WO1986006240A1

WO1986006240A1 - Video transmission system

Info

Publication number: WO1986006240A1
Application number: PCT/GB1986/000202
Authority: WO
Inventors: Steven Raggett; Thomas Terry; Reginald Swain
Original assignee: Paytel Limited
Priority date: 1985-04-12
Filing date: 1986-04-11
Publication date: 1986-10-23
Also published as: EP0218703A1; GB8509390D0

Abstract

A television system in which, at the transmitter, the video signal is scrambled by inverting the signals of groups of lines down the picture, the transmitted signal also including a key number, and, at the television receiver, decoding apparatus which, in combination with a plastic card incorporating electronic circuit components, utilises the key number to produce a desired number relating to the numbers of lines whose signals are inverted, whereby the decoder may reinvert the signals to produce a usable video waveform.

Description

VIDEO TRANSMISSION SYSTEM

The present invention relates to a video transmission system which is preferably particularly applicable to a system for restricting the viewing of received signals to a particular television receiver or to a

5 particular viewer, the viewer in one preferred arrangement being a subscriber to a pay-television service which may be received either by way of a

' cable or by trans ission from terrestrial or

10 satellite transmitter.

The method and apparatus of the invention provides a modulated video signal which when applied to a standard television receiver produces a scrambled 15 picture, a decoding apparatus bei.ng required to decode the received video signal to produce a proper picture.

There are a number of aspects of the present 20 invention as follows.

Because there is no necessity for -real time interaction between the television system operator and the viewer it is possible to transmit the signals

25 by terrestrial or satellite communication rather than through a closed cable system. According to this aspect, therefore, the method of the invention comprises a method whereby a video signal is transmitted (either alone or impressed on a suitable

30 carrier signal) containing scrambled video information such that when directly used to operate a

standard television receiver will produce a scrambled picture, the transmitted signal also including a coded signal which, when applied to a decoding apparatus associated with a television receiver, will produce an unscrambled video waveform whereby the television receiver will display an unscrambled picture. The invention also includes apparatus for carrying out the aforesaid method.

The relevant coded signal may be sent during the field blanking interval, and may be provided in the form of a binary number, which may take the form of a data pulse carrying sub-pulses of various time periods.

In effect, therefore, the coded signal provides a key and the decoder incorporates a lock and the combination of key and lock will allow the signal to be unscrambled.

The invention also provides according to a further aspect a security system comprising an electric signal (such as a video signal) including in encoded form, a key number, an electronic decoder apparatus and an electronic circuit part (preferably in the form of a plastic card including electronic components) selectively connectable to the electronic decoder apparatus, the electronic decoder apparatus and the electronic circuit part each including means for carrying out a series of different operations on said key number to produce a desired number from said key number. Clearly such an arrangement has widespread security uses as it enables one to separate two parts of the decoding apparatus, that is the electronic decoder apparatus and the electronic circuit part,_ so that the decoding can only take place when those two parts are connected to one another.

In use of this system in a television system, the electronic signal may comprise a video signal transmitted from a transmitter, the video signal being scrambled, and incorporating a key number, the electronic decoder apparatus and electronic circuit part being provided to subscribers to the television service whereby the desired number produced by the decoding apparatus from said key number may be used to unscramble the scrambled video signal to allow the video signal to be viewed.

In this particular case,- the scrambling arrangement of the video signal may be changed at regular intervals and the key number changed in synchronism with that. Furthermore, the electronic circuit part may be replaced at regular intervals, for example, weekly, monthly or three monthly.

The key number may be included in the electric signal by means of a data pulse, the actual number being encoded in the form of a binary code, the components of the binary code being determined by the width of sub-pulses within the data pulse. In use with a television system, the electronic circuit part may incorporate a memory and the data pulse may also incorporate information regarding a particular programme being transmitted so that the electronic circuit part or card can store within its memory information regarding the particular programmes which have been decoded by the decoding apparatus.

Where the electric signal is a video signal, the video signal may be scrambled by means of inverting the signal values with respect to some of the lines making up the television picture, the inversion being carried out in respect of groups of lines at spaced intervals down the picture. The desired number produced by the decoding apparatus relates to the numbers of the lines which are inverted.

The present invention provides a television system in which the video information is scrambled or coded, the video signal including data to provide a key number, decoding apparatus comprising in combination an electronic decoder apparatus and an electronic circuit part, preferably in the form of a plastic card including electronic components, selectively connectable to the electronic decoder apparatus, the electronic decoder apparatus and electronic circuit part each including means for carrying out a series of different operations on said key- number to produce a desired number from said key number, said desired number enabling the decoding apparatus to decode or unscramble the video signal to provide a video signal which may be viewed by a standard television receiver.

The present invention may also provide a method of scrambling a television signal comprising inverting the signals in respect of alternate groups of lines, the groups of lines each comprising a predetermined number of lines, and the television signal containing information relating to the numbers of lines in each group.

By line inversion of the signal in the above paragraphs we mean changing the value of the composite luminance/chrominance signal so that a black signal becomes white and a white signal becomes black with corresponding changes therebetween and for example blue becomes yellow and vice versa.

