DE2406023C3 - Television system - Google Patents

Description

The invention relates to a television system having a central station, at least one therewith in Connection seeing subscriber device and at least two channels for the transmission of television programs from the central station to the subscriber device, with a first switching device, which a program one after the other alternately switches to the two channels, a second switching device that Subscriber device alternately connects to the two channels one after the other, and with one with the two Switching devices electrically connected device for synchronizing the switching frequencies.

Such a television system is known from US Pat. No. 3,054,857. In the known television system, the television signal is alternately switched on to the two channels in such a way that the signal is switched to the other channel before it is switched off from the one channel. As a result, both channels are temporarily occupied with the same signal. The receiver is switched at the point in time when the signal is present on both channels. Since both channels are temporarily occupied by the same signal, they can only be used to transmit a single, continuous useful signal. If necessary, an interference signal can be transmitted as the second signal. The need to provide two transmission channels for each program requires a great deal of effort and makes the known system uneconomical, especially when a large number of different programs are to be transmitted.

Accordingly, the invention is based on the object of providing a television system of the type described above to train so that as many programs can be transmitted as there are channels, with two Channels so two television programs are transmitted at the same time and optionally received one of them can be.

This object is achieved according to the invention in that both channels for transmission each have one Television programs are set up and the device for synchronizing the switching frequencies a first arrangement for deriving a first signal from one of the programs, a second arrangement for Deriving a clock signal from the other program, one on the first signal and the clock signal attractive third arrangement for generating a channel switching code, a decoding arrangement, a fourth Arrangement for transmitting the channel switching code to the decoding arrangement which is derived from the channel switching code dependent switching signals generated, and a fifth arrangement for supplying the switching signals to the second Switching device includes.

The particular advantage of the television system according to the invention is not only that the channel capacity is fully utilized, i.e. a television program can be transmitted per channel, but also a perfect choice between the two broadcast television programs is possible and the synchronization signals required for this can be derived from the program signals themselves. In order to

the television system according to the invention is different also advantageous from all the other known television systems that use encryption on the transmission side the program signals is thereby achieved, Jaß the picture tube of the subscriber device interfering signals are supplied so that the horizontal synchronization pulses are suppressed and the video signals are shifted in phase be that the amplitude of the synchronization component of the composite video signal to the Gray level reduced and various control pulses and signals as well as program and interfering audio signals be generated that the polarity of the video and / or audio television signals reversed or that certain Properties of the television signal are changed during the spaced successive intervals because all of these television systems not only have complicated facilities for encryption and decryption of the program signals, but also used to control the decryption devices Signals have to be transmitted that use up the channel capacity.

Since in the system according to the invention, the television signal is not actually changed, but only with one If a predetermined frequency is switched between two channels, there is no significant and In particular, there is no noticeable deterioration in the signal quality for the viewer, rather it is the representation of the decrypted signal essentially with the quality possible that it was before Had encryption.

The television system according to the invention can be used with any number of channels. Included a switching of a plurality of programs between a plurality of channels can take place. It is of particular advantage that the number of channels encrypted according to the invention is minimal Effort can be increased.

The encryption can be done on the sending and receiving side either by switching the immediate image and audio signals or intermediate frequency or high frequency signals to which the image and Sound signals are modulated on. Furthermore, it is possible in a simple manner to do all the usual on the receiver Techniques pu use such as cards, keypads, key switches or the immediate Approval through the sale of the device in order to allow the subscriber to receive encrypted programs to enable.

Another advantage of the system according to the invention is that it can be used in any television system applied to one-way or two-way traffic between the central station and the subscriber devices can be.

Further details and configurations of the invention emerge from the following description the Ausführungsbeispieie shown in the drawing. The description and the drawing too In other embodiments of the invention, the extracting features can be used individually or as well several can be used in any combination. Show it

1.2 and 3 show three different block diagrams Embodiments of encryption devices on the transmitter side of a system according to the invention,

F i g. 4 shows the block diagram of a television system according to the invention;

F i g. 5 and 6 are diagrams of signals for explaining the in FIG. 4 illustrated system,

F i g. 7 shows the block diagram of the authorization unit of the system according to FIG. 4,

F i g. 8 shows the block diagram of the switching device of System according to FIG. 4,

9 shows the block diagram of the arrangement for automatic frequency control of the system according to FIG. 4 and

10 shows the block diagram of the arrangement for automatic gain control of the system according to FIG Fig. 4th

The F i g. 1, 2 and 3 illustrate three different ones Embodiments for the encryption of television programs A and B on the transmission side, so in the Central station of a television system. Although these embodiments regardless of the type of Transmission of television signals can be used, so both wireless and wired Signal transmission, the invention is described below using a CATV system. Using these three exemplary embodiments, it is shown that programs A and B can be switched over by switching their Fundamental frequencies, their intermediate frequencies or high frequencies can be encrypted. How to Hand Fig.4 is further described, includes the Invention also facilities which authorized participants, individually or in groups, the possibility give to watch and listen to selected decrypted television programs.

In the case of the in FIG. 1, programs A and B are from program sources 11 and 13 are each fed to a processing device 15 or 17, which the corresponding video mix in Frequency range from 0 to 4 MHz and sound signals generated. When one of the program sources 11 or 13 is on is a local studio, the assigned processing device only needs the audio signal and the To amplify video mix separately. If, on the other hand, the program source is a television antenna or a cable to receive the program information, the associated processing device would have to use the composite video and generate the audio signal, in which the received RF television signals, the modulated video carriers, Contain, demodulate or otherwise process color and sound carriers.

The video composite and audio signals supplied by the processing devices 15 and 17 are fed to a switching circuit 19 in which they are simultaneously switched one by one each time a later to be explained switching signal is supplied. The circuit 19 shown schematically can easily can be implemented by means of electronic or other known switches.

During a first switching time, the video mix and the audio signal of the Ptogram A are in one Kanai-X transmitter 21 modulated on appropriate carriers. During the same switching time there are; Video mix and the audio signal of the program B via the circuit 19 to a channel V transmitter 23 supplied and modulated there on appropriate carriers. During a second switching time, the program A-Si signals switched to channel V transmitter 23 while the program B signals on channel A Transmitter 21 are switched. During each subsequent switching time, the program Α and program E Signals from the circuit 19 are each switched to the other of the two transmitters 21 and 23.

The output signals of the channel X and channel V set the 21 and 23 are normal television signals with de

Video carrier frequencies f \ and h, apart from the alternately switched program sources. The output signals of the channel X and channel V transmitters 21 and 23 are therefore referred to in the following as f 1 and h , respectively. It will be understood, however, that the output signals / i and k characterize television channel spectra which comprise sound carrier, optionally color carrier and video carrier frequencies. The output signals h and / 2 are given in frequency division multiplex on a main line so that they can be sent to subscribers of the television system. In the case of wireless television systems or CCTV systems, the output signals / 1 and h are transmitted in the manner that is common in such systems.

Fig. 2 illustrates an embodiment for switching (encrypting) the program Α and program B signals at their intermediate frequencies. In more detail, in this embodiment the program Α and program B IF signals, each of which includes the associated video mix, the audio signal and, if applicable, the color information, are fed to a circuit 25. The circuit 25 forms Program A / B and Program B / A IF output signals in the channel X transmitter 21 a and are supplied to an up-converter 29 in the channel Y transmitter 23a to an up mixer 27th In the up-converter 27, the program A / B-IF signal is superimposed with the output signal of the local oscillator 31, which is also contained in the channel X transmitter 21a , in order to form the channel X signal / i. At the same time, in the up-converter 29, the program B / A-IF signal is superimposed with the output signal of a local oscillator 33 which is located in the channel Y transmitter 23a to form the channel Y signal h .

Each time a switching signal is supplied to the circuit 25, the program Α and program B IF signals are alternately switched between the up-converters 27 and 29. That way will. if the program A-ZF-Signa! the channel X transmitter 21a is supplied, the program B-IF signal is supplied to the channel Y transmitter 23a and vice versa. The embodiment according to FIG. 2 could also be modified in such a way that the audio IF and composite video IF signals of each program are switched over separately and then combined and transformed into a higher frequency range by mixing in a transmitter, similar to what is shown in FIG. 1 is illustrated.

