CA1276288C - Restricted access television transmission system - Google Patents

Abstract

RESTRICTED ACCESS TELEVISION TRANSMISSION SYSTEM
Abstract of the Disclosure A Television Signal Transmission Security System which provides a higher degree of security then previously possible by utilizing encoding and decoding technology which effectively prevents pirating by amateurs or experimenters. The television signal transmitted is encoded by injecting one or more interfering carriers in specific frequency slots in close proximity to the video carrier. Interfering carriers can be injected either above or below the video carrier between harmonics of the horizontal frequencies. Combinations of interfering carriers provide coding techniques making the system virtually pirate proof. The interfering carrier is removed by very narrow band rejection filters having a rejection band attenuation greater than about 40dB over bandwidth of up to 4000Hz and a 3dB bandwidth of approximately 5kHz. The filters are constructed using advanced technology to produce the quartz crystal bulk resonators or surface acoustic wave filters. Pre-compensation at the transmitter improves overall amplitude and phase response to provide a nearly perfect decoded picture.

Description

`1` 1276288 I¦l RESTRICTED ACCESS TELEVISION TRANSMISSION SYSTEM

2 1; SPECIFICATION
I¦ Field of the Invention 3j _ _ 4 ¦I The invention relates to television transmission security 51 systems and more particularly relates to a television encoding 6l and decoding apparatus and system which substantially prevents 7 unauthorized access to television signal transmissions.
81 Backqround of the Invention 9¦ Since the advent of Cable Television a variety of methods have been proposed and imp~emented to secure Pay Television 111 against unauthorized access.

12,¦ The present practical security systems offer both active I1 and passive systems which are designed to prevent non-'1 subscribers from watching a Pay T.V. Premium Channel. In a ¦1 passive system the signal is filtered out by a trap at each non subsciber's location; in an active system the signal is mutilated or coded such that only a subscriber with a decoder supplied by the Pay T.V. company can restore the signal such that it can be viewed.
The traps in a passive system are typically mounted at the ta~e-off point of the cable to the subscriber. Those traps are often illegally removed or tampered with by other means. Each non-subscriber needs a trap, resultlng in a high capital investment for those Premium programs which have a ~elatively low saturatlon. It is further physically impossible in practice l1 to remove or add traps just for a Pay-Per-View programs, for il which the subscriber has to pay a special charge. In an active ¦I system the signal is scrambled before it is distrlbuted. Two tyPes of active systems are in use. In one type the ¦j subscriber receives from the Pay T.V. company a decoder which 30~1 decodes the signal all the time. In the second type, either by lj addressable decoders or by other means the subscriber can only ~ lZ76288 ~ ~atch those specific programs for which he pays.
2 It has been proven very difficult to design a secure TV

3 coding system for a reasonable cost. As a result, in many 41 systems simple decoders can be bough~, made by entrepeneurs 5 I not re,lated to the Pay T.V. company, which decode the signal 6 ¦ such that the viewer does not have to pay the Pay TV Company.
7 ¦ In other systems which are more secure, the decoders are 81 extremely complex and therefore very expensive.
9l Another complication arises in the case of Pay-per-View 10¦ programs. In that case thé Pay TV Company has to install an 11 ¦ expensive decoder ln the home of each potential customer, 12 ¦ without any assurance that the customer will ever buy a 13 ¦ program. This'requlres a very large capital investment by the 14 I Pay-T.V. operator.
This invention relates specifically to active security 16~ systems and the invention offers a solution to provide secure 17 decoders at a low cost with the added advantage that Pay-per-18 ¦ Vlew programs can be easily accomodated with a minimum I 19 I investment, by the Pay TV company. ' In one particular actl~ve system the synchronization signals 21¦ are modlfied such that a regular television receivor cannot ~221 synchronize to the signal. The disruption of synchronizing 23 s1gnals is inadequate because it may be easily circumvented. A
24 ~ technlcian of moderate abillty can construct a decoder to ~2s~l stabi1ize the synchronization of the receiving television set.
¦ Systems whlch in~ect an interfering or scrambllng signal 27¦l between the video and audio carrier are also known. Such 28¦ systems are disclosed and described in U.S.'Patents 29 ' 3,896,262 and 4,074,311. The advantage , of the system of the latter patent is that it is very 3l 1 effective in interfering both with the video and audio carrier to prevent reception of a usable television signal at the user "~,.i.,..~