Preferably there is provided apparatus for decoding the scrambled video signal, said decoding apparatus including means whereby, in respect of inverted lines, the composite luminance/chrominance signal is changed into a signal of opposite polarity of corresponding value, (ie +0.5 volt becomes -0.5 volt) and a standard voltage (usually 0.7 volt) is added to that reversed signal.

Means may also be provided to compare, after this signal processing, the value of inverted black signal on the inverted lines with the porch signal level and the "standard" voltage value added may be varied so as to determine that the inverted "standard" and porch level are identical. Preferably, subsequently, the signal is passed through a variable amplifier in which the gain is varied so as to provide a correct voltage level for inverted white on the inverted lines to provide therefore the required linearity.

A preferred apparatus of the invention utilising a preferred method of the invention will now be described by way of example only and with reference to the accompanying drawings in which:

Figure 1 is a diagrammatic illustration of the overall method and apparatus of the invention,

Figure 2 is a perspective view of an encoder , apparatus ,,

Figure 3 is a perspective view of a decoder and electronic card adjacent a television receiver for showing an unscrambled picture,

Figure 4 is an enlarged view of the decoder and card apparatus of Figure 3,

Figure 5 illustrates the waveform of a normal video signal during the frame blanking interval.

Figure 6 illustrates the waveform of a video signal corresponding to part of the video signal of Figure 5 but as modified by apparatus of the invention,

Figure 7 is an enlarged view of part of Figure 6, Figure 8 is a block circuit diagram of the timing and phase lock circuit portion of the encoder,

Figure 9 is a circuit diagram of the part of the encoder shown in Figure 8,

Figure 10 is a block diagram of the video signal path through the encoder,

Figure 11 is the circuit diagram of the part of the encoder shown in Figure 10,

Figure 12 is a block diagram of the timing circuits of the decoder,

Figure 13 is a circuit diagram of the part of the decoder shown in Figure 12,

Figure 14 is a block diagram of another part of the decoder,

Figure 15 is the circuit diagram of the part of the decoder shown in Figure 14,

OUTLINE OF SYSTEM AND MANAGEMENT

Referring first to Figure 1 there is shown a pay- television system in which a programme produced, for example, in a studio 10 and viewed by a camera 11 produces a video signal which is encoded by an encoder 12 and, after being used to modulate an rf signal is transmitted by a terrestrial transmitter 13 or via a satellite 14 or after modulation by a suitable signal is passed along a cable network 16. The relevant signal is received at the receiving end by an aerial (antenna) 17, dish aerial 18, or cable receiver 19 and the signal is passed to a decoder 21, the decoded signal being^" passed to the television receiver 22. The decoder 21 is controlled by means of a card 23, the purpose of which will be explained later.

In broad principle the apparatus of Figure 1 operates as follows. A normal video signal from the camera 11. is encoded by the encoder 12 so that a normal television receiver receiving the signal from the transmitter 13, satellite 14 or cable 15 will produce a scrambled picture unless the signal is decoded by the decoder 21. It will be understood that the video signal may alternatively be derived in other ways, for example, from a film (ie movie).

A variety of methods of scrambling video signals has been proposed but in this case we use so called "line inversion".

In the arrangement to be described the decoder 21 will not by itself unscramble the video signal received via the aerial 17 dish aerial 18 or cable receiver 19. The decoder 21 only operates when the card 23 is inserted in the decoder. The card 23 contains electronic components such as a memory and a combination of decoding logic in the decoder 21 and card 23 operate in such a manner as to properly decode the video signal received and to produce an unscrambled signal for the television receiver 22.

Such an arrangement allows for the transmission of suitable television type signals in a secure manner. This has many uses including security uses but the primary use is for a pay-television channel.

Thus there may be transmitted programmes such as movies, which may only be viewed by viewers who have subscribed to the pay-television operator and the provision of both the encoder 21 and card 23 enables the television programme operator to control usage of the system by providing suitable cards 23 and encoders 21 to subscribers only. The card 23, which may be returned to the television operator from each television user through the post enables the operator to simply change the coding arrangement and also, if necessary, to subsequently calculate which programmes a particular viewer has been watching and therefore the charge to be rendered to that viewer.

Both the card 23 and decoder 21 include electronic components which operate together to properly decode the signal. The card 23 which is shown in Figures 3 and 4 is of approximate dimensions of a conventional credit card but includes electronic components within the plastic. The electronic components incorporate memory as well as logic elements.

In order to further reduce the possibility of fraudulent unscrambling of the incoming signals the transmitted signal incorporates a further element of information (a "key number") which causes the decoder 21 and card 23 to decode the incoming signal in a particular way.

The apparatus thus far described can be used to operate a pay-television system having the following functions. The subscriber to the pay-television service is given a decoder 21 for insertion in the line between his aerial and television receiver.22. This decoder 21 is substantially permanently installed although of course can be removed if the subscriber ceases to pay for the service. At regular intervals the subscriber is provided with a new card 23 and has to return- the used card 23 back to the television system operator. The decoder 21 will only operate with the card 23 inserted.

Thus during normal use, the signal received by the decoder 21 contains not only the video information, some of which is scrambled, but a key number which enables the card and decoder 21 to operate to unscramble the television signal, information regarding the programme being transmitted (each programme may carry an identification number) the information regarding the time and date and if different types of service are to be provided, information regarding the type of service provided. If the subsscriber wishes to watch a particular television programme, the card 23 is inserted in the decoder 21 and the system set in operation. As well as decoding the scrambled video signal, the information regarding the number of the programme being watched, the date and time, is passed from the decoder 21 into an electronic memory in the card 23.

