulator circuit; and

FIG. 5 is a graphical representation of waveforms (a)-(d) useful in explaining the various stages of scrambling of the RF signals.

In FIG. 1, which shows a first embodiment of the invention, there is shown a transmitter (scrambler apparatus as at 1 surrounded by the broken line, and a receiver (descrambler apparatus) as at 2 surrounded by the broken line. Firstly, at the transmitter 1, the RF video signal is partially extracted from a distributor circuit 5 and then applied to a horizontal synchronizing signal separator circuit 7. The horizontal synchronizing signal component of 15.75 KHz is extracted by the horizontal synchronizing signal separator circuit 7, and the resulting extracted synchronizing signal component is applied to a phase and amplitude regulator circuit 10 through an amplifier 8 and a filter 9 which allows 15.75 KHz to pass therethrough. In this circuit 10, the sine wave signal has been regulated in phase so that the phase assumes n .pi. (where n is a whole number) relative to the synchronizing signal. Signal waveforms obtained at that time are shown in FIGS. 5(a) and (b). That is, FIG. 5(a) shows the normal RF video signal containing the horizontal synchronizing signal, and FIG. 5(b) shows the sine wave signal or encode signal, which has been regulated in phase so that the phase assumes n .pi. (where n is a whose number) at the position of the synchronizing signal contained in the RF video signal. The thus formed sine wave signal is applied to a drive circuit 11, where the sine wave signal is subjected to impedance conversion, after which the signal is applied to a logarithmic amplitude modulation circuit 6 composed of elements having an exponential function characteristic, the RF video signal being scrambled by the amplitude modulation circuit 6. Simultaneously with the scrambling modulation of the RF video signal, the accompanying RF aural signal is also modulated. Signal waveforms obtained at that time are shown in FIG. 5(c) and (d). That is, FIG. 5(c) shows a condition where the sine wave shown in FIG. 5(b) is passed through the exponential function circuit, and FIG. 5(d) shows the RF video signal amplitude-modulated by the signal shown in FIG. 5(c).

The signal scrambled in a manner as described above is transmitted to the receiver through a truck transmission line 12 such as a coaxial cable or the like, but as may be understood from those waveforms as shown in FIG. 5(d), the scrambled RF video signal has horizontal synchronizing signal amplitudes much lower than picture component amplitudes to such extent that if the signal should be reproduced in an image receiver without modification, the horizontal synchronization would not be attained, as a consequence of which a sufficient scramble effect may be obtained.

Next, at the receiver 2, the scrambled signal obtained through the trunk transmission line 12 passes a logarithmic amplitude demodulator circuit 13 (which is identical with the above-mentioned circuit 6) to separate only the RF aural signal by a distributor circuit 14. This RF aural signal is amplified by a narrow band amplifier 15 and is further detected by a negative detector circuit 16 to obtain a decode signal displaced in phase by 180.degree. from that of the sine wave shown in FIG. 5(b). (It should be noted in this case that since the scramble modulation of RF aural signal is in the form of AM, the signal negatively detected by the detector circuit 16 may be utilized without modification). The thus obtained signal is amplified by a narrow band amplifier 17 of 15.75 KHz into a modulation input of the amplitude demodulator circuit 13. Since the provision of a high degree of amplification of the amplifier 17 among a loop composed of the above-mentioned structure which forms the receiver 2 may follow at an extremely high speed, a complete AGC circuit is formed, and the incoming scrambled signal may be subjected to a remodulation or compensatory modulation process by the logarithmic amplitude demodulator circuit 13 in a mode of operation entirely opposite the case where scrambling takes place, to thereby obtain an output signal, which is the same in quality as that of the original signal.

FIG. 2 shows a second embodiment is accordance with the present invention, which is different in construction from that shown in FIG. 1 in such a point that at the transmitter 1, the sine wave of 15.75 KHz obtained in a phase and amplitude regulator circuit 24 is passed through an exponential function circuit 25 to obtain beforehand the signal as shown in FIG. 5(c), which uses as a modulation input of an amplitude modulation circuit 20 composed of square-law characteristic elements such as field-effect transistors (FET) or the like. The scrambled signal thus obtained results in the same signal as the signal scrambled at the transmitter 1 of FIG. 1. The scrambled signal enters a receiver 4 through a trunk transmission line 27 to first separate the RF aural signal by a distributor 28, and is then converted into a signal having a phase opposite to that shown in FIG. 5(c) through a negative detector circuit 34. This signal is converted into a sine wave having a phase opposite to that shown in FIG. 5(b) by an amplififer 33 having a first characteristic of 15.75 KHz and further passes a phase and amplitude regulator circuit 32 and converterd into a waveform having the relation in phase opposite to that shown in FIG. 5(c) by an exponential function circuit 31. This signal obtained by the exponential function circuit 31 is subjected to impedance conversion by a drive circuit 30, after which the signal is applied to an amplitude demodulator circuit 29 composed of square-law characteristic elements such as FET and thus descrambled with respect to the scrambled RF video signal.

