High security subscription television system employing real time control of subscriber's program reception

What is claimed is:

1. A method of operating a subscription television system, comprising the steps of:

I. At a central transmitting station:

(a) generating a standard television signal having a video signal component with predetermined amplitude and polarity relationships;

(b) scrambling and amplitude modulating successive fields of the standard television signal in a time-coded way so as to provide a complete scrambled amplitude modulated television signal which has said predetermined polarity reversed for at least some fields thereof, and which is amplitude modulated on a channel carrier having a first frequency in the CATV channel assignments;

(c) generating a coded control signal stream having first portions containing respective subscriber addresses, second portions identifying the channel carrier frequency, and third portions containing scrambling signals which identify the polarity of each field of the complete scrambled television signal;

(d) modulating the coded control signal stream on a code carrier having a second frequency different from said first frequency, in the CATV channel assignments; and

(e) transmitting the complete scrambled television signal and the modulated coded control signal stream to at least one subscriber control unit.

2. The method of claim 1, wherein the standard television signal includes vertical sync pulse intervals appearing at a vertical sync frequency and a blanking period preceding each vertical sync pulse interval, and step I.(b) comprises:

providing the standard television signal at baseband to a first input of a scrambling commutator;

inverting the standard television signal at baseband;

generating an amplitude reference pulse during each blanking period preceding the vertical sync pulse interval of the standard TV signal;

summing and providing to a second input of the scrambling commutator the inverted standard television signal and the amplitude reference pulses;

actuating the scrambling commutator to alternately provide at an output thereof the signals provided to the first and second inputs thereof, the scrambling commutator being actuated at some multiple of the vertical sync frequency of the standard television signal to produce a time-chopped scrambling sequence in synchronism with a frame rate of the standard television signal, the time-chopped scrambling sequence having an arbitrarily long repetition cycle; and

amplitude modulating the complete scrambled television signal appearing at the output of the scambling commutator on said channel carrier to produce a signal of the vestigial sideband suppressed type.

3. The method of claim 1, wherein step I.(b) comprises:

generating said channel carrier;

amplitude modulating the standard television signal on said channel carrier to produce a signal of the vestigial sideband suppressed type;

phase shifting said channel carrier by 180 degrees;

level controlling and supplying the phase-shifted channel carrier to a first input of a scrambler commutator;

actuating the scrambler commutator to alternatively provide and not provide the level-controlled and phase-shifted channel carrier as an output signal thereof, the scrambling commutator being actuated at some multiple of a vertical sync frequency of the standard television signal to produce a time-chopped scrambling sequence in synchronism with a frame rate of the standard television signal, the time-chopped scrambling sequence having an arbitrarily long repetition cycle; and

algebraically summing the modulated standard television signal and the output signal of the scrambler commutator to produce the complete scrambled television signal.

4. The method of claims 1, 2, or 3, further comprising the steps of:

II. at said subscriber control unit:

(a) receiving the complete scrambled amplitude-modulated television signal and the modulated coded control signal stream;

(b) demodulating the coded control signal stream;

(c) determining whether one of said first portions of the coded control signal stream matches a local address code, and providing an enabling signal when a match is found;

(d) tuning a local oscillator to a first local oscillator frequency corresponding to said channel carrier frequency, in dependence on said second portions of the coded control signal stream, when said enabling signal is provided;

(e) mixing the modulated completed scrambled amplitude modulated television signal with the first local oscillator frequency to produce an intermediate-frequency scrambled television signal;

(f) mixing the intermediate frequency scrambled television signal with a second local oscillator frequency to produce the complete scrambled television signal amplitude modulated on an RF carrier detectable by a conventional TV receiver;

(g) regulating said first local oscillator frequency such that the complete scrambled television signal amplitude modulated on the RF carrier is maintained at a constant frequency and is phase-stable; and

(h) descrambling the complete scrambled television signal amplitude modulated on the RF carrier to produce said standard television signal modulated on the RF carrier in a form detectable by a conventional television receiver.

5. The method of claim 4, wherein step II.(h) comprises:

amplitude envelope detecting the video signal component of the complete scrambled television signal amplitude modulated on the RF carrier;

producing a channel carrier information signal; and

modulating the channel carrier information signal with the amplitude-envelope detected video signal component to produce said standard television signal amplitude modulated on the RF carrier in a form detectable by a conventional television receiver.