F i g. 3 illustrates a modification of the embodiment according to FIG. 2, which leads to the third exemplary embodiment, in which the switching (encoding) of the program Α and program B signals takes place in the high-frequency range. In this exemplary embodiment, the program Α and program B IF signals are sent directly to an up-converter 35 in the channel X transmitter 21 b and an up-mixer 37 in the channel Y transmitter 23a is supplied with the outputs of the local oscillators 39 and 41 ft in the transmitters 21 and are arranged 23ft, a circuit are supplied to 43 and at every switching signal switched between the up-converters 35 and 37 The switching or encryption consequently leads in this embodiment to a change in the frequencies of the output signals of the up-converters 35 and 37 during each switching time. In this way, for each switching signal, the frequency of the output signal of the Y channel transmitter 21 ft is either changed from Λ to h or from h to Λ, while at the same time the frequency of the output signal of the Y channel transmitter 23b is either changed from / 2 is changed to f \ or from Λ to h . As a result, whenever program A is transmitted in channel X with frequency / i, program B is transmitted in channel Y with frequency k and vice versa.

While the basic encryption technique on the basis of the three different embodiments according to FIGS. 1, 2 and 3 was explained, the in Fig. 4 illustrated arrangement for decoding an encrypted television channel for all three Forms of encryption useful. The invention will now be described in the following on the basis of an overall television system with encryption and decryption of the television signals by the encryption device makes use of FIG. 2, explained with reference to FIG.

F i g. 4 illustrates a cable television network in which the invention is accomplished. Television programs A and B are from program sources 51 and 53 one Encryption device fed in the central station, not shown, so that they are there processed and encrypted. Specifically, programs A and B become processing facilities 57 and 59 supplied. These processing devices form the input signals Program-A- and program B-IF signals. For example, if the program source 51 is a local studio the video mix and the audio signal of the program would be in two, not shown, separate Modulators modulated on. The resulting composite video and audio IF signals would then be converted into an IF filter, not shown, combined to form the program IF signal. If against it the program source 51 either a television antenna or a cable for receiving program information the processing device 57 would convert the program RF signal into the desired program IF signal reduce. It goes without saying that the above statements also apply to the program source 53 and the assigned processing device 59 apply.

The program Α and program B IF signals are fed to an IF amplifier and detector 61 and 63, respectively. The detectors of these arrangements form the video mixes A and B, which are fed to the corresponding control logics 65 and 67. The control logics 65 and 67 have the same structure and the same mode of operation. However, different output signals of the two circuit arrangements are used.The function of the control logic 65 is to generate a vertical synchronization interval signal VS / which indicates a predetermined period of time within the vertical blanking interval of the video composite A. In contrast, the control logic 67 is used to generate clock signals which indicate the exact location in each horizontal line and identify the line in composite video B. The VSASigna! Supplied by the control logic 65! and the clock signals supplied by the control logic 67 are fed to a switching control 69. The switching control 69 uses the input signals supplied by the control logics 65 and 67 in order to determine the switching time and the switching frequency. Furthermore, the switching controller 69 terminates the VSA signal. by feeding an end signal IVi: (window end) to the control logic 65. The switching control 69 forms a channel switching code which indicates the position of the programs A and B in the channels as well as the switching times

A typical channel switching code is thirteen bits in length. These thirteen bits include three start bits, four program bits, five channel bits, and one parity bit. The three start bits (001) are used to prepare two Manchester decoders 79 and 111 for processing the ten following bits. Without the supply of correct start bits, the Mtinchester decoders do not process any of the following bits.

The parity bit is used to give odd parity to the ten bits that follow the three start bits issue in order to increase the reliability of the functioning of the system. A parity check on these ten bits of one Codes is covered later.

The first four bits of the five channel bits indicate which pair of a group of adjacent television channels is used to encrypt the associated television programs. The fifth bit of the five channel bits identifies the channels on which the programs appear. For example, if the fifth channel bit is in the 0 state, program A is on channel X and program B is on channel Y. Similarly, if the fifth channel bit is in the 1 state, program A is on channel Kund that Program B on channel X.

The four program bits contain program information via the television channel that can be selected by a subscriber. The first program bit in Code provides information on whether the assigned channel is a free or a chargeable channel. Is this first program bit in the 1 state, the assigned channel is a free television channel that does not require that a subscriber pays a fee to decrypt the program assigned to this channel State to see. If, on the other hand, the first program bit is in the 0 state, it is a chargeable one Channel that requires the subscriber to pay a fee before using the associated Program can receive decrypted. Whether the first program bit is in the O state (fee-based program) or in! -status (free program) is confirmed by the staff at the control center of the cable television system determined according to a prepared plan. The remaining three program bits indicate the amount of the fees, which must be paid if the assigned program is to be received. The combinations 001,011 and 111 of the program bits indicate the amount of the Charge with one, two or three units. The costs for one unit can be for one type paid television program may be higher than another. For example, a unit can correspond to an amount of $ 0.50, $ 1.00, $ 1.50, or any other dollar amount.

The channel switching code, supplied by the switch controller 69 is of conventional Manchester encoding technique used in a Codesendex 71 for pulse code modulation in accordance with a carrier frequency h sent The code signal h is given in the frequency multiplexing on the main line 73, so that it is supplied to the participants. It will be understood that in other embodiments of the invention the channel switching code could be modulated onto the carrier signal h by any other modulation method, including amplitude modulation, frequency modulation, frequency shift keying, phase modulation and phase shift keying. Furthermore, it would also be possible to transmit the channel switch with the same carrier that is used to transmit the composite video.

The carrier signal h is fed from the main line 73 to a code receiver 77 via a tap 75 and is then decoded in a Manchester decoder 79. The alternating writing and counting signals NRZ (non-return to zero) and DCK (downclocks) obtained in this way are fed to a decoder 81. The decoder 81 forms a switching signal which is fed to a switching circuit 83.
At the same time that the program Α and program B IF signals are supplied to the detectors 61 and 63, they are also supplied to the circuit 83. The supply of the switching signal to the circuit 83 consequently causes the circuit 83 to provide program A / B and program B / A-IF output signals. This means that when program A is available at program A / B output, program B is available at program B / A output and vice versa. The program A / B and program B / A-IF signals are supplied to channel X and channel V transmitters 85 and 87. The transmitters 85 and 87 form signals Λ and h. which are given via taps 89 and 90 to the main line 73 so that they can be fed to subscriber devices. In addition to the signals f \, h and h , the main line 83 also contains the signals from other encrypted and unencrypted, that is, free, not shown television signals, which are also supplied by the central station.

The signals / 1, k and h from the encryption device 55 of the central station and other encrypted and unencrypted channels supplied by the central station are received by a frequency converter 97 in the decryption devices of the subscriber devices, such as the decryption device 93 shown in FIG. A filter 95 in the decryption device 93 also allows the Signa! h reaches a code receiver 99 with the signal switching code. The frequency converter 97 is electronically controlled, for example tunable using capacitance diodes. In general, all frequency converters which can be tuned by applying a tuning voltage V can be used for the invention. When a given tuning voltage V is applied, the frequency converter 97 will essentially select a signal frequency band or a channel from the multitude of frequency bands or channels in the range from 54 to 270 MHz and convert the signal of this channel into a predetermined fixed frequency band at the output of the frequency converter 97. The output frequency band or channel of the frequency converter 97 falls within a normal VHF channel. which can be received by a conventional television receiver. If a frequency converter 97 equipped with varactor diodes is used, the television set 129 is tuned to this channel and its tuning does not need to be changed. All the necessary tuning or channel selection takes place by means of a suitable setting of the tuning voltage V, as shown below with reference to FIG. 5 will be explained.

5 illustrates the above-mentioned properties of the frequency converter 97. The ordinate 101 indicates the amplitude of the tuning voltage V which is required in order to set a specific television input signal to a fixed output channel on the abscissa 103 . The curve 105 reproduces the curve of the channel 8, for example. Point 107 on curve 105 shows that the tuning voltage must have the value V. thus the distorted channel X signal with

¹ 10

the frequency / ι is converted by the frequency converter 97 into the channel 8. In the same way, point 109 on curve 105 shows that the tuning voltage must have the value Vy so that the encrypted channel V signal with the frequency h is converted to channel 8.