At the end of a prescribed period, for example a month or three months, or when the card 23 is full, the card is returned to the television system operator and then the information regarding the number of the particular programmes watched and the date and time can be extracted from the memory in the card 23. This enables the subscriber to be billed for the programmes which have been viewed.

There are other features which are available. For example many subscribers will wish to preview a particular television programme before it is decided whether to subscribe to that programme -and the decoder 21 may include facilities for allowing an initial one or two minute view of a programme before charging commences. There may thereby be provided a preview button which is pressed and of course the system may be arranged so that the preview button may only be pressed once during the transmission of a particular television programme, a feature which is facilitated by the transmission with the video signal of a number corresponding to that particular programme .

Furthermore there may be provided either at different times or simultaneously more than one type of programme. In some cases an individual programme 12

will be paid for (for example a recently released or even pre-released movie) but other programmes may be viewable by a subscriber without further payment being covered by a basic subscription charge.

One feature of the system is that all of the cards 23 may be identical in which case when the card 23 is first inserted into the decoder 21, the decoder 21, which includes a particular number identifying the subscriber, causes that number to be impressed on the memory in the card 23. This enables the television operator's computer to identify from whom the card 23 has been returned and also means that the card 23 cannot be inserted into another decoder. Thus fraudulent use of the cards 23 in more than one decoder 21 can be eliminated.

Such an arrangement of pay-television service has considerable advantages over those which are presently used and proposed which generally rely simply on cable links and a real-time two way communication between the television system operator and the subscriber.

Various ways of distributing the cards 23 and paying for the service may be used. In the above system the television subscriber pays retrospectively for programmes viewed. An alternative arrangement allows for the card 23 to be prepaid and may include in its memory a certain amount of credit which will allow viewing until the credit within the memory of the card 23 has been used up. The system may also be arranged so that the cards 23 will be valid for a particular period of time, say one month or three months. An advantage of this is that if the cards 23 include a certain amount of logic which is used in unscrambling the video signal, then the unscrambling system can be changed at regular intervals which improves the security of the system.

ENCODER, DECODER AND CARD

Figure 2 illustrates the encoder 21. It comprises an electronic unit 24, the contents of which will be described in more detail later together with a cathode ray tube 26 which displays the exact video waveform being transmitted, a first monitor 27 which displays the unscrambled picture and a second monitor

28 which displays the scrambled picture produced.

The decoder 21, card 23 and television receiver 22 of a typical installation are illustrated in Figure 3. The decoder and card are illustrated in greater detail in Figure 4 from which it will be seen that the decoder 21 incorporates a slot 29 into which the card 23 may be inserted, the card incorporating in one edge thereof electrical contacts (not shown) which cooperate with electrical contacts in the slot

29 so as to interconnect the card 23 and decoder 21. The decoder further incorporates a channel selection switch 31 which enables the viewer to select one of a variety of pay-television channels, a switch 32 for selecting between 3 types of received signal, that is a free broadcast channel 33 a subscription channel 34 and a "pay-per-vie channel 35". There is also provided a power switch 36.

WAVEFORM OF VIDEO SIGNAL

We shall now describe the video signal which is transmitted and received and the manner in which this is scrambled. As is well known, a television picture comprises a series of horizontal lines and the colour and brightness of a particular point along each line is determined by the "chrominance" and "luminance" values respectively at each particular point along each line. So far as signal values are concerned we will generally only be referring to the composite chrominance/luminance signal.

The scrambling of the video signal in the present apparatus comprises inverting the signal value of groups of lines through the picture. Thus, for example, the first few lines of the picture from the top will not be affected, the next few lines will have inverted signal values, and the next group of lines normal signals values, and the next group of lines inverted signal values. We prefer to invert the signal values for groups of lines up to about 4 and to have normal signals for groups of lines also up to about 4. The effect, if used through a television receiver without the benefit of decoding by decoder 21 is a picture with bands of meaningless scrambled information which renders the picture unviewable .

The number of lines in each band is different down the height of the picture and as an additional complication the number of lines for each group is freuqently varied. Thus it is not possible to unscramble the picture electronically without knowing the prearranged coding arrangement because by the time a particular scrambling pattern is unscrambled the scrambling pattern will have changed. The scrambling pattern in the preferred arrangement ^' is changed every six fields (ie 3 pictures). Furthermore, for succesive fields the scrambling pattern is inverted. Thus if in field 1 the lines 1 to 4 are normal, 5 to 7 carry inverted signal 1, 8 to 10 normal, etc, then in field 2 lines 1 to 4 will carry an inverted signal, lines 5 to 7 will be normal, lines 8 to 10 will carry an inverted signal and so on.