6. The method of claim 5, wherein the step of producing the channel carrier information signal comprises

amplitude clipping and filtering the complete scrambled television signal amplitude modulated on the RF carrier to obtain the channel carrier information signal.

7. The method of claim 6, wherein step II.(h) further comprises:

peak detecting and maintaining substantially constant the peak amplitude of the amplitude-envelope detected video signal component.

8. The method of claim 4, wherein step II.(h) comprises:

amplitude clipping and filtering the complete scrambled television signal amplitude modulated on the RF carrier to obtain a channel carrier information signal;

phase shifting the channel carrier information signal by 180 degrees; and

algebraically combining the complete scrambled television signal amplitude modulated on the RF carrier with the phase-shifted channel carrier information signal to restore said standard television signal amplitude modulated on the RF carrier in a form detectable by a conventional television receiver.

9. The method of claim 8, wherein step II.(h) further comprises:

amplitude envelope detecting the video signal component of the complete scrambled television signal amplitude modulated on the RF carrier;

peak detecting the amplitude-envelope detected video signal component; and

regulating the amplitude of the complete scrambled television signal amplitude modulated on the RF carrier to maintain substantially constant the peak amplitude of the amplitude-envelope detected video signal component.

10. The method of claim 4, wherein step II.(h) comprises:

producing a channel carrier information signal,

amplitude envelope detecting and providing to a first input of a descrambling commutator the video signal component of the complete scrambled television signal amplitude modulated on the RF carrier;

inverting and providing to a second input of the descrambling commutator the amplitude-envelope detected video signal component;

actuating the descrambling commutator in dependence on said third portions of the coded control signal stream to provide the amplitude-envelope detected video signal in descrambled form as an output of the descrambling commutator; and

modulating the channel carrier information signal with the descrambled amplitude-envelope detected video signal to restore the standard television signal amplitude modulated on the RF carrier in a form detectable by a conventional television receiver.

11. The method of claim 10, wherein the step of producing the channel carrier information signal comprises:

amplitude clipping and filtering the complete scrambled television signal amplitude modulated on the RF carrier to obtain the channel carrier information signal.

12. The method of claim 11, wherein step II.(h) further comprises:

peak detecting and maintaining substantially constant the peak amplitude of the amplitude-envelope detected video signal component.

13. The method of claim 4, wherein step II.(h) comprises:

amplitude clipping and filtering the complete scrambled television signal amplitude modulated on the RF carrier to obtain a channel carrier information signal;

detecting the phase of the channel carrier information signal in a phase lock loop to obtain a phase-coherent channel carrier;

phase shifting by 180 degrees and providing to a commutating switch the phase-coherent channel carrier;

actuating the commutating switch in dependence on said third portions of the coded control signal stream to alternately provide and not provide as an output signal thereof the phase-shifted phase-coherent channel carrier; and

algebraically combining the complete scrambled television signal amplitude modulated on the RF carrier with the output signal of the commutating switch to restore the standard television signal amplitude modulated on the RF carrier in a form detectable by a conventional television receiver.

14. The method of claim 13, wherein step II.(h) further comprises:

amplitude envelope detecting the video signal component of the complete scrambled television signal amplitude modulated on the RF carrier;

peak detecting the amplitude-envelope detected video signal component; and

regulating the amplitude of the complete scrambled television signal amplitude modulated on the RF carrier to maintain substantially constant the peak amplitude of the amplitude-envelope detected video signal component.

15. A method of operating a subscription television system, comprising the steps of:

at a subscriber control unit:

(a) receiving a complete scrambled television signal which has a video signal component with reversed polarity for at least some fields thereof and which is amplitude modulated on a channel carrier having a first frequency in the CATV channel assignments;

(b) receiving a coded control signal stream having first portions containing respective subscriber addresses, second portions identifying the channel carrier frequency, and third portions containing scrambling signals which identify the polarity of each field of the complete scrambled television signal, the coded control signal stream being modulated on a code carrier having a second frequency, different from said first frequency, in the CATV channel assignments;

(c) demodulating the coded control signal stream;

(d) determining whether one of said first portions of the coded control signal stream matches a local address code, and providing an enabling signal when a match is found;

(e) tuning a local oscillator to a first local oscillator frequency corresponding to said channel carrier frequency, in dependence on said second portions of the coded control signal stream, when said enabling signal is provided;

(f) mixing the modulated complete scrambled amplitude modulated television signal with the first local oscillator frequency to produce an intermediate-frequency scrambled television signal;

(g) mixing the intermediate frequency scrambled television signal with a second local oscillator frequency to produce the complete scrambled television signal amplitude modulated on an RF carrier detectable by a conventional television receiver;

(h) regulating said first local oscillator frequency such that the complete scrambled television signal amplitude modulated on the RF carrier is maintained at a constant frequency and is phase-stable; and

(i) descrambling the complete scrambled television signal amplitude modulated on the RF carrier to produce a standard television signal amplitude modulated on the RF carrier in a form detectable by a conventional television receiver.