In the case of the in FIG. 4, the code receiver 99 demodulates the signal h supplied by the filter 95 and supplies the resulting Manchester output data to a Manchester decoder 111. The variable letters NRZ and the counting signals DCK, which are supplied from the Manchester decoder 111, are supplied to a decoder 113. The decoder 113 uses the four most significant bits CSl to CSA of a channel selection code selected with the aid of a channel selection switch 119, which identify a selected channel pair, together with the NRZ data and the counting signals which are fed to it from the Manchester decoder 111 to the last-digit bit CH5 of an input channel code to generate the four program bits PRi to PR4 derived from the NRZ data, a signal “Data OK” and a switching signal and to supply an authorization unit 115. One of the input program bits tells the authorization unit 115 whether the selected program is a free or a fee-based program. The remaining three bits of the input program code contain information about the cost of the program if it is a paid program. The "Data OK" signal is generated after the decoder 113 has checked the parity of the Λ / ÄZ data. The switching signal tells the authorization unit 115 the times at which programs A and B are to be switched between channels X and Y. With the help of an input device 117, the subscriber can also send a signal to the authorization unit 115, for example by using a punch card, by pressing keys or the like Television programs can be purchased for a fee. In this way, the broadcaster of a television program can receive a fee for allowing any subscriber to freely view and listen to a program. When the subscriber inserts the purchased card into the input device 117, the authorization unit 115 is supplied with information bits derived from the card. In the exemplary embodiment shown, the bits include five bits C1 to CS of a channel code and three bits CB to CS of a card program code. The card channel code is used to indicate to the authorization unit 115 the special channel that the subscriber wishes to receive. The card program code is used to indicate to the authorization unit 115 the costs or fees charged for viewing the undisturbed program. In addition to the input signals described above, the authorization unit 115 also receives the entire code of the selected channel with the bits CS1 to CSS. The code of the selected channel is a binary code that indicates which channel the subscriber has selected for reception.The authorization unit 115 compares three of the bits of the input program code with the card program code that is added by the input device 117 and also that of the input device 117 The card channel code supplied with the code supplied by the channel selection switch 119 for the selected channel. If the subscriber has selected the channel indicated by the card channel code and there is a match between the card program code and the corresponding three bits of the input program code supplied by the decoder 113, the authorization unit 115 generates signals through which an encrypted , paid television program is decrypted. The authorization unit 1115 contains

ίο also circuit arrangements that allow the subscriber to receive undisturbed programs without charge, which can be either a free operation or a preview operation. In the free mode, the subscriber can watch a complete free television program without charge. In the preview mode, the subscriber can watch a paid television program for a specific period of time. At the end of this period, normal encryption takes place, unless the subscriber pays the fee for the remainder of the fee-based program or declares his willingness to pay. These signals, which are generated by the authorization unit 115, are marked with XF. YF, diff. Gain and Ref. Gain. The signals XF and YF are fed to a switching device 121. The switching device 121 also receives the four most significant bits CSi to CS4 of the code for the selected channel from the channel selector 119. Depending on these four bits of the code of the selected channel, the switching device 121 internally generates analog voltages for each of the two channels X and Κ, which are used for Formation of the tuning voltages fed to the frequency converter 97 are used. When the signal XF is received by the switching device, the tuning voltage for the channel X is established. On the other hand, when the switching device receives the signal YF , a deflection voltage for the channel Y is established.

The tuning voltage formed by the switching device 121 is fed to the frequency converter 97 in order to adjust its frequency so that the frequency which is present at the output for the channel 8 of the frequency converter 97 is determined by the four bits of the channel code selected by means of the channel selection switch 119 and that of the signals XF or YF is determined, which is being fed to the switching device 121. The output signal of the frequency converter 97 is fed to an amplifier 127 serving for gain control, the output of which is connected to the television device 129 of the subscriber

Since two channels are used in the encryption and decryption system discussed here, it is important that the switching device 121 generates the correct tuning voltages in order to alternately tune the frequency converter 97 to the frequencies Λ and / 2 if Λ is the video carrier frequencies for the channel X and h is the video carrier frequency for channel Y if program A has been selected and enabled as desired, or alternately tuned to frequencies h and Λ if program B is desired, selected and enabled.It is assumed that, like ai hand fi g. 5, the output of the frequency converter 97 is tuned to channel 8. It is therefore very important that the input video carrier frequency for the television receiver 129 is precisely matched to the same frequency and amplitude in channel 8

regardless of which of the channels X and Y has been selected by the frequency converter 97. In other words, it is necessary that the same RF bandwidth and the same RF voltage level is offered to the input of the subscriber's television receiver 129 on channel 8, so that no reduction in the picture quality is noticeable on the subscriber's television set when the selected encrypted program is decrypted. In order to meet these requirements, the output signal of the amplifier 127 used for gain control is fed to an arrangement 131 for frequency control. This arrangement 131 is selectively responsive to the XF and YF signals supplied by the authorization unit 115 to selectively form error signals for the / V and Y channels. When the signal XF is fed to the arrangement 131 , the error signal for the channel X is formed. In the same way, the arrangement 131 forms the error signal for the channel Y when the signal VF is fed to it. The error signals for channels X and Y are fed to switching device 121 in order to correct the tuning voltages when they change between one level for channel X and another level for channel Y. In this way, the output frequency of the frequency converter 97 is controlled very precisely by the tuning voltages so that it always delivers the same frequency at its output, even if the frequency of the input signal fluctuates between the channels A ″ and V ″.

As explained above, it is also important that the voltage of the video carrier at the output of amplifier 127 be stable in order to avoid any noticeable flicker caused by channel switching. For this purpose, a composite video is fed from the output of the arrangement 131 for frequency control to an arrangement 133 for gain control. The arrangement 133 for gain control responds to the video mixture fed to it and the signals of the gain and ref. Gain which are fed to it from the authorization unit IU in order to form a control signal which controls the level of the output signal of the amplifier 127 when the system is running toggles between channels X and Y. The arrangement 133 for gain control also forms an IF control signal which is fed to the arrangement for frequency control 131 in order to stabilize the voltage level of the composite video output signal of the arrangement 131 for frequency control when the signal Ref. Gain has been selected by the authorization unit 115

The switching functions of the system shown in FIG. 4 will now be explained in more detail with reference to FIG. Curve 135 shows that the program content is switched between channels X and Y at the fluctuating times fi, ti, f4, fc, te, fe and h 1. It goes without saying. that the switching times given above have only been chosen for the purpose of explanation and that other times can be used in any arbitrary and even random sequence instead.

Changes at each of the switching times given above. the level of a switching signal 137 changes, for example, from 0 to + 5 V or from + 5 V to 0 V. The circuit 83 forms two complementary signals 139 and 141 which serve to program programs A and B between channels Λ 'and V to switch to the switching times indicated by curve 135. The signal 139 is in phase with the switching signal 137, while the signal 141 is 180 ° out of phase with the switching signal 137.

In the design of the circuit 83 used here, the voltage of the complementary signals 139 and 141 changes between +2 V and -2 V. By using the signals 139 and 141 to switch between the programs A and B within the circuit 83, the programs A. and B selectively encrypted at the outputs of the channel U-Y and channel V transmitters 85 and 87, as indicated by the divided blocks 143 and 145 . The signals transmitted over channel X , illustrated by block 143 , consist of time segments of different duration which are assigned to program A and program B alternately. As block 145 shows, the signal transmitted over channel Y exists! during the times when program A is transmitted via channel X , from program B, and during the times when program B is transmitted via channel X, from program A.

The encryption is therefore achieved both in channel X and in channel Y by alternately switching programs A and B to this channel. The extent of the encryption is related to the duration of the time periods used and the fluctuations in the time periods that are assigned in the individual switching sequences. At very low switching frequencies (less than 1 Hz), the encryption that results from the destruction of the recognizability is relatively low, but the encryption will prove to be unpleasant for the viewer. By using higher and more fluctuating switching frequencies, the extent of the encryption can be increased to such an extent that the recognisability of the content of conventional television programs is completely lost.