i276~88 11 or subscr~ber end. An additional advantage of this system is 2¦ the low cost of the decoder needed for removing the interfering 31 or scrambling signal. However his system suffers from the 4j disadvantage that the decoders, though inexpenslvely produced, 51 can be also produced by pirates with only moderate technical 6 ! knowledge and ability. The decoders are easily reproduced 7 1¦ since they use circuitry made up of standard components, which 8 il can be easily obtained, to remove the interfering signal. This ! restriction is the result of the necessity of placing the 10¦ scrambling signal a sufficient distance from the video and audio ! carriers to allow removal without removing a substantial amount 12i o, and degrading, the video signal. The reason for this is 13¦ that circuitry made up of standard discrete components can only 14 j! produce an inefficient notch filter which will remove too much 15 1 f the video information if the interfering carrier is too close 16,l to the video slgnal. The system of the latter patent is thus 17 ~! limited to using an interfering carrier near the center of the 18 1! band between the video and audio carriers.
191l Pirating of scrambled T.V. signals is a serious problem in 20ii the Pay T.V. industry. There is at present no econo~ical fully 21i secure system and the need remains for a secure, economical 22¦ Pay T.V. Transmission system to make Pay T.V. and particularly 231 "Pay-per-View" a desirable business proposition.
24 ' Noting that an economical, near fully secure 25!1 Pay T.V. system would be an outstanding advance in the 261j industry the present invention seeks to provide a low 27 1l cost television signal transmission security system 28il which may not be circumvented using standard, easily 29jl available components.
301~ Further the present invention seeks to provide 3Ijj a television signal transmission system secure against 32il pirating by even skilled artisans.

lZ76288 1 Still ~urther the present invention seeks to 2 provide a restricted access television si~nal 3 transmission system in which the technology for the 4 system decoders is valid even beyond VHF freguencies.
Yet further the present invention seeks to 6 provide a secure television transmission system 7 utilizing decoder technology which permits sophisticated 8 coding techniques to be used. The coding techniques g used involve multiple different interfering frequencies at different times or from program to program. Decoding 11 efficiency allows the use of coding techniques such as 12 using several interfering carriers, once thought 13 impractical.
14 The present invention also seeks to produce a system having a high degree of security in which picture 16 degradation, when removing an interfering carrier or 17 carriers, is minimized. High efficiency and narrow band 18 sophisticated decoders reduce the loss of video 19 information to an insignificant amount.
Still further, the present invention seeks to 21 minimize temperature and aging effects by using stable, 22 solid state technology. The critical elements of the 23 decoder are constructed of Quartz or possibly other very 24 stable materials such that no significant drift due to temperature and aging is anticipated.
26 Brief Description of the Invention 27 The invention in one broad aspect pertains to 28 a television signal transmission security system 29 comprising television signal generating means generating a television signal having a video carrier and an audio 31 carrier, encoder means for injecting at least one 32 interfering signal into the television signal, each 33 interfering signal being injected into the television 34 signal about midway between harmonics of the horizontal 12~76~88 sweep frequency of the video carrier where spectral 2 intensity of the harmonics is not substantial and in a 3 frequency band including the video carrier and on either 4 side or both sides of the video carrier. Transmission means is provided for transmitting the television signal 6 with the interfering signal to a plurality of television 7 receivers, and decoder means has notch filter means with 8 at least one notch at each television receiver receiving 9 and removing each interfering signal. Each notch of the notch filter means has a bandwidth at 3db down of less 11 than about the frequency bandwith between adjacent 12 harmonics of the horizontal sweep frequency wherein the 13 notch filter means does not remove a significant amount 14 of video sideband information, allowing the television lS signal to be viewed at the plurality of television 16 receivers without substantial degradation.
17 Another aspect of the invention pertains to a 18 method of securing television transmission comprising 19 generating a television signal having a video carrier and an audio carrier, injecting at least one interfering 21 signal into the television signal, each interfering 22 signal being injected into the television signal about 23 midway between harmonics of the horizontal sweep 24 frequency of said video carrier where spectral intensity of the harmonics is not substantial and in a frequency 26 band including the video carrier on either side or both 27 sides of the video carrier, transmitting the television 28 signal with each interfering signal to a plurality of 29 television receivers, removing each interfering signal at the plurality of television receivers with decoder 31 means having notch filter means with at least one notch.
32 Each notch of the notch filter means has a bandwidth at 33 3db down of less than about the frequency bandwidth 4a ~ zt76z8~
between adjacent harmonics of the horizontal sweep , frequency so that the notch filter means does not remove 3 a significant amount of video sideband information, r~ allowing the television signal to be viewed at the cj plurality of television receivers without substantial 6 degradation 7 More particularly, this invention relates to a 8 television signal transmission system which provides a g higher degree of security than previously thought possible by utilizing encoding and decoding technology 11 which effectively prevents pirating by amateurs or 12 experimenters and reduces the potential for pirating by even the most sophisticated technicians by the extremely stringent 14 requirements of the decoders.
The spectrum of the luminance part of a television signal 16 is characterized by a periodic structure of bands of frequencies 17 with a high spectral intensity interleaved with bands of 18 ~requencies with a low spectral intensity. The spectral 19 intensity is high in bands which are separated from the video carrier by a whole number cf multiples of the horizontal 21 scanning frequencies (15,734 Hertz in a color T.V. signal) 22 and low for bands which are in-between those high intensity 23 bands and which are centered around half multiples of this 24 horizontal frequency. This is true for signals up to about t MHz above the video carrier, 26 Color signals are modulated on a subcarrier such that their 27 ~pectral intensity is highest at half harmonlcs of the 28 horizontal frequencies, but their magnitude is typically quite 2~ low in a band o~ +/- 1 M~z above and below ~he ~i~eo'cdrL~e~.
Another characteristic of the video signal is that the 31 spectral intensity due to this luminance signal decreases 32 rapidly for spectral components far away from the carr~er. The f~