16. A method of operating a subscriber control unit for use in a subscription television system using coded control signals originating at a central transmitting station comprising:

(a) receiving any one of a plurality of channels, at least one of which may be in scrambled form;

(b) receiving and processing coded control signals;

(c) converting said at least one scrambled channel to a predetermined frequency band in part by tuning a variable frequency oscillator in response to said coded control signals; and

(d) descrambling said converted channel in response to said coded control signals rendering the resultant descrambled channel directly usable on a conventional television receiver, wherein said step of descrambling said converted channel comprises:

receiving a converted channel signal comprised of an inverted video television waveform;

amplifying and detecting said inverted video signal to obtain a detected video signal;

clipping and filtering said inverted video signal to obtain a continuous wave carrier signal; and

modulating said continuous wave carrier signal with said detected video signal to obtain a descrambled channel directly usable on a conventional television receiver.

17. The method of claim 16, further including the step of peak detecting said detected video signal for maintaining a substantially constant peak amplitude video signal input to the modulator.

Description Submit all comments and votes

BACKGROUND OF THE INVENTION

The present invention relates to subscription television systems and more particularly to methods and apparatus for implementing a high security pay television system wherein real time control of each subscriber's control unit is exercised by the central transmitting station.

It is well known in the subscription television field to provide means for insuring that only authorized subscribers may gain useful access to transmitted programming materials. Since its inception, the pay television industry has employed a wide range of techniques to assure that the proper revenue is derived in payment for programs provided. Initially, periodic flat rate charges were levied on all subscribers connected to a particular network, and the subscribers were then provided with a straightforward conversion device enabling reception of all programming transmitted. As the industry developed it became obvious that the early systems were too susceptible to abuse by unauthorized use of the converters, and equally importantly did not provide the degree of control required to differentiate between premium, higher cost, programs and more routine subject matter. To meet the new requirements imposed by the subscribers' desire to receive a wider mix of programs, and to assure proper pro rata revenue on a per-program basis, a number of more selective equipment devices evolved. The devices afforded the individual subscriber a wide choice of programs and provided the means whereby the CATV operator could exercise an increased degree of control against the unauthorized use of the programming.

Illustrative of prior art systems wherein a moderate degree of security is employed, is U.S. Pat. No. 3,885,089 to Callais et al which describes a system wherein scrambling of television programs is achieved by transmitting in an alternating sequence a pair of television programs on a pair of channels. The receiving stations then assemble a complete program by deinterleaving the transmitted waveforms. An exemplary disclosure of a prior art effort to exert positive control of the channel frequency in use at the subscriber's location is shown in the U.S. Pat. No. 3,914,534 to Forbes. Forbes teaches the use of control signals originating at the central station to enable coding means already resident in the subscriber's terminal to determine the channel frequency to be enabled. Other illustrative prior art is found in the U.S. Pat. No. 3,852,519 to Court, and in U.S. Pat. No. 3,919,462 to Hartung et al.

SUMMARY OF THE INVENTION

The present invention is primarily directed towards simple and effective methods and apparatus for providing a high degree of security in, and positive control of a subscription television system wherein a large number of subscribers are cable connected to a central transmitting facility. In general terms, the desired high degree of security is achieved by a combination of techniques including scrambling and descrambling the television signals in a random manner controlled by the central facility. In the video inversion techniques employed, which may be implemented at baseband video or RF levels, the equipment at the subscriber's location has no a priori knowledge of the scrambling sequence being employed at the central facility. Positive system control is exercised in part by a technique wherein the central facility maintains real time control of the channel tuning of each subscriber's frequency converting unit when pay television channels are being received.

Therefore it is a primary object of the present invention to overcome the limitations and disadvantages of the prior art and to provide improved apparatus and methods for transmission and reception of television programming in CATV-like distribution systems.