The decoding of programs A and B is carried out by blocks 147 and 149 in FIG. 6 illustrates. Circuit arrangements 93 (Fig.4) located within the Entschlüsselurigseinrichtung, decrypt the data transmitted over the channels X and Y shows, by switching between the channels X and VZU the right times to cause at the output of the frequency converter 97, either the Program A or Program B is made available.

In the following, the circuit arrangements of the system according to FIG. 4 are used for a more detailed explanation need to be described in more detail.

It should be remembered that the output signals Λ and h of the channel X and channel V transmitters 85 and 87, together with other transmitted channels in the frequency band from 54 to 270 MHz, are fed to the frequency converter 97 , while the output signal h of the code transmitter 71 passes a filter 95 passed , demodulated in the code receiver 99 and decoded by the Manchester decoder 111 and the further decoder 113. Here, the additional decoder 115 performs 113 different signal information, including program and channel information, the authorization unit. The authorization unit 115, which is used to enable a specific subscriber to view a specific program , can be implemented in various ways, depending on the sales technique used and the type of charging. However, it is essentially always necessary that the authorization unit 115 receives an encoded input signal which it as

recognize valid authorization of the individual participant to view a certain selected program can. Upon receipt of this signal, the authorization unit 115 must decrypt the selected Allow program.

: Some specific methods of building and operating authorization units are below listed:

a) By prior agreement with the operator of the cable television system or the seller of pay TV programs, the subscriber can indicate his desire to purchase a specific program or programs or to use one or more pay TV channels on any time basis, for example per day, per week or per month to pay. Such an agreement can be made by telephone, letter or in person in the seller's shop. In this case, at the instigation of the seller, a coded address would be sent to all subscribers via the coded channel (Λ). Each participant would be assigned a unique address, which is electronically built into the authorization unit using suitable wiring and digital logic circuits. When a signal is received on a coded channel that contains the coded address, the authorization unit indicates coincidence. The address signal is followed by a coded signal that indicates "On" or "Off". The authorization unit recognizes the "On" or "Off" code, which immediately follows the receipt and checking of the subscriber address, and sends a signal to the DQ to enable or disable it, depending on the purpose of the signal, namely a program trigger or terminate. Accordingly, the subscriber can be authorized to watch a program on a specific channel for a period of time specified by the seller. Invoicing and payment can take place before or after the program is sent out.

b) A second method of selling paid television programs may be: that an authorization card will be sold for the program. This card comes in a suitable Card reader introduced, which is part of the decryption device of the participant is. The authorization card is provided with a code through which the Numbers of the desired channel and the desired program are entered in the usual way, for example by means of punched holes, electrical contacts and connections, optical or digital signs. Suitable sensors and circuit arrangements then determine this information and generate a static digital output signal that is fed to the authorization unit. If the authorization unit receives the same channel and program number from decoder 113, it sets them Agreement and triggers the decryption process for the selected channel, for the one Authorization exists.

It is also possible for the admission card to have the unique address in addition to the channel and program number of the subscriber to limit the use of the card to a specific subscriber line. In this case, in addition to a match between the channel number and the program number, the card address matches the subscriber's address before decryption takes place can.

c) A third method of program release and calculation is also made from authorization cards Use. With this method, the authorization card bears a coded indication of a maximum amount of money. trags, which is expressed by unity steps. For example, the card can have a total of Specify 10 dollars, which is recorded in 20 partial amounts of 50 cents. The card reader would then be sensors contain the total amount of the paid

ίο fee and the number of unused Would determine partial amounts that are still available on the card. The authorization unit would then periodically communicated by the decoder 113 of the amount of the fee for the viewing of each to a paid television program broadcast at a certain time is required, and the assigned Channel number. After having selected the channel for the desired program, the subscriber would have to has to press a switch on his device to trigger the sale of the program. The actuation of the Switch would cause the authorization unit to to communicate the digitally encoded amount of the fee for the program in question to the card reader. the The card reader would then destroy such a number of the partial amounts on the card that the required fee for the program results. The destruction of the partial amounts can be electrical, for example are done by essentially burning out sensitive electrical conductors embedded in the card but also mechanically by punching holes in the card or other mechanical ones Changes are made, or otherwise. The card reader then becomes the credit tag determine which remains on the card for further purchases, and then appropriate partial amounts of the cancel remaining credit if additional programs are purchased. As part of the subscriber device A digital display can be provided which shows the subscriber the remaining balance.

Instead, at the request of the seller, the fee can also be charged in such a way that parts of the Program one after the other. For example, if for a program of 60 minutes a Fee of 3 dollars is payable, the debit code can be chosen so that at intervals 10 minutes each partial amounts of $ 0.50 are deleted.

d) A fourth method of releasing and calculating programs requires neither a card nor a card prior approval. With this method, the participant's device is equipped with a key switch, a number switch and a channel selector switch. The participant has a key for the key switch to allow unauthorized persons to use the subscriber's decryption device impede. To buy a program, the subscriber must press his key switch Set the channel selector switch to the desired channel and press the number switch. This will make the Authorization unit sent an authorization signal that enables the authorization unit to trigger the decryption process and allow the participant to view the desired program to allow. With this method, which is primarily suitable for cable television systems. the information required for invoicing is obtained through feedback to the central station. When you have a cable television system where

Ιό

i also a signal transmission from the subscriber devices to the central station is possible, a processing center is set up in the central station. The processing center periodically polls the subscriber devices and receives digitally coded responses from them. The answers are evaluated by a specially programmed computer in the processing center.

When the encryption and decryption system described herein is used as an integral part of a two-way cable television system, the transmission of billing information to the vendor of the programs can be accomplished through the following steps. If a participant is authorized to receive a certain television program, the activation of the payment switch by the participant not only triggers the decryption process but also transmits the participant address and the number of the selected channel to logic and memory circuits located in the participant device. When the subscriber device is then queried by the processing center, it delivers charge information as a response. This information is processed by the computer at the processing center, which also adds the time of the program sale, in order to record a complete invoice line on magnetic tape or other storage means as required for the preparation of an invoice.

For the explanation of the invention, the second is the Chosen options for the construction and operation of an authorization unit discussed above been. It is understood, however, that each of the other options described above as well other, similar methods and devices could be used in the system according to the invention.

Such an embodiment of the authorization unit 115 will now be treated with reference to FIG. This Embodiment allows four different modes of operation. In a first operating mode, programs A and B not encrypted. In a second operating mode, programs A and B are encrypted, but they are at least one of the two programs is free. In this case, the authorization unit 115 effects a Decryption of the selected free program without charging the subscriber. In a Third mode, programs A and B are encrypted, and it is at least one of the two Programs payable. In this case, the participant must be entitled to use the chargeable, to receive encrypted program so that the authorization unit 115 can decrypt the selected encrypted program allows, In a fourth operating mode, the selected channel is encrypted and broadcasts a paid program, however, the subscriber has not paid for the program paid and is therefore not entitled to receive it. In this case the selected channel stays in its encrypted state. These four modes of operation will now be discussed in detail.

In the first operating mode, in which the programs are not encrypted, the output is a AND gate 701 in the O state. The working states of this AND element 701 are detailed later explained. The output of the AND gate 701 becomes immediately AND gates 703 and 705 and after a neeation by NOT gates 707 and 709 the inputs of an AND gate 711 and an OR gate 713. The output signal of the AND element 705 is fed to a second input of the OR element 713 supplied Since the output signal of the AND gate 701 during the first operating mode im Is 0 state, AND gates 703 and 705 are blocked by negating the 0 state on The output of the AND element 701 through the NOT element 707 becomes the lower input of the AND element 711

ίο prepared. The state at the output of the AND element 711 is therefore determined by the state of the bit CS5 of the code of the selected channel that is fed to the upper input of the AND element 711. A 1 state of bit CS5 indicates that the subscriber has selected channel X , while the 0 state indicates that the subscriber has selected channel Y. In the first operating state, the output signal of the AND element 711 is consequently in the! State if the subscriber has selected channel X , and in the O state,

ao if the subscriber has selected channel Y. The output signals of the AND gates 703 and 711 are fed to an OR gate 715. The output of OR gate 715 is signal XF. The signal XF is also passed through a NOT gate 717 in order to avoid the

to form the signal VF. As a result, it can be seen that when the channel A is selected, the signal XF is in the 1 state. while the signal YF is in the 1 state when the channel Y is selected. As a result, the state of signal XF at the output of OR gate 715 indicates whether channel X or channel Y has been selected. It follows that the state of the output signal XF of the OR gate 715 is determined by the binary state of the bit CS5 , because the AND gate 701 produces an output signal in the 0 state at this time.