The scrambling pattern (the number of lines in each successive group of lines) is derived from a key number which is transmitted with the scrambled video signal received from the transmitter. The key number is extracted by the decoder 21 and used as a starter number in an algorithm, part of which algorithm is provided by the decoder 21 and part of which is held in a memory in the card 23 to produce a series of numbers which correspond to the combination of numbers of lines in the groups of lines down the screen. In use the part of the algorithm in the card 23 is transferred to the decoder 21 and the processing is carried out in the decoder 21. Thus, for example, the key number may be 2, and this is fed to the algorithm in the decoder 21 and card 23 to produce a series of numbers for example 4, 3, 2, 1, 4, 2, 3, 4, 1,... . This gives the sequence of numbers of lines in each successive groups of lines, ie the first four lines of a particular frame include normal video signals, the next three lines include inverted video signals, the next two lines normal video signals, the next one line inverted signals, the next four lines normal video signals, the next two lines inverted video signals and so on to the bottom of the frame.

As mentioned above, the key number can be changed as frequently as desired, for example, every six fields which means that the series of numbers produced by the algorithm will change every six fields.

A variety of algorithms may be used but by way of example only the following may be considered.

ALGORITHM

D Deec<cooddeerr 2211 Card 23

1 plus 3

2 multiply by 2

3 minus 4 4 4 ddiivviiddee bbyy ^"- 2

5 (ignore decimals )

6 multiply by 7 last significant figure

8 is it greater than 4

9 if yes substract 4 and return to step 8

10 if no remainder is required number 11 insert number produced by step 6 into step 1

Thus assuming that the key number is 14 operating on that key number by the algorithm provides the following.

1. 14 + 3 = 17

2. 17 x 2 = 34

3. 34 - 4 = 30

4. 30 divided by 2 = 15

5. Ignore decimals.

6. 15 x 1 = 15

7. Take last significant figure = 5.

8. Is it greater than 4?

9. Yes, Deduct 4, 5 - 4 = 1.

8. Is it greater than 4?

10. No. Thus required number is 1. The first group of lines is one line only.

11. Use figure in step 6 above as a new figure to be inserted in the algorithm so that algorithm proceeds as follows.

1. 15 + 3 = 18

18 x 2 = 36

3. 36 - 4 = 32

4. 36 divided by 2 = 16

5. Ignore decimals.

6. 16 x 1 = 16

7. Take last significant figure = 6.

8. Is it greater than 4?

9. Yes, so deduct 4, 6 - 4 = 2

8. Is it greater than 4?

9. No, thus required number is 2.

Therefore the second group of lines is two lines

The above algorithm is very simple and in practice a much more complicated algorithm can be used. Furthermore, the mathematics would be in binary form rather than decimal. The important feature however is that the scrambling pattern can only be calculated by knowing the key number from the signal, and both parts of the algorithm from the electronic components

(or software) in the card and in the encoder.

To aid security the key number is changed every few fields and the part of the algorithm in the card can be changed every month or so with the card.

We shall now describe the video waveform which incorporates the key number and line inversion.

Figure 5 shows the signal waveform during the field blanking interval of a normal video signal. The first part of the field blanking interval signal incorporates pre-equalising pulses 41 followed by the field sync pulse 42 followed again by post equalising pulses 43. The relative periods of the pulses are illustrated on the drawing.

It will be known that so far as the television picture is conce'rned the 0 volt level is black and the 0.7 volt level is white and the grey levels are between 0 volt and 0.7 volt.

Figure 6 comprises an enlargment of the part of Figure 5 to the right of the line A when modified in accordance with the preferred aspect of the invention. To the left of Figure 6 there is shown the end of the field sync pulse 42. The first five post equalising pulses 43 remain as before, but thereafter the post equalising pulses 43 are changed as shown. The next signal comprises a timing reference 4,6. This timing reference signal 4,6 is a full line of inverse bla-ck (ie 0.7 volt).

The timing reference pulse 46 is followed by five amplitude calibration pulses which comprise four successive voltage levels, the first 48 being at 0.7 volt, the second 49 being at 0.49 volt, the third 50 being at 0.21 volt, and the fourth 51 being at 0 volt. The next pulse 52 comprises a data pulse in which 24 bits of data are transmitted in the form of sub pulses 53 of differing width, a narrow sub pulse 52 indicating a binary "0" and a wider sub pulse 53 indicating a binary "1".

The timing reference pulse 46 is provided because, as is well known, each picture on a television screen comprises two interlaced fields, and the signal for the second field is half a line out of phase with the signals of the first field. An accurate timing reference is therefore required which is provided by the pulse 46.

Figure 7 shows one of the amplitude calibration pulses 47 in greater detail. In those lines in which the signal is not to be inverted, it will be understood that as mentioned above the video signal is transmitted in the normal way with the dark areas of the line having a voltage level close to or at the black level, 0 volt and the lighter or whiter areas of the line having voltage levels close to or at 0.7 volt. In respect of those lines in which the signal is inverted, however, in principle we aim to turn that part of the line which should be black into white and that part which is white into black and blue into yellow and vice versa. The way in which this is done is to invert the voltage (ie turn a positive voltage value into the corresponding negative value) and then add a fixed value of 0.7 volt.

Thus let us now consider the effect this has on five voltage levels.

Firstly, let us consider what would normally be a white level of 0.7 volt. The signal is inverted becoming -0.7 volt and to this is added θ^'.7 volt thereby leaving a signal level of 0 volt that is black level. Thus "inverted white" level equals black level.

Consider black level 0 volt. This is inverted and so remains 0 volt and to this is added 0.7 volt leaving a signal level of 0.7 volt, that is white level. Thus "inverted black" level equals white level.