Il 1;~76:288 1¦ frequency bands ranging from 20 to 30 harmonics of the 2 horizont~l below the video carrier to 20 to 30 harmonics above 3, the video carrier contain the bulk of the energy contained in 4l the luminance signal. Disrupting any of those bands around the 5 ¦ harmonics will do severe harm to the demodulated signal.

6 Furthermore scrambling carriers injected into those bands w~ll 7¦ be very effective in disturbing the T~V. signal.
~¦ If a scrambling carrier is injected in close proximity to 9ll an harmonic of the horizontal, subsequent filtering in the 10l decoder to remove the scrambling signal may also remove some of Il the important spectral components due to the luminance signal 121~ and the resùlting picture will be irreparably damaged. The 13''1 scrambling carriers therefore should be placed in-between the 14 lil harmonics of the horizontal frequency and the filters in the IS!! decoder should be so narrow in the frequency domain or so 16'¦ highly selective that the spectral component due to the ~7¦¦ luminance signal around harmonics of the horizontal will not 18~¦ be materially disturbed.
lg The purpose of the present invention is therefore to encode 20 ¦ the signal by in~ecting one~or more interfering carriers in 21 I specific frequency slots in close proximity to the video carrier 22 1¦ into a television signal transmission while those carriers can 23l¦ only be removed by highly selective, high technology decoders.

24,¦ Those injected carriers therefore cannot be removed by the ,; usual standard component clrcuits as that disclosed and 26 1I described in United States Patent Number 4,074,311. More specifically carriers will be in~ected into a band of about 600 kHz, centered around the video carrier corresponding to a bandw~dth of ~hout 10% i~ relation to the bandwi~t~ of 6 ~u~
¦l of a TV channel.

31 ll Generally this system is applicable to Cable T.V. Systems ',¦ which very often utilize Pay channels, but the system may also !l ~27628fl ,1 1¦1 be used for over the air transmission.
2 ll The system described and disclosed herein is a low cost, 3 1! versatile and extremely reliable, almost fully secure television 411 transmission system that has significant advantages over 5,1 existing systems. The system uses a scrambling scheme similar 6 ¦ to the one proven in the above identified patents. In common 7 ¦ is, therefore, the addition of an interfering carrier to 81 obliterate both visual and audio signals and the removal of 9¦ that interfering carrier to a sufficient degree to restore 10j proper operation. However the system disclosed herein utilizes , interfering signals far outside the possible or practical 12'1 range of interfering signals possible with the system 13 l¦ disclosed in that,patent.
14 ll The problem with the previously disclosed systems is, while ~¦ effective in creating an interfering signal which can be removed 16 ! to a reasonable degree, that practical compromises were ~7 ll required, plaguing the system and preventing widespread 1~11 acceptance. Among these problems is "soft" security that must 9l~ be accepted because the system could be easily circumvented 20'i and so the system had to be~minimal in cost.