In either method according to FIGS. 1 and 2, the signal waveform scrambled at the transmitter 1 is identical with that scrambled at the transmitter 3, and hence, the receivers 2 and 4 can be exchanged with each other.

FIG. 4 specifically illustrates the above-mentioned logarithmic amplitude modulation circuit 6 and logarithmic amplitude demodulator circuit 13, which will be described hereinafter. As shown in FIG. 4, this circuit comprises on onde side an RF signal input terminal, and on the other side an RF signal output terminal and a power source terminal to transistor Q. The circuit further comprises resistors R1 through R4, capacitors C1 through C6, and a coil L1, of which inductance is so selected that tuning may be made to the carrier frequency of RF signal between the coil L1 and the capacitor C4 connected in parallel therewith.

In the logarithmic amplitude modulator circuit, in the case that the transistor Q is used in a region where collector current I.sub.C is I.sub.C = 1 .times. 10.sup.-9 A, the voltage V.sub.BE across base and emitter and the collector current I.sub.C have a function as given by

with input and output characteristics illustrated in FIG. 3. At this time, the function gm (V.sub.BE) of amplification degree of transistor Q is given by ##EQU1## Let V.sub.o represent the dc bias voltage and V.sub.1 sin .theta.t the encode signal, then the function of amplification degree when the bias is aided by the encode signal is given by

Also, let V.sub.1 cos (.omega.t + .phi.) represent the RF input signal; the RF output voltage, i.e., the scrambled signal V.sub.o, is given by

where R.sub.L is the load resistance. If the above-mentioned signal is the RF input at the receiver and a circuit similar to the scrambler circuit is the descrambler, and let V'.sub.o represent the dc bias voltage and -V.sub.1 sin .theta.t the decode signal, then the function of amplification degree thereof is given by

and hence, the output voltage v.sub.o ' is given by

Since B.sup.2 .multidot.R.sup.2.sub.L .multidot.exp {K(V.sub.o ' + V.sub.o)} in equation (5) is a constant, V.sub.1 cos (.omega.t + .phi.) may be obtained from the result. As a consequence, the ripple component does not remain as in the prior AM gray sync system mentioned above.

Next, in the second embodiment as shown in FIG. 2, encloding may be accomplished in a manner such that the dc voltage V.sub.o and sine wave V.sub.1 sin .theta.t are passed through elements (for example, such as diode and transistor) having an exponential function characteristic to obtain exp {K(V.sub.o + V.sub.1 sin .theta.t)}. This signal serves as one input of the amplitude modulator, but the amplitude modulator herein used comprises elements having a square-law characteristic and in this case the relationship of amplification degree of the modulation element in the bias is in proportional relation so that let

then the encoding equation becomes

Thus, let V.sub.1 cos (.omega.t + .phi.) represent the RF signal, then the RF output voltage v.sub.o encoded is given by

Since this equation (8) is of the same nature as equation (3), it is obvious that reproduction may be accomplished even in the descrambler 2 in FIG. 1. Further, the descramble method shown in FIG. 2 is such that the decode signal is inverted in phase from that of the encode signal, and decoding may be accomplished in the exactly same method as that of encoding so that the output voltage v.sub.o ' at that time becomes

and it will be obvious from this equation that the output voltage is the same in nature as that of equation (5).

From the foregoing, it will be apparent that the signal scrambled in FIG. 2 may also be reproduced by the descrambler in FIG. 1.

As described above, the present invention provides apparatus which is characterized in that at the time of scrambling the video signal, logarithmic (or exponential function) amplitude modulation is applied thereto by the encode signal, whereas at the time of descrambling, logarithmic (or exponential function) amplitude modulation is applied to the RF video signal by the decode signal, to thereby eliminate the production of residual rippled components or the like in the RF video signal as in a conventional AM gray sync system and to minimize the deterioration in image quality caused by the utilization of a scrambler and descrambler.