Consider the half way case, that is 0.35 volt, a mid grey. By inversion this becomes -0.35 volt and by adding 0.7 volt becomes 0.35 volt. In other words the exact mid grey remains the same. However a value slightly above the mid range, say 0. 5 volt that is a slightly brighter than mid grey, when inverted becomes -0.5 volt, and adding 0.7 volt leaves 0.2 volt that is a value somewhat darker than mid grey.

Similarly a value of mid grey towards the black level, say 0.2 volt becomes 0.5 volt and so becomes closer to white than to black.

One of the advantages of this.part icular method of line inversion, (rather than simply inverting the signal about a mid point of 0.35 volt) is that it is possible to compensate in the decoder for errors in voltage level which have taken place during transmission. Thus the calibration pulses 47 allow one to vary the basic 0.7 volt added to the inverted signal so as to compensate for these voltage errors. This is done by making sure that the inverted level 48 is 0 volt. Thus if the level 48 when inverted produces a voltage level of, 0.01 for example then the voltage to be added to the inverted signals during line inversion will be 0.69 rather than 0.7 volt. This means that when the inverted black level is inverted it becomes 0 volt exactly. Having calculated this correct voltage to be added it is then possible using the levels 49, 50 and 51 to provide a ratio factor which will compensate for other transmission errors so that not only will inverted black be inverted to the correct 0 volt value but the other voltage signals when inverted will be inverted to the correct values. It is well known that it is normally necessary for the signal level to remain^' at black during the field blanking interval because this is the period during which the beam on the cathode ray screen flies back from the bottom corner to the top corner and if the level was other than black then it would be visible as it passed across the screen. In practice modern television sets should switch off during this' interval but some older television sets do not do so and in any case even some modern television sets do not suppress the signal as much as they might.

However in the present instance we can use , this particular interval to transmit various signals because the signal which reaches the television receiver is not -that which has been transmitted, the decoder at the television receiver stripping off this signal information. We are thus free to use this part of the video signal for our own purposes.

We now refer in more detail to the data pulse 53. As mentioned above this data pulse includes the key number and is for example 8 bits long in which case there could be 256 possible key numbers. The key number is changed for example every six fields (other multiples of two fields may be used) so that it is not possible to unscramble the picture without the proper apparatus.

The data pulse 52 incorporates more than the key number. Other numbers can be used for other purposes. For example the pulse should include a number which relates to the particular programme being broadcast. As described above this information is necessary for accounting purposes and is stored in the memory of the card 23. Thus if the card is inserted in the decoder 21 and the programme is being watched the programme serial number will be inserted into the memory of the card 23 and when the card 23 is at a later date read by the operator's computer then there is an indication of which programmes were watched.

This number may also be used to allow previewing of a particular programme as has already been described.

The data pulse 52 may also include information regarding whether the particular programme is a subscription programme or is a pay-per-view programme.

The data pulse 52 may also include details of the price of the particular programme so that the user may see how much that particular programme is. In addition the data pulse may include the date and time and other information as required.

ENCODER BLOCK DIAGRAM (FIGURES 8,9,10,11)

Figure 8 shows a block diagram of ^'the encoder. It is clearly necessary in this system for there to be very accurate timing signals. For example, referring back to Figure 6, the ti ing pulse 46 must be very accurately timed as must the amplitude calibration pulse 47 and the data pulse 53. This requires therefore a rather more accurate timing base than that normally provided in a television syste . Clearly the timing reference must be based on the timing pulses (ie the line sync and field sync pulses) derived from the television camera or other video signal source in use.

Referring therefore to Figure 8 it will be seen that the video signal from the camera 11 (or other source of video signal) is passed through a filter 61 which removes the chrominance signal from the video signal. The output signal from the filter 61 is then being passed to a sync extractor 62. The signal from the sync ^'extractor is a digital signal corresponding to the sync signal received by the sync extractor 62 and is passed to two circuits, the first of which produces a timed output in respect of the line sync and the second of which produces a timed output in respect of the field sync.

With regard to the first of these functions, the output from the sync extractor 62 is applied to a IC 63 which removes those sync pulses which are at twice the line rate and the remaining pulses are passed to phase lock circuit 64. This produces an output which controls an oscillator 65 so that the oscillator 65, which is a nominal 4 mHz produces an exactly accurate oscillation rate depending upon the rate of signal received from the phase lock circuit 64. The output of the oscillator 65 is passed to several counters, one of which is illustrated at 66 which thereby produces a count up to 256 for each interval between successive line pulses. The counter 66 therefore divides the period between the beginnings of successive line pulses into 256 equally spaceds intervals and it is thus possible by reading the output of counter 66 to accurately determine any one of the 256 equally spaced points through the line interval from the beginning of the line. The line rate output of the counter 66 is fed to a microprocessor 67.

It will be understood of course that the count rate from output line 8 of counter 73 is in fact at line rate.

Similarly the signal from the sync extractor 62 is passed to a field sync detector 68 which detects the field period and outputs to the microprocessor 67 so that the microprocessors can establish time intervals within the field interval the timing output^'s being given on lines 70 in Figure 8.

The microprocessor 67, by looking at the particular values passed to it by the signals from counter 66 and field sync detector 68 is able to determine at any time the number of the line and the field.