21i, The problems of the old system have been obviated by ¦ applying new Sol$d State, High Technology to produce decoders which achieve new cost and performance breakthroughs and al~ow ~ more versatile and nearly fully secure scramble method at the 24jj ,¦ head end. The operating principles are relatively simple ~ut 25Ij ! the subtleties of the system and the stringent requirements , necessary for successfully removing the interfering carrier , !l discourage tampering and piracy.
29,1 In the present invention the interferlng carrier is m~
i1 so close to the video carrier that it is very difficult to 31 ! remove that carrier with standard components wlthout removing 32 1 at the same time a substantial part of the video carrier.

!l I

]ll ~ithout the carrier, detection becomes very difficult. Even if 2,j enough carrier is maintained to perform detection, so many 3 i! critical modulated video components will be removed that the 411 resulting picture will be heavily distorted and will not 5 ¦¦ properly synchronize. The security afforded by the new 6 1¦ invention will thus effectively prevent tamperinq by amateurs 7!l and experimenters and discourages efforts by even 8 ¦ technologically able pirates.
91 An important but uni~ue feature of the invention is that 10¦ several interfering carriers can be used in combination which ~ will provide coding techniques which are virtually pirate 12 1I proof. For example, at least one or more interfering carriers 13 ¦I could be in~ected in the before mentioned 10~ frequency interval around the carrier while other interfering signals 5ll could be anywhere between the video and audio carrier beyond 16 1 the 10% range. The large number of possible locations for ~7 1¦ lnterfering carriers would allow coding techniques with a very l8'1 large number of combinations. If a single carrier would be ~ used, the before mentioned 600 kHz band will allow about 38 20 i different code~. If two ca~riers are used about 1400 codes are 1 available while three carrlers gives more than 50,000 22 1 combinations. If in addition to this, additional-carriers 3 1l would be used outside this 600 kHz band the number of Il available codes would become very large indeed.

25,l A different code or interfering technique could be used for each program, thus the system would lend itself to be used ' very effectively for "Pay-per-View" programs.

28l In the "Pay-per-View" area the encoder can be very quickly modified to produce particular scr~mbling signa1s. Decoders at ¦ the subscriber's receiver will have receptables or slots for receiving modules containing filters to remove the intexfering I signals coded for that particular program. The large number of ~1 1 `I 127628fl ~¦1 codes available and frequent changing from one code to another 211 will effectively prevent the subscriber from attempting to 3¦ capture the scram~led television si~nal transmission. Even if a ¦
41 potential pirate would have all modules available, it would be 51 very difficult to find out which particular set would be needed 61 for the specific program.
7 The filters necessary to decode the signal will be very 8l narrow band rejection filters. The relative bandwidth of those 91 filters, defined as the ratio of the 3 ds bandwidth to that of 101 the center freqùency, will range from about 0.0001 for the ll! lower T.V. channels to about 0.00002 for the high VHF

l2l channels. This will require selective devices with a Quality ~3 1I Factor Q of respectively 10,000 to 50,000. Those Q's are 14'l unobtainable with standard components which have a Q which 5ll seldom exceeds a few hundred. Only acoustic devices, made on 6 1I single crytals can be used and currently Quartz is the only 37 1! commercially'available material to have the necessary temperature and time stabillty. However any expert in the field can identify other solid state materials, as for ! instance Berlinlte, which c$ould be used in the future, 21 1i Initially the filters are made of Quartz and a variety of !¦ technologies can be used to fabricate the filters.- A sin~le ¦I pole filter can be made from a single Quartz crystal resonator ¦ and a few very simple components. A filter made of'several 24il resonators can be fabricated on a single crystal element by a Il technology which is known as monolithic filters.
j Due to the high frequencies involved the Quartz elements 27 !~ `
j are very thin ~of the order of a few thousands of an inch for ¦ the lower VHF Preguencies) while the other dimensions of the l crystal are a very small fra'ction of an inch. The amount of ¦¦ Quartz needed is therefore very small and the material cost is ¦l low but the required precision ma~es it next to impossible for a , i Il !