It is a further object of the present invention to provide a subscriber's control unit for use in a subscription television system which provides a high degree of security by the inclusion of a plurality of encoding and enabling techniques operative in combination.

It is a further object of the present invention to provide simple means for scrambling and descrambling television signals to prevent reception by unauthorized subscribers without introducing unwarranted equipment complexity, or critical subscriber operating adjustments.

It is a still further object of the present invention to provide a positive control subscription television system wherein the central transmitting station controls the channel selection of subscriber's control unit during reception of pay channels.

It is a still further object of the present invention to provide a positive control subscription television system wherein the central transmitting station provides real time coded program scrambling and descrambling information for use throughout the cable network.

A yet further object is to provide an ultra stable subscriber's control unit which is inexpensive to produce and which requires a minimum of manual intervention by the subscriber during program selection and viewing.

BRIEF DESCRIPTION OF THE DRAWINGS

These objects as well as additional features and advantages of the present invention will become apparent to those skilled in the art as the description proceeds with reference to the accompanying drawings wherein:

FIG. 1 is a simplified block diagram of a central transmitting station of a subscription television system;

FIG. 2 is a simplified block diagram of a preferred embodiment of a subscriber's control unit according to the present invention;

FIG. 3 is a block diagram of a frequency converter unit illustrating the local oscillator control method;

FIGS. 4A-4B are block diagrams of video scrambling modulators wherein the inversions are accomplished at the baseband video level and the RF level respectively;

FIG. 5 is a block diagram of a descrambler showing descrambling at the baseband video level;

FIGS. 6A-6D show the key TV waveforms associated with the baseband video descrambling method of FIG. 5;

FIG. 7 is a block diagram of an alternate embodiment of a descrambler which operates at the RF signal level;

FIGS. 8A-8C show the key TV waveforms associated with the RF descrambling method of FIG. 7;

FIG. 9 is a block diagram of an alternate embodiment of a baseband video descrambler, similar to FIG. 5, showing the addition of a dynamic descrambling capability according to the present invention;

FIG. 10 is a block diagram of a preferred embodiment of an RF level descrambler, similar to FIG. 7, including the features of phase lock loop carrier regeneration, and dynamic descrambling capability according to the present invention;

FIG. 11 is a schematic diagram partly in block of a preferred embodiment of a control logic processor;

FIG. 12 is a block diagram of an FSK control receiver;

FIG. 13 is a block diagram of a high stability embodiment of the subscriber's control unit illustrating the multiple use of a master crystal oscillator.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, there is shown a simplified block diagram of an illustrative embodiment of a central transmitting station associated with a subscription television system. Briefly, the central transmitting station assembles, processes and transmits a plurality of television programs simultaneously via a broadband RF cable to a plurality of subscribers as is well known and conventional in the CATV art. Both recorded programs available on video tapes, and programs obtained by reception of the conventional VHF/UHF broadcast signals are typically provided on the cable. The diagram outlines in functional form those elements needed to implement the positively controlled transmission of multiple channel programming to the CATV network. Hereinafter for brevity, the central transmitting station will be referred to as the CTS or alternately as the head end.

A plurality of automatic video tape reproduction systems, (hereinafter video tape machines) 10A-10N are shown as the primary source of television signals for which positive control is required so that specific charges to authorized subscribers may be made. The output signals from the video tape machines 10A-10N are applied to a corresponding plurality of scrambling modulators 12A-12N and thereafter via a plurality of directional couplers 14A-14N, whereby the scrambled television signals are impressed on a broadband coaxial cable 16 for distribution throughout the CATV network. Also impressed on the cable 16 are broadcast television signals which have been locally received by means of well placed antenna systems (not shown), and broadcast television signals which have been patched into the CATV CTS via landlines (not shown) and the like. These broadcast television signals are shown as simply entering the CATV CTS via a line 18 where they are also impressed on, and available via the cable 16.

As is well known in the CATV art these multiple sources of television signals are transmitted at frequencies outside of the broadcast television frequency bands, on channel allocations reserved for CATV operations. Also, the transmitted signals are complete and standard in that they contain the usual video, sync and audio signal components as defined by the National Television Standards Committee (NTSC). These standard signals relationships are required for proper operation when used in conjunction with the conventional VHF/UHF television receivers. Hereinafter, use of the phrase "standard television signal" and similar phrases, is meant to designate those characteristics and parameters as set forth in the NTSC specifications and in the subsequent generally accepted revisions to and modifications of the specifications.