The. detailed circuit generally corresponding to Figure 8 is shown in Figure 9. It is not believed necessary to describe in detail all of the components of the circuit as all the components are drawn in standard notation. The operation of all the individual components will not be described but we will describe some of the more important features of the detailed circuit diagram.

The particular filter for filtering chrominance in the filter 61 is shown at 71, and the IC for removing twice line rate signals is shown at 72.

Referring to the field sync detector 68 in more detail shown in Figure 9 an edge trigger bistable 81 is triggered 48 microseconds after the start of each line sync signal. Now if the voltage at that particular time is low then there must be a field sync signal present but if the voltate is high then there is not a field sync signal present. Thus the bistable 81 detects whether there is a field sync signal present or not.

A bistable 82 of and a counter 83 produce a time window which removes transient signals and spurious field pulses.

The microprocessor 67 can therefore define within each field any particular line by counting the field pulse input from the counter 66.

Referring now to Figure 10 this shows the output end of the encoder in block diagram form. There is illustrated a switch 91 which is timed to operate by timing input on line 92 controlled by the microprocessor 67 and associated hardware. A switch 91 has as inputs the video signal on line 93 and a preset black level voltage signal on line 94. There is also provided an input line 95 from a combiner 96, the combiner acting to combine an input signal on line 97 and 98, line 97 being connected to a voltage signal from the white calibration source ie level 49 in Figures 6 and 7 and data on line 98 from the microprocessor 67. Thus the combined data and white calibration voltage is applied; to the input line 95 of the switch 91. The switch 91 is timed by the input on line 92 so that during the transmission of picture information the video input 93 is connected to the output 100, and during the field blanking interval either the black level voltage signal from line 94 or the white level voltage signal and data from line 95 is passed to the output 100.

Switch 104 not only inverts complete lines but also part if the calibration waveform to give inverted white and inverted black. The output signal on line 100 is passed to a differential amplifier 101 which has two outputs 102, 103 which produce complementary signals. In other words, line 102 carries one signal and line 103 carries an inverted signal.

The amplifier 101 also has an input from amplifier 110 which provides the basic 0.7 volt level from a controlled voltage input 111 and from the output, to be described.

The two outputs 102, 103 are applied to a switch 104 which is again controlled by a timing output from the microprocessor 67 via line 105. The timing along line 105 controls (a) whether the picture information for a particular line is to be the normal signal from line 102 or the inverted signal from line 103 and (b) whether the black level, or inverse black level, or white level, or inverse white level signals are to be passed to line 106.

Thus switches 104 and 91 effectively produce the amplitude calibration pulse 47 shown in Figure 7. Referring to Figures 7 and 10 at point 131 of Figure 7 the switch 91 is switched so as to connect the black level signal from line 94 to line 100 and the switch 104 is switched to pass the inverted signal from line 103 along to line 106. Thus an inverted black signal value 48 is produced. At a point of time 131A in Figure 7 timing pulses along lines. 92 and 105 switch the switch 91 and switch 104 so that switch 91 passes the white level signal from 99 to line 100 and switch 104 passes the normal signal from line 102 to 106 so that a white level signal 49 is produced.

At time point 131B of Figure 7 a timing pulse is passed along line 105 to switch the switch 104 and thereby cause switch 104 to pass the inverted white signal from line 103 to line 106 and thereby produce signal level 50.

At time 131C of Figure 7 a timing pulse is passed to switch 91 and 104 so as to cause switch 91 to pass the black level signal from line 94 to line 100 and the switch 104 to pass normal level signal from line 102 to line 106 thereby producing the normal black level 51.

The output from line 106 is passed to an output amplifier 107 and this then provides the video output which will be modulated for transmission.

Figure 11 shows a circuit diagram for the arrangement shown in the block diagram of Figure 10. Points ^"to note are that amplifier 101 sets the voltage level about which it inverts the signal received by looking at the calibrating signal output from amplifier 110. The actual 0.7 volt level is set by the amplifier 110 and is varied by means of the variable potentiometer 114. It is also apparent from Figure 11 as to how the black level voltage signals produced at 90 are accurately aligned with the relevant voltage level of the video input from line 93. A feedback clamp 115 is arranged to compare the black level voltage signal produced by component 90 and fed to point 116 with the porch level of the incoming video signal, the comparison being carried out by the differential amplifier 117 which operates to make the two levels the same. As the black level voltage signals from 90 are set in practice the amplifier 117 moves the voltage level of the video input so as to match the black level voltage signal produced at 90.

It should also be noted that the outputs for the monitors 27 and 28 are taken from the video input in the top left hand corner of Figure 11 and the output in the bottom right hand corner of Figure 11.

DECODER FIGURES 12,13,14,15

Clearly many of the requirements in the decoder are similar to that in the encoder. Figure 12 is a block diagram of part of the decoder.

Referring to the upper part of Figure 12 which provides the accurate vertical and horizontal sync signals (field and line sync signals) and extracts the data, the video signal which has been removed from the rf signal is applied to the video input 120 and passed from there to a filter 121 which corresponds generally to filter 61 of Figure 8. Filter 121 filters out the chrominance part of the signal. It is then passed to .a sync extractor 122 which corresponds generally to. sync extractor 62 of Figure 8 where accurate square pulses are produced in synchronism with the line sync pulses received or with the field sync pulses, the pulses produced being accurately timed both with respect to repetition rate and to width. Outputs 123 and 124 from the sync extractor 122 provide vertical sync and field sync signals respectively. A further output from the chrominance filter 121 is passed to the data extractor 125 where the data is stripped from the signal and the data is passed to the data output 126.