The output signal of the OR gate 713 is the signal Ref. Gain. This signal Ref. Gain is also passed through a NOT gate 719 to the signal Diff. Gain to form. It is therefore easy to see that the State at the output of the OR gate 713 determines whether the signal Ref. Gain or the signal Diff. Gain in ! State is generated. Since it was stated above that the output signal of the AND gate 701 is in the 0 state and the signal is negated by the NOT gate 709 is, the output of the OR gate 713 is in the 1 state. This means that in the first mode of operation without encryption, the Ref. Gain signal is always selected.

In the second operating mode, programs A and B are encrypted, but at least one is from "free" them. It is assumed that the selected channel contains a free program. In this case it will the switching signal supplied by the decoder 113 is differentiated by a differentiator 721, the output of which is connected to a monoflop 723, which can be a monoflop of the type SN 74 122, the manufactured by Texas Instruments Incorporated and between pages 6-79 and 6-83 of the manual published by Texas Instruments Incorporated: »The Integrated Circuiis Catalog for Design Engineers «. The monoflop 723 leads the lower input of the AND gate 701 to a signal in the 1 state when there is a transition from 0 to at its input 1 state. The period of time during which the output signal of the monostable 723 remains in the! State, is much larger than the longest switching interval, which is determined by the fact that the switching signal from 1 state to 0 state and then back to

1 state changes as long as the monoflop 723 sees 0 to 1 transitions that occur every time the Switching signal changes its state, the monoflop 723 remains in the 1 state. Only when the gearshift signal during a time that is longer than the clock period of the Monoflop 723, its state does not change, the output signal of the monoflop 723 goes into the 0 state back and thereby causes that the output signal of the AND gate 701 also assumes the O state. This would cause the authorization unit 115 to return to the first operating mode discussed above.

Since encrypted programs are present in the second operating mode, the AND element 701 is prepared in the 1 state by the signal present at the lower input. As already stated, decryption is possible when the output signal of AND element 701 is in the 1 state. The upper input of the AND element 701 is connected to the output of an OR element 725 with two inputs. One input of the OR element 725 is connected to one of the four outputs of a program register 727, while the other input of the OR element 725 is connected to the output of an AND element 729. The program register 727 receives and stores the four bits PR 1 to PR 4 of the input program code. This input program code is entered into the program register 727 in the following manner.

It is assumed that when the subscriber has set the channel selection switch 119 to channel X, the last bit CS 5 of the code of the selected channel is in the 1 state, while this bit is in the 0 state. if the subscriber has set the channel selector switch to channel Y. From the previous treatment of the format of the channel switching code in the switching controller 69, it emerged that whenever a code is received in which the last bit CH 5 of the input channel code is in the 0 state, the decoder 113 is notified of the program A on channel X and program B on channel Y and that the program bits of the code apply to program A. If CH 5 is in the 0 state and CS5 is in the 1 state, it is necessary that the "Data OK" signal assigned to the code permits the input program bits PR 1 to PR 4 to be stored in the program register 727 because these bits contain information associated with channel X. It can therefore be seen that when the binary states of bits CH 5 and CS5 are different and the signal "Data OK" is generated, the program register 727 receives the program bits PR 1 to PR 4. For this purpose, the signals CH 5 and CS5 are fed to a logic element 731, which performs the function of an "exclusive OR" and only generates an output signal in the 1 state if its input signals are different. The output signal of the logic element 731 is fed to an input of an AND element 733. The "data OK" signal is fed to a second input of the AND element 733 and enables the AND element 733 to generate a "load" signal whenever the "data OK" signal is present and the input signals of the logic element 731 are different . The "load" signal is fed to the program register 727 by the AND element 733 and allows the program register to _receive the input program bits PR 1 to PR 4 for 6 seconds.

Since signal CH5 changes its binary state every time programs A and B are switched, program register 727 is only loaded every other switchover. As a result, the information stored in program register 727 is only valid for the program that is assigned to the selected channel. In the same way, the data stored in program register 727 are assigned to channel V if bit CH 5 is in the 1 state and bit CS 5 is in the 0 state.

The program register 727 can basically consist of four separate flip-flops, not shown, for each data input, each of which is supplied with a bit of the input program code. The "load" signal from AND element 733 is fed to the clock inputs of all flip-flops, while the assigned bits of the input program code are stored at the corresponding Q outputs of the flip-flops.

If the Q output of the flip-flop, to which the PR 1 bit has been fed, assumes the 1 state or remains in this state after the "Data OK" signal has been generated, the selected channel contains a free program. This signal “free” in the 1 state is fed from the program register 727 via the OR element 725 to the upper input of the UN D element 701. Since the programs A and B are encrypted at this time, trigger: the switching signal continuously the monoflop 723, so that the resulting output signal in the 1 state is fed to the lower input of the AND element 701. As a result, the output of AND gate 701 is in the 1 state. This signal enables the decryption process.

The output signal in the 1 state of the AND element 701 is negated by the NOT elements 707 and 709 in order to block the AND element 711 and the lower input of the OR element 713. The signal in the 1 state of the AND element 701, however, prepares the upper input of the AND element 703, so that the state at the output of the AND element 703 is determined by the output signal of a comparator 735 which is the lower input of the AND element 703 is fed. It can be readily seen that when the output of comparator 735 is 1, then signal XF 'is 1 and channel X is selected. Similarly, when the output of comparator 735 is 0, signal VF is 1, indicating that channel Y has been selected. The comparator 735 compares the binary states of the signal CS5 unc of the switching signal. If the signal CS 5 and the switching signal have the same binary state, a signal in the 1 state is generated at the output of the comparator 735 in order to form the signal XF. If, on the other hand, the two signals have different binary states, a signal in the 0 state is generated at the output of the comparator 73f in order to form the signal YF zi. It can therefore be seen that when there: Signal CS 5 is in the 1 state, this indicates that channel X has been selected and that the participant: would like to receive program A, and vice versa. E: it should be remembered that when the switching signal is in the 1 state, program A on channel Xundda; Program B is on channel Y. If, on the other hand, there is: the switching signal is in the O state, program A is on channel Y and program B is on channel X.

It should also be remembered that it has been shown that the output signal of the comparator 735 determines the state at the output of the OR gate 715. Therefore, if channel X has been selected and as a result, since bit CS5 is in the 1 state, the output signal de

η /

The comparator 735 and, as a result, the signal XF follow the binary state of the switching signal. In the same way, if the channel Y has been selected so that the bit CS 5 is in the 0 state, the output signal of the comparator 735 and consequently the signal XF will have the negated binary state of the switching signal. In this way, the service unit decrypts programs A and B. However, since the binary state of bit PR 1 indicated that the program was free, no fees are charged for this decryption.

The 1 state at the output of AND element 701 also prepares the upper input of AND element 705 before. As a result, the state at the output of AND element 705 and thus also the binary state of the signals Ref. Gain and Diff. Gain during this second operating mode from the binary state of the Switching signal dependent. Accordingly, the 1 state of the switching signal has a 1 state of the signal Ref. Gain Consequence, while a 0-state of the switching signal an ao 1-state of the signal Diff. Gain causes.