A central controlling subsystem 20 is shown as containing a computer 20A having a number of input/output devices, such as a disc file 20B, a line printer 20C and a teletype terminal 20D. The central controlling subsystem 20 does not form part of the instant invention per se, although reference is made herein to routine signals or functions originating within the subsystem. Briefly, the control subsystem provides a coded control signal stream consisting of a series of subscribers' addresses, scrambling signals, and channel frequencies which are routed via a line 22 to an FSK control transmitter 24. The output of the FSK control transmitter 24 is applied via a directional coupler 26 to the cable 16. An auxiliary output from the FSK control transmitter 24 is applied via a line 28 to all of the scrambling modulators 12A-12N. This path provides the control signals for time-varying the scrambling such that dynamic control is exercised.

Thus, the cable 16 carries a plurality of radio frequency signals comprising: (a) television programs derived from reception of broadcast signals; (b) television programs reproduced locally by video tape means, which may or not be scrambled; and (c) a stream of coded control signals originating at the CTS--all of which are transmitted to the subscribers connected to the CATV network. The novel means for scrambling the video tape generated programs, a portion of the inventive concept of the present invention, will be described in more detail below.

Referring now to FIG. 2, there is shown a simplified block diagram of a subscriber's control unit according to the instant invention. Hereinafter, for brevity, the subscriber's control unit will be referred to as the SCU. The diagram outlines in functional form those elements needed to implement the positively controlled reception of multiple channel programming material at an individual CATV subscriber's location.

An input signal consisting of a plurality of television programs in complete radio frequency signal form, and auxiliary coded control signals also at radio frequencies, are routed from the CTS via the coaxial cable to each subscriber's location. The input signal is thereafter connected via an input line 30 to the input of a converter 32, and to the input of an FSK control receiver 34. The converter 32 has as its primary function the conversion of one particular CATV channel to a predetermined single frequency band suitable for direct use via an unused channel of a conventional (standard) television set (not shown) at the subscriber's location. As previously noted, the term "standard" implies having characteristics compatible with the NTSC specifications. Typically, the output frequency of the converter 32 is adjusted to utilize the band reserved for either channel 2, or 3 of the VHF broadcast television spectrum. An output of the converter 32 is routed via a line 36 to a first input of a program type selector 38 hereinafter referred to as the Pay/Regular selector, which may be characterized functionally as a single-pole-double-throw switch, having its movable pole connected to an output terminal 40. Line 36 is further connected to an input of a descrambler 42, whose output is in turn routed via a line 44 to a second input of the Pay/Regular selector 38. For reception of free CATV programming material, that for which no charge is made, and hence does not require scrambling, the Pay/Regular selector 38 is retained in the position as shown and provides the frequency converted television programs to the subscriber's TV set without further signal processing. For reception of scrambled television programs, which is restricted to specifically authorized users, the descrambler 42 is utilized to descramble the program prior to being routed through the Pay/Regular selector 38 and output terminal 40 to the subscriber's TV set. In either position of the Pay/Regular selector 38, standard composite television signals are available (descrambled when necessary) at the output terminal 40 for direct connection and use by the subscriber.

The FSK control receiver 34 receives control signals directly from a selected frequency portion of the input signal supplied on the cable via the line 30, and is used to generate the enabling control signals which allow the reception of scrambled television signals at a particular subscriber's location. The receiver extracts the coded control signals, which are frequency shift keyed modulated on a VHF carrier frequency, corresponding to the digital logic signals originating at the head end. The output of FSK control receiver 34 is connected via a line 46 to the input of a control logic processor 48. The control logic processor 48 performs an address comparison to assure that the program material is being routed to the proper subscriber; performs a function decoding which effectively establishes the local oscillator frequency for the converter 32 via a line 50; and further provides a jam code signal to the descrambler 42 via a line 52.

In subsequent sections hereinbelow, a detaileed description of each of the generalized blocks described will be provided.