In the lower part of Figure 12 the video input 120 is also connected to an amplifier 130 which provides a normal video and inverted video output on lines 131, 132 respectively. The signal on line 131 is passed through a buffer 133 and the signal on line 132 is passed through a variable gain buffer 134 and the two signals are then passed to a switch 136 which switches between lines 131 and 132 to provide an output to output amplifier 137 and thence to provide the video output 138. There is also provided a comparator 139 and a further comparator 141. The comparator 139 is provided to compare from lines 131, 132 after the buffers 133, 134, the inverted inverse black with the normal porch value (indicated at 131 in Figure 7) and by providing an output on line 142 to the input of amplifier 130 varies the amplifier 130 so as to make them equal. The comparator 141 again compares the outputs on lines 131 and 132 after the buffers 133, 134 and'it is_. arranged to compare the normal white signal level with the inverted inverse white signal level and makes them equal by a control signal along line 143 to the variable gain buffer 134.

The comparators 139,141 are controlled to carry out the comparisons at times dictated by signals along lines 144, 146 from the microprocessor 67 which is necessary because the comparators will only be receiving their relevant signals on the lines 131, 132 at particular times during the field blanking interval .

The comparator 139 as a result of the comparison will change the DC level of the video signal so as to make the voltage of the porch level and the voltage level inverted black of the inverted signals the same. As mentioned above with respect to Figures 6 and 7, this means that the base voltage level of the video signal can be corrected to compensate for any errors during transmission. The comparator 139 thus produces a corrected video output.

Similarly the comparator 141 compares the normal white signal with the inverted white on an inverted signal and as a result of that comparison changes the gain of the amplifier 134 so as to correct the linearity.

The switch 136 of course is controlled by timing pulses on line 147 from microprocessor 67 to switch the video signal between normal and inverted video in accordance with whether the line is to be inverted or not. The timing signal on line 147 is derived from calculating the algorithm.

The circuit diagram of the circuit corresponding to Figure 12 is shown in Figure 13 with similar parts numbered the same.

As has already been made clear the data which is extracted at data output 126 contains the key number and is fed to the microprocessor 150 in the encoder which, in combination ith the memory in the card, produces the necessary information regarding which lines are inverted, and this is produced as a timing signal on an output line 152 to switch the switch 149 to select either normal video or inverted video as appropriate.

We refer now to Figure 14 which shows another part of the decoder. The output 124 of Figure 12 is used as an input to the part of the circuit shown in Figure 14, Figure 14 including similar parts to that of part of Figure 8, that is the signal is passed to a circuit 165 (corresponding to circuit 63) which removes the signals of twice the line rate frequency, to a phase lock circuit 166 which corresponds to phase lock circuit 64 of Figure 8, and which is used to control the output of a 4 mHz oscillator 167 corresponding to the oscillator 65 of Figure 8. This output is passed to an 8-bit counter 168 corresponding to the counter 66, and outputs therefrom are passed to the microprocessor 67 the circuit diagram for this part of the apparatus is shown in Figure 15. The inputs for the microprocessor are shown at 169.

In the above description most of the electronic components in respect of the decoding system are provided in the decoder 21 itself with the card 23 merely holding memory. However, it is possible to arrange for more active electronic elements to be provided in the card 23 if necessary.

The above description and drawings illustrate the hardware of the apparatus in some detai. However much of the operation of the apparatus is controlled within the microprocessor 67 and associated memory ^•element in the card by means of a software programme.

The advantage of such an arrangement is that the operation of the system can within wide limits be varied to suit a particular operator by means of changes to the software rather than to the hardware.

Furthermore, a number of the hardware items are connected directly to the microprocessor, for example, the switches 31 and 32 on the decoder provide inputs to the icroprocessor which accordingly provides suitable entries into the memory in the card.

It has already been described of course that the algorithm is calculated by means of software in the microprocessor and the memory in the card and indeed part of the algorithm is in the card itself.

Thus the data relating to the programme, for example, the identification number is stripped from the signal in the decoder and is passed to the microprocessor and thereby to the memory of the card in the relevant manner as determined by the television station.

Details of the subscriber and the date and time are also inserted into the electronic memory of the car in a mariner controlled by the software. The use of a preview button which allows for previewing a programme will also be controlled by the software so that the time may be varied as_ required by the television station.

The arrangement is such that, to prevent unauthorised decoding of the scrambled signal, there is insufficient information in the signal during the field blanking interval to determine whether the successive line will comprise a normal signal or an inverted signal and indeed it is not possible to tell from the signal itself whether that signal is inverted or not.

In a modification of the above apparatus, to prevent continued use of the decoder by a non-permitted operator, further information is impressed upon the video signal. As has already been described each decoder has a particular identification number and the encoder is arranged so as, along with the other information already described, to transmit numbers of decoders which are not permitted to operate. It will conventially be possible to send one or two such numbers during each field blanking interval and so it will take a little time to pass through the complete list of barred identification numbers of the decoders to be switched off.