In the third operating mode, programs A and B are encrypted and the selected channel is not free. As a result, the authorization unit 115 requests that the payment or the commitment to pay a fee be confirmed before it allows the decryption process. In this mode of operation, bit PR 1 of the input program code, which is stored in program register 727, is in the 0 state to indicate that the program transmitted on the selected channel is not free. As a result, the lower input signal of the OR gate 725 is in the 0 state. The output signal of the OR gate 725 is therefore determined by the binary state of its upper input signal, which is supplied by an AND gate 729. The inputs of the AND element 729 are connected to the outputs of two comparators 737 and 739. The channel group comparator 739 compares the card channel code bits C1 to C4 with the bits C51 to CS4 of the code of the channel selected by means of the channel selector switch 119. If there is an exact match between the compared, mutually corresponding bits, the channel code comparator 739 forms an output signal in the 1 state, which is fed to the lower input of the AND gate 729. The comparator 737 compares the card channel code bit C5 with the bit CS5 of the code of the selected channel and also compares the card program code bits C6 to CS with the input program code bits PR 2 to PRA contained in the program register 727. If there is an exact match between the compared bits, the comparator 737 forms an output signal in the 1 state, which is fed to the upper input of the AND gate 729.

If both comparisons 737 and 7 are found to match, their output signals are set in the 1 state, the AND element 729 is able to generate a signal "card valid" which is sent to the upper input of the OR gate 725 is supplied to cause the decryption as described above for the second operating state has been treated.

The "card valid" signal from AND element 729 is also fed to the first input of an AND element. _{The 6s} bit PR 1 of the input program code in the 0 state, which is stored in the program register 727, is negated by a NOT element 743 and fed to the AND element 741 as a second input signal.

Finally, the output signal of the monoflop 723 is fed to the third input of the AND element 741, the is in the 1 state during this time because programs A and B are encrypted. The three Input signals in the 1 state at AND gate 741 tend to indicate that the selected channel is encrypted, that it is chargeable and that the subscriber has inserted a valid card into the input device 117 If there are three input signals in the 1 state at its inputs, the AND element 741 forms a "Card accepted" signal, which is fed to the card reader in the input device 117, so as not to To operate the validation device shown, which destroys the code bits on the card when the card is removed from the card reader by the participant. This prevents the Participants who use the card for the following programs In the fourth operating state, programs A and B encrypted, chargeable and not by generating a "card valid" signal, the one Fee payment confirmed, released. As a result, the AND gate 701 is through the output of the OR gate 725, which is in the 0 state, blocked in order to prevent a decryption process. Consequently the selected channel remains encrypted.

In the following, with reference to FIG. 8 the switching device 121 of the system according to FIG. The four highest-digit bits of the code of the channel selected by means of the channel selection switch 119 are fed to two digital-to-analog converters or D / A converters 751 and 753 for short. The D / A converters 751 and 753 are programmed in such a way that they form the tuning voltage V «for channel X and the tuning voltage Vy for channel Y , depending on the four most significant bits of the code of the selected channel. Since four bits are used for the channel pair, one of sixteen different channel pairs can be selected. This means that thirty-two different transmission channels can be encrypted in pairs in the central station and different tuning voltages V 1 and V 1 can be used for each pair.

The tuning voltage V »is intended for one channel and the tuning voltage V> for the other channel of a selected pair. The tuning voltage Va supplied by the D / A converter 751 is fed through a resistor 755 to an X-tuning voltage selector 757, while the tuning voltage Vy from the output of the D / A converter 753 is fed through a resistor 759 to a V-tuning selector 761, which is also is designed and operates like the X tuning voltage selector 757. The tuning voltage V «is fed to the non-inverting input of an operational amplifier 763, which is located in the A" tuning voltage selector its output is fed back to the inverting input.

The error signal for channel X supplied by the arrangement 131 for frequency control is fed to the non-inverting input of the operational amplifier 763 via a resistor 765. The X error voltage is used to correct inaccuracies in the output signal of the D / A converter 751. The error signal for channel Y, which is supplied by the arrangement 131 for frequency control, is fed via a resistor 767 to the non-inverting input of the operational amplifier 763 in the V tuning span.

Voltage selector 761 is supplied to correct any inaccuracies in the tuning voltage Vy. The output of the operational amplifier 763 is connected to the emitter of its field effect transistor or FET 769 for short and also to the gate electrode of the FET 769 via a pull-in resistor 771. The purpose of the pull-in resistor 771 is to pull the potential of the gate electrode essentially to the potential of the emitter when the FET769 is switched on.

It should be mentioned at this point that the FET 769, like other field effect transistors to be described, operated as switches that are either closed or open. A FET is closed, when the gate potential can be essentially the same as the emitter and collector potential. If a »5 FET is closed, the internal resistance between emitter and collector is relatively low. This one treated field effect transistors are opened by the potential of the gate electrode with respect to the Emitter and collector potentials are made negative, ao by applying a voltage to the gate electrode which is equal to the reverse voltage of the FET. When an FET is open, its emitter-collector resistance is very high.

The collectors or two field effect transistors _a5 769 of the two tuning voltage selectors 757 and 761 are connected to one another in order to supply the tuning voltage for the frequency converter 97. In each tuning voltage selector, a diode 773 has its anode connected to the gate electrode of the FET 769. The signals XF and YF, which are supplied by the authorization unit 115, are fed to the cathodes of the diodes 773 in order to selectively close or open the field effect transistors of the tuning voltage selector.

As discussed above, the signal KF is in the 0 state when the signal XF is in the 1 state, and vice versa. For purposes of explanation, it is assumed that the signal AF is in the 1 state and the signal VF is in the 0 state. The signal XF in the 1 state acts on the diode 773 of the X-tuning voltage selector in the reverse direction and thereby enables the associated FET 769 to feed the X-tuning voltage at the output of the amplifier 763 of the X-tuning voltage selector 757 to the frequency converter 97. At the same time, the signal VF, which is in the 0 state, passes through the diode 773 in the y-tuning voltage selector 761 and opens the associated FET 769 to prevent the Y- tuning voltage from reaching the frequency converter 97. In this way, the tuning voltage allows ^ ₀ At the output of the X tuning voltage selector 757, the program which is located in channel X at this time is to appear at the output of the frequency converter 97. In the same way, when the signal YF is in the 1 state and the signal XF in the 0 state, the y-tuning voltage selector 761 allows the V-tuning voltage to pass to the frequency converter 97 so that the program present on channel Y at this time is output of the frequency converter 97 may appear. At the same time, the signal XF, which is in the 0 state, prevents the X tuning voltage from being supplied to the frequency converter 97.
For a more detailed explanation of the arrangement 131 for frequency regulation, reference is now made to FIG. 9 referenced. The basic function of this arrangement consists in the voltages characteristic of frequency deviations in the channels X and Y or X- and yFc ^v *? Rs; g!? AiG »?! Ektrv- ifes ^ Ä ·· - and τα 3 * si These X and K error signals are selectively fed to the switching device 121 so that this switching device forms the correct tuning voltages for the channels X and Y in order to precisely tune the frequency converter 97, so that the video carrier in the output channel 8 of the frequency converter 97 always exactly the has the same frequency, regardless of whether the frequency converter 97 is tuned to the channel X or the channel ^.

The output signal in channel 8 of the amplification; 127 is fed to a mixer 801 in which the Input signal in channel 8 is superimposed with the output signal of a local oscillator 803. The local oscillator 803 is tuned to a frequency approximately 45.7 MHz higher than the video carrier frequency of channel 8. The output signal of mixer 801 is the IF of channel 8, which is a standard video IF amplifier and detector 805 is supplied. The amplifier and detector 805 can be RCA module No. 132 586 act, although any other suitable video IF amplifier and detector can be used.

The IF signal of channel 8 is fed to pin 16 of the IF amplifier and detector 805 supplied. The control voltage for the IF amplification, which is used by the Arrangement 133 for gain control is supplied to pin 15 of the IF amplifier and Detector 805 supplied. The level of this control voltage is determined by the arrangement 133 for gain control adjusted so that the voltage between the tips of the video mix at pin 8 is about 6 to 8 volts. Recall that the one supplied by the 805 IF amplifier and detector Video composite is used by the arrangement 133 for gain control to determine the amplitude of the Set the control voltage for the IF gain. The arrangement 133 for gain control is later with reference to F i g. 10 will be explained.