Referring now to FIG. 3, there is shown a detailed block diagram of the converter 32. Basically, this is a dual conversion receiver of the superhetrodyne type and provides the required selectivity and sensitivity for the frequency conversion portion of the SCU. The input signal from the cable is applied via the line 30 to a band pass filter 110, and thereafter via a line 112 to a first input of a first mixer 114. A logically derived DC voltage used to establish the pay channel selecting frequency (as will be described below in connection with the control logic processor 48) is applied to a second input of the converter 32, via a line 50, and thereafter directly to a first input of a Pay/Regular selector 116. (As with its counterpart, element 38 of FIG. 2, this Pay/Regular selector 116 may also be functionally characterized as a single-pole-double-throw switch, both of which are actuated in concert). The movable pole of Pay/Regular selector 116 is applied to a first input of a VCO 118. The output of the VCO 118 is applied via a line 120 to an amplifier-buffer 122, and thereafter via a line 124 to a second input of the first mixer 114. The heterodyned output of the first mixer 114 is available on a line 126 for application to an IF filter 128 whose output is routed to an IF amplifier 130. Functionally, the band pass filter 110 serves to minimize any possible coupling of the VCO 118, the first local oscillator signal, back into the cable system thereby preventing possible disruption of the reception at other subscriber's locations. The first mixer 114 is of conventional double-balanced type and accepts the RF signal input from the line 112, the local oscillator input from the line 124, and provides the usual sum and different frequencies at its output via the line 126. Thereafter, the proper one of these two output signals is filtered and amplified by IF filter 126 and IF amplifier 130 respectively. The output of IF amplifier 130 appears on a line 132 where it is fed to a first input of a second mixer 134.

An oscillator 136 supplies a fixed frequency output via a line 138 to a second input of the second mixer 134. The output of second mixer 134 is coupled via a line 140 to an output filter 142. The output filter 142 delivers the selected predetermined television signal, which illustratively may be adjusted to be in the bands associated with either channel 2 or channel 3, to the subscriber's TV receiver via the output line 36. An additional output from the output filter 142 is routed via a line 144 to a phase lock loop/AFT circuit 146. It should be noted that the oscillator 136, the second local oscillator signal, is of fixed, predetermined frequency. Various frequencies of an illustrative embodiment of the converter 32 are provided below at the end of the detailed description.

The phase lock loop/AFT 146 is shown as comprised of an AFT section 146A and a phase lock loop section 146B. Common to both sections is a limiter 148 which receives its input via the line 144 and whose output is applied to a first input of a phase detector 150. The output of a fixed frequency crystal oscillator (not shown) is applied via a line 152 to a second input of the phase detector 150 whose output is connected directly to a DC amplifier 154. The output of DC amplifier 154 is applied via a line 158 to a first input of a summing junction 160 and thereafter via a line 162 to a second input of the VCO 118. The output of limiter 148 is also applied to a discriminator 164 whose output is coupled via a line 168 to a second input of the summing junction 160. Functionally, the phase lock loop/AFT 146 serves to provide a closed loop vernier tuning of VCO 118 such that the signal output from the output filter 142 is maintained at precisely the frequency established by the crystal oscillator as applied to the converter 32 via the line 152. Also, the phase lock loop 146B assures a phase stable output signal on the line 36. This is accomplished by means of a DC control voltage out of the phase detector 150 which represents the difference in phase between the fixed frequency oscillator via line 152 and the output signal from output filter 142. This DC voltage is applied to VCO 118 in the conventional manner so as to provide the required vernier tuning of the converter portion of the SCU.

Typical frequencies associated with the various elements described above, which may be considered as an illustrative embodiment only, are as follows: The band pass filter 110 may have a bandwidth of from 50-300 MHz; the VCO 118 may produce frequencies in the range of 433-700 MHz; the IF filter 128 may have a center frequency of 375 MHz, with a 6 MHz bandwidth; the second local oscillator 136 may have a frequency of 433 MHz; and the fixed frequency crystal oscillator from line 152 may have a frequency of 55.25 MHz. The frequencies described herein would produce an RF output signal on the line 36 having a video carrier frequency at 55.25 MHz, which then would be receivable by a standard television receiver by tuning to VHF channel 2. Obviously, other combinations of frequencies may be used herein equally successfully.

Referring to FIG. 4A, a detailed block diagram of a baseband video scrambling modulator is shown. In brief, the purpose of this portion of the CTS is to provide the required system security by scrambling the pay television programs, and more specifically to scramble the signals in a dynamic manner. The dynamic scrambling method consists of the scrambling of successive fields of the television signals in a time coded way such that--at least for moderately long intervals--a simply repetitive scrambling scheme is not recognizable. Thus, the output of the scrambler will be a complete television program