Furthermore, the decoder includes means to extract this information regarding the barred identification numbers and if the barred identification number equates with the identification number of the particular decoder then that decoder will switch itself off and will not be operable thereafter.

Although it may take a little time for the identification number of a particular decoder to be received owing to the number of identification numbers which must be transmitted, it is not likely that this period of time will be longer than that of a typical programme, say half an hour and so at some stage during the use of the barred decoder, it will switch off.

Claims

1. A television system comprising means for producing a scrambled video signal including, in encoded form, a key number, an electronic decoder apparatus, and an electronic circuit part selectively connectable to the electronic decoder apparatus, the electronic decoder apparatus and the electronic circuit part each including electronic logic means for carrying out a series of operations on said key number to produce a desired number from said key number, the decoder including means to use the desired number to decode the scrambled video signal to produce an unscrambled video signal.

2. A television system as claimed in claim 1 in which the .electronic circuit part comprises a plastic card incorporating electronic circuits therein.

3. A television system as claimed in claim 2 in which the plastic card includes, in one edge, connector means for connecting two cooperating connector means on the electronic decoder apparatus, whereby the card may be selectively connectable to and disconnectable from the electronic decoder apparatus.

4. A television system as claimed in claim 1 in which the electronic logic means for carrying out a series of operations in said electronic circuit part and said electronic decoder apparatus comprises electronic parts containing a programme, the programme comprising an algorithm whereby the application o^'f the key number to the algorithm will produce said desired number.

5. A television system as claimed in claim 4 in which operators (that is multiply, divide, plus or minus) are contained in one of the electronic decoder apparatus or electronic circuit part and the number on which the operator operates is stored in the other of the electronic decoder apparatus or electronic circuit part.

6. A television system as claimed in claim 1 in which a plurality of desired numbers are produced in sequence from said key number.

7. A television system as claimed in claim 1 in which the key number is in the form of a binary number.

8. A television system as claimed in claim 1 in which the key number is in the.form of a data pulse carrying sub-pulses of various time periods.

9. A television system as claimed in claim 1 in which the means for producing the scrambled video signal includes means to change the video signal and key number in synchronism at regular^• intervals.

10. A television system as claimed in claim 1 in which the .key number comprises a data pulse, the actual number being encoded in the form of a binary code, the components of the binary code being determined by the width of the sub-pulses within the data pulse. 40

11. A television system as claimed in claim 12 in which the electronic circuit part incorporates a memory and the data pulse also incorporates information regarding a particular programme being transmitted so that the electronic circuit part can store within its memory information regarding the particular programmes which have been decoded by the decoding apparatus.

12. A television system as claimed in claim 11 in which the scrambled video signal producing means includes means for inverting the signal values with respect to some of the lines making up the television picture, the inversion being carried out in respect of groups of lines at spaced intervals down the picture, said desired number produced by the decoding apparatus relating to the numbers of the lines which are inverted.

13. A television system as claimed in claim 1 in-which there are provided a plurality of electronic decoder apparatus, a television receiver or group of television receivers being associated with each decoder, and a respective electronic circuit part being provided for each electronic decoder apparatus, each electronic circuit part being identical.

14. A television system as claimed in claim 13 in which each electronic decoder apparatus includes means containing identifying information identifying the particular electronic decoder apparatus, and when said electronic circuit part is first connected to said electronic decoder apparatus, ^"- the identifying information is passed to the electronic circuit part and is stored in memory therein.

15. A television system as claimed in claim 13 in which computer means is provided, the computer means including means whereby one or more of the electronic circuit parts may be connected to the computer means, and including means to interrogate the electronic circuit part whereby to determine which programmes have been decoded and from this information provide an indication of the charge due.

16. A television system as claimed in claim 15 in which means may be provided on each electronic decoder apparatus to decode the scrambled video signal for a predeter i ed period of time without entering information onto the electronic circuit part usable by the computer to produce a charge.

17. A television system in which the video information is, scrambled or coded, the video signal including data to provide a key number, decoding apparatus comprising in combination an electronic decoder apparatus and an electronic circuit part selectively connectable to the electronic decoder apparatus, both the decoding apparatus and the electronic decoder apparatus incorporating electronic logic elements for carrying out a series of operations on said key number in accordance with a programme to produce a desired number from said key number, said desired number enabling the decoding apparatus to decode- or unscramble the video signal to provide a video signal which may be viewed by a standard television receiver.

18. A method of scrambling a television signal comprising inverting the signals in respect of alternate groups of lines, the groups of lines each co prising a predeter ined number of lines, the television signal containing information relating to the number of lines in each group and, in respect of inverted lines, the composite luminance/chrominance signal is changed into a signal of opposite polarity of corresponding value and a standard voltage is added to that reversed signal.

19. A method as claimed in claim 18 in which, after this signal processing, the value of inverted black signal on the inverted lines is compared with the porch signal level and said standard voltage value added is varied so as to determine that the inverted standard voltage value and porch level are identical.

20. A security system comprising means for producing an electric signal including, in encoded form, a key number, an electronic decoder apparatus, and an electronic circuit part selectively connectable to the electronic decoder apparatus, the electronic decoder apparatus and the electronic circuit part each including means for carrying out a series of operations on said key number to produce a desired number from said key number.

21. Apparatus as claimed in claim 20 in which the electronic circuit part comprises a plastic card including electronic logic components.