Under the operating conditions noted above, the voltage difference between pins 4 and 5 of amplifier and detector 805 is an indication of the extent to which frequency converter 97 is detuned from the channel 8 video center frequency. For example, if pin 5 is positive with respect to pin 4, the frequency converter is tuned to a frequency that is too high, and vice versa. The signals at pins 4 and 5 are fed through resistors 807 and 809 to the non-inverting and inverting inputs of an operational amplifier 811, respectively. The output of the amplifier 811 is fed back to the inverting input via a resistor 813. A series circuit of resistor 815 and Zener diode 817 is connected between a voltage source with +30 V and ground so that a reference voltage is generated at Zener diode 817 that is not via a resistor 819 inverting input of the operational amplifier 8111 is supplied. The operational amplifier 811 is connected so that it acts as a differential amplifier and amplifies the differential voltage between pins 4 and 5 of the IF amplifier and detector 805 and brings the amplified differential voltage to a level that is besitimmi by the reference voltage supplied by the Zener diode 817.

The error signal at the output of the operational amplifier 811 is via an insulation resistor 821 sample and hold circuits 823 and 825 for the channel λ • ύ- ^ η channel VrUgCfEr .-- The sample and Kalielkreise

823 and 825 have the same structure and function. The error voltage supplied from the operational amplifier 811 through the insulation resistor 821 is supplied to the emitter S of an FET 827 in each of the sample and hold circuits. The collector of each FET is connected to the non-inverting input of an operational amplifier 829 and a capacitor 831 , the other terminal of which is connected to ground. The output of the operational amplifier 829 is connected to its inverting input and, via a resistor 833, to the Gau of the FET 827 and the anode of a diode 835 . The signals XF and YF supplied by the authorization unit 115 are fed to the diodes 835 in the load and hold circuits 823 and 825 in order to enable these circuits to receive the error voltage from the operational amplifier 811 and thereby selectively the error signals for the channels X and K to form.

When the signal XF \ m 1 is applied to the diode 835 of the sample and hold circuits 823 in the reverse direction, so that the output of the operational amplifier 829 of the sample and hold circuit 823 is transmitted through the resistor 833 to the gate of the FET 827 can, so that the potential at the gate rise to the potential of the emitter and collector and thereby the FET 827 in the sample and hold circuit 823 can be closed. When the FET 827 is closed by the 1 state of the signal XF , the associated capacitor 831 can be charged or discharged to a potential which is substantially equal to the differential error signal at the output of the associated operational amplifier 81 1 at that time. As a result, the channel X sample and hold circuit 823 samples the X error signal during the time it is supplied with the 1 state signal XF. The X error signal is fed from the output of operational amplifier 829 of the sample and hold circuit 823 to the switching device 821 to enable the switching device 121 to generate the correct X tuning voltage for the operation of the frequency converter 97 during the time during which the desired program is transmitted via channel X.

When the signal XF is in the 0 state, it passes the diode 835 in the sample and hold circuit 823 and blocks or opens the associated FET 827 by bringing gate G of the FET 827 to a negative potential with respect to the emitter and collector. As a result, no X error signal is generated at the output of the operational amplifier 829 in the sample and hold circuit 823 when the signal XF'ith is 0

The operation of the sample and hold circuit 825 for the channel K is identical to the operation of the sample and hold circuit 823 for the channel X, except that the signal YF is used to form the error signal for the Y channel. As described above, the signal VF is in the 1 state when the signal XF is in the 0 state and vice versa. As a result, when the error signal for the channel X is sampled, the error signal for the channel Y s is high and vice versa the X error signal is also fed to the Y error signal of the switching device 121 so that the switching device 121 can generate the correct Y tuning voltage for the operation of the frequency converter 97 during the time during which the desired program is on the channel

The arrangement 133 for gain control, to which the video mixture is fed from the arrangement 131 for frequency control, will now be explained in detail with reference to FIG. It should be remembered that when the arrangement for frequency control according to FIG. 9 was discussed, it was stated that the control voltage for the IF amplification supplied by the arrangement 133 for the gain control is stabilized at such a value that the arrangement 131 for the frequency control mixes the video with a voltage of about 6 to 8 volts measured between the peaks of the signal. This composite video, which is supplied by the IF amplifier and detector 805 in the arrangement 131 for frequency control, is fed in the arrangement 133 for gain control to a peak detector 851 , which also separates the synchronization signals and forms an output voltage that is proportional to the amplitude of the Horizontal and vertical sync pulses included in the composite video. The output of the peak detector 851 is applied to a reference sample and hold circuit 853 and a difference sample and hold circuit 855 . The authorization unit 115 also sends the signals Diff. Gain and Ref. Gain supplied. The structure and operation of the sample and hold circuits 853 and 855 is identical to those of the DEM and discussed above with reference to Figure 9 sample and hold circuits 823 and 825. Consequently, a detailed explanation of the sample and hold circuits 853 and 855 not required.

It will now be explained how the control voltage for the IF amplification, which is fed to the IF amplifier and detector 805 for encrypted and unencrypted programs, is formed.

From the block diagram according to FIG. 7 of the authorization unit 115 it can be seen that the output of the AND element 701 is always in the 0 state when no decryption is initiated. This can either be the case if the desired program has not been encrypted or no fee has been paid for an encrypted program. Recall that the Ref. Gain signal is in the 1 state and the Diff. Gain is in the 0 state when the output signal of the AND gate 701 is in the 0 state. The signal Ref. Gain in the 1 state has the effect in the arrangement 133 shown in FIG. 10 that a video reference voltage is formed by the sample and hold circuit 853 in the same way as is described for the sample and hold circuit 823 in FIG has been. This video reference signal is fed to a gain controller 857 and an IF gain controller 859. It is the IF gain controller 859 which forms the control voltage for the IF amplification which is fed to the IF amplifier and detector 80S. As a result, IF gain controller 859 will now be discussed

The video reference signal is from the sample and hold circuit 853 via a resistor 861 of the base; of a pnp transistor 863. The emitter of the transistor 863 is connected to the junction between two series connected resistors 865 and 86Ϊ connected, their outer ends with a voltage source of + 30V or ground are connected. The collector of transistor 863 is across a resistor 869 connected to a voltage source of -12 V The IF gain controller 859 is used as an inverting

I)

An amplifier designed in which the output signal of the transistor 863 is applied from its collector via a resistor 871 to a connection point 873 , from which the control voltage for the IF amplification is taken. The wiper of a potentiometer 875, which is connected between the voltage source of -12 V and ground, generates a reference voltage which is fed to the connection point 873 via a resistor 877. The video reference signal is inverted and amplified by transistor 863 before it reaches connection point 873. The reference voltage, which is supplied by the wiper of the potentiometer 875 , is used to set the output level of the control voltage by adding the voltage tapped by the wiper of the potentiometer 875 to the signal at the collector of the transistor 863 at connection point 873 . As a result, the control voltage for the IF amplification fluctuates around the reference level which is determined by the position of the wiper of the potentiometer 875 .

If the voltage of the composite video decreases during operation, the video reference voltage also decreases, so that the level of the control voltage for the IF amplification is increased and the IF amplifier and detector 805 in FIG. 9 is caused to increase the amplitude of the composite video, which is fed to the peak detector 851. This causes an increase in the video reference voltage and thus a stabilization of the loop at a fixed voltage level.

In the case of operation with decryption, this loop also remains in effect during the decryption process. However, the sampling frequency of the sample and hold circuit 853 is lower because the system samples between the sample and hold circuits 853 and 855 alternately. This is due to the fact that the signals Ref. Gain and Diff. Gain alternately change their binary states. When the signal Ref. Gain is in the 1 state, the sample and hold circuit 853 samples the output of the peak detector 851 , while the sample and hold circuit 855 samples the output of the peak detector 851 when the signal Diff. Gain is in the 1 state. The binary states of the signals Ref. Gain and Diff. Gain are determined by the switching signal that is fed to the authorization unit 115 from the decoder 113 . The sample-and-hold circuit 853 for the reference signal is also required to form the video reference signal, albeit with a lower sampling frequency, in order to cause the IF gain controller 859 to generate a fixed control voltage for the IF , which controls the gain of the IF amplifier and detector 805 fixed to a certain value, regardless of whether channel X or channel V is currently selected.

In addition to the requirement to set the frequency of the video carrier signal fed to television set 129 stabilize, there is a further requirement in the decryption operation that the amplitude of the video carrier signal fed to the television set 129 is stabilized. Furthermore, in this operating mode, when the switching signal supplied by the decoder 113 im 1 state is the polarity of the signals Diff. Gain and Ref. Gain in such a way that the signal Ref. Gain im Is 1 state to enable the reference sample and hold circuit 853 to sample the output of the peak detector 851 during the sample and hold circuit 855 holds the previous value. In a corresponding way, if the switching signal is in the 0 state, the signal Diff. Gain in the 1 state and the signal Ref.

Gain in the 0 state. At this time, the sample and hold circuit 855 is selected to sample the output of the peak detector 851 while the sample and hold circuit 853 is holding the previous value. As a result of this independent sampling and holding of the video level of the selected channels X and Y , the difference between the video differential voltage and the video reference voltage is the difference between the signal span.

ίο the two video carrier frequencies of the encrypted Channels Ä 'and K proportional.

Any voltage difference between the video difference and video reference signals is corrected by the operation of the gain controller 857 to which these signals are applied. The mode of operation of this gain controller 857 is explained below. A resistor 879 is connected in series to a potentiometer 881. The outer ends of resistor 879 and potentiometer 881 are connected between a positive voltage source and ground. The wiper of the potentiometer 881 is connected to the non-inverting input of an operational amplifier 883, which is connected as a voltage follower. The inverting input of the operational amplifier

883 is connected to its output so that the operational amplifier 883 can form a reference voltage which is determined by the position of the potentiometer 881 . When the signal Ref. Gain is in the 1 state, a diode 885 is applied in the reverse direction.

so that an FET 887 can feed the reference voltage from the operational amplifier 883 to the amplifier 127 used for voltage regulation. This reference voltage is set to a value at which the amplifier 127 provides optimum gain control

allows. A drag resistor 889 is connected between the output of operational amplifier 883 and the gate of FET 887. The mode of operation of the FET 887 is the same as that of the FET 769 treated with reference to FIG. 8 and therefore does not need to be explained again here.

The video difference and video reference signals supplied by the sample and hold circuits 853 and 855 are fed via resistors 891 and 893 to the inverting and non-inverting input of a differential

Amplifier 895 supplied. A feedback resistor 897 is connected between the output and the inverting input of the differential amplifier * 895. The reference voltage generated at the output of the operational amplifier 883 is

was 899 fed to the non-inverting input of the differential amplifier 895. The differential amplifier 895 and resistors 897, 899, 891 and 893 form a circuit arrangement that corresponds to the circuit arrangement of the 811 operational amplifier and resistors

the 813, 819, 809 and 807 in the arrangement 131 is equal to the frequency control according to Fig.9. The output of differential amplifier 895 is a differential voltage equal to and equal to the difference between the video differential and video reference signals

the magnitude of the reference voltage is shifted. As a result, if the video differential voltage is more positive than the video reference voltage, the differential voltage at the output of the differential amplifier 895 less positive than the reference voltage on

Output of amplifier 883, and vice versa

If the signal Diff. Gain is in the 1 state, it is applied a diode 901 in the reverse direction, whose anode with the

ΙΌ

Gatt of a FET 903 is connected. A resistor 905 connects this Galt to the output of the differential amplifier 895 and is used to ensure the correct function of the FET 903. The mode of operation of the FF.T 903 corresponds to that of the FET 887. When the diode 901 is blocked, the FET 903 is able to supply the differential voltage generated at the output of the differential amplifier 895 as a control voltage to the amplifier 127. Any difference in amplitude between the differential voltage and the reference voltage forces a change in the output voltage of the controlled amplifier 127 and thus the voltage level of the video mixture in such a direction that any difference between the video differential and video reference voltages becomes 0 when the arrangement 133 for automatic voltage regulation between the Signals diff. Gain and Ref. Gain is switched. The output level of the controlled amplifier 1 27 changes when the signals Diff. Gain and Ref. Gain can be switched in such a way. that the Videodifferenz- and video reference signals are substantially equal to each other The result of this automatic voltage control inspects the fact that the signal level of the video carrier frequency at the output of the amplifier 127 which is supplied to the TV i29 and also the arrangement 131 to the automatic frequency control remains constant when the The system is switched between channels Λ and V 'during the key tune process.

The invention therefore em system is specified, be switched at least two television programs intermittently between two transmission channels, so that the output signal of each Übcrtragungskanals never contains a sequence of each of the two television signals abwech sclnden Fernschprogranv $ *. At the same time as the two television programs are switched between the two transmission channels, or shortly before, an encoded signal is sent to all subscriber devices in the system, which enables authorized subscribers to receive a selected channel in which the encrypted program is decrypted . After a subscriber has obtained authorization to receive a desired program in the decrypted state by paying a fee or by agreeing to the payment of a fee, which can be done by pressing a push button, key switch * or other suitable device. a decryption device is triggered which switches between the transmission channels essentially at the same time as the program is switched between the transmission channels.

After the essential features of the invention have been described and illustrated above, is it is easily recognizable for a person skilled in the art that compared to the illustrated embodiments in the frame Changes lying within the invention are possible. For example, the system could be designed in this way become that only one channel reveals an understandable program. The second channel could be a confusing one contain a highly disruptive signal in order to make the desired television program understandable interrupt to a greater extent. It should also be noted that the Dane: the one described here Programs the duration of normal television programs of a few seconds. Minutes or longer, but also only needs to have the duration of a television picture or even a field. The second channel could also contain high frequency audio signals and moving audio patterns. Although the invention on hand of a pair of adjacent channels has been described. \ it arises that the \ star with more than one l'Yogrammpaar and with more than two channels in each Schalignippc as well as with non-adjacent channels can work. Even if it is easier to realize the system / u when the channels are adjacent, then there are It is not a requirement caused by the invention that made it necessary for two channels to be adjacent should be. The invention was finally related to the encryption of television signals described. It can also be used in voice connections or Other communication links are used where reception is disturbed by unauthorized persons People is desirable.

In addition 7 sheets of drawings

Claims (4)

Patent claims: 1. Television system with a central station, at least one associated subscriber device and at least two channels for the transmission of television programs from the central station to the subscriber device, with a first switching device that switches a program alternately to the two channels, a second switching device that the subscriber device alternately connects to the two channels one after the other, and with a device electrically connected to the two switching devices for synchronizing the switching frequencies, characterized in that both channels are set up for the transmission of a television program each and the device for synchronizing the switching frequencies has a first arrangement (61 , 65) for deriving a first signal from one of the programs, a second arrangement (63, 67) for deriving a clock signal from the other program, a third arrangement (69) for ore which is responsive to the first signal and the clock signal eugung a channel switching code, a decoding arrangement (95, 99, 111, 113), a fourth arrangement (71, 73) for transmitting the channel switching code to the decoding arrangement (95, 99, 111, 113) which generates switching signals dependent on the channel switching code, and a fifth arrangement (115) for supplying the switching signals to the second switching device (121) . 2. Television system according to claim 1, characterized in that the subscriber device comprises a frequency converter (97) which converts each selected channel to a predetermined channel, and that the second switching device (121) generates first and second tuning voltages in dependence on the switching signals and the Frequency converter (97) feeds, which effects the conversion of the selected television program on the predetermined channel as a function of the tuning voltages. 3. A television system according to claim 2, characterized in that an arrangement (131) for automatic frequency control is coupled to the fifth arrangement (115) which is responsive to the switching signals and the signals of the desired television program in the predetermined channel, generating first and second frequency error signals and supplies said second switching means (121) for correcting said first and second tuning voltages so as to accurately maintain the frequency of said predetermined channel. 4. Television system according to claim 3, characterized in that the fifth arrangement (115) generates further signals as a function of the switching signals and the frequency converter (97 ) comprises a voltage-controlled amplifier (1X7) and that between the fifth arrangement (115) and the amplifier (127) an arrangement (131) for automatic gain control is connected, which responds to the further signals of the fifth arrangement (115) and a video signal supplied by the arrangement (131) for frequency control and generates a control signal and feeds it to the amplifier (127), to control the voltage of the output signals of the amplifier (127) while the frequency converter (97) is switched between the two channels for the transmission of television programs.