127628~
1I pirate to fabricate those parts without the necessary equ$pment 2 I and know-how.
3 ¦ For the higher VHF frequencies and for frequencies beyond 4 ¦ VHF Surface Acoustic wave ISAW) Filters may be more appropriate.
5 I SAW filters are fabricated by lithographic processses on 6 I relatively thick and sturdy substrates and are therefore more 71 suitable for higher frequencies.
81 Expressed in electrical terms those filters will have a 9¦ rejection band attenuation greater than 40 dB over a bandwidth of 500 to 4000 Hertz and a 3 dB bandwidth which is ideally S
~ Hz or less, but which can be as large as 20 kHz. The lower 12 ll values for bandwldth are for the low channels in the VHF band 13 ll while the high values pertain to the high VHF TV channnels.
14,1 The values mentioned will allow for reasonable fabrication 15~1 tolerances, temperature and aging variation, though they are 16 i known to be small. The temperature dependence of Quartz is ~7 !~ less than +/- 1 part per million for a temperature range of 15 18 1! C to 30 C corresponding to a variation of about 50 Hertz at 19ll T.V channel ~2. Ageing can be expected to be considerable 20~¦ le~s. Quartz crystals can ~e fabricated to an accuracy of 21 1 better than 100 ~ertz.
22~1 Even for the filters with a very small 3 dB bandwidth some 23 1I distortion of the bands with the high spectral intensity, 24'i centered at the harmonics of the horizontal frequency, will take 25il place. Even if the 3 dB bandwidth is 6 kHz, the 1 dB bandwidth 26;1 may be 15 kHz. Furthermore it is known that the phase distortion 27 ' is ~ust as important in video transmission and phase distortion i! can often be noticed before amplitude distortion is noticeable.

2~ To counteract the effect of amylitude and phase distortion ¦¦ an amplitude and phase compensation will be used in the ¦¦ transmitter. The use of such amplitude and phase compensation ¦¦ together with very stable notch filters will assure a near Il .

lZ76Z8~3 lI perfect decoded signal while providing "hard" security.
2I Quartz devices can be manufactured to very high tolerances.
3 ~y manufacturing the devices for a target frequency slightly 4l above the final desired frequency and by trimming the devices to i 51 their final value, very high yields can be reached. This 6I trimming is done with automated, computer controlled 7I equipment.
~¦ The very distinct advantage of this system is that it 9¦ offers to the television cable industry a low cost secure 10I television transmission system that will make "Pay-per-View" a 11¦ practlcal alternative.
12 The above and other novel features will be more fully 13 ¦ understood from the following detailed description and the l4I accompanying drawings, in which:
5 ! Brief Description of the Drawinqs 6 1¦ Fig. 1 is a graph of the frequency response of a television ,7~I transmission system Fig. 2 i8 an expanded view of the frequency spectrum around II the video carrier for a television transmission system.
il Fig. 3 is a block diag~am of a fully secured system Ii constructed according to the invention.
22!1 Fig. 4(a) is the desired frequency response for à decoder I filter of a typical system~ as shown with Fig. 1.

24i¦ Fig. 4(b) is the desired frequency response for a decoder I filter of a system with high intermodulation; as shown with Fig. 1.
25Ij 26~i Figs. 5(ad) illustrate several solid state elements which j exhibit very high selectivity 27 !' 28 1I Figs. 6(af) are schematic diagrams of typical filter !I configurations for decode~s according to the invent~on.

30I Flg. 7 is a schematic diagram of two fllter elements with 31I mutual resistor coupling for use in a decoder accodig to the 32I invention.
Il ' ' , 11 1~76Z8~3 Il Figs. 8(a) and (b) are graphs of typical filter responses 21 constructed according to the invention.
3j Figs. 9(a) and (b) are block diagrams of compensation 41 networks using negative feedback for use in the system of the 5 !1 invent,ion , 6I Figs. 10(a) and (b) are graphs illustrating total response 71 of filter and compensation,network according to the lnvention.
8j Figs. 11(a) and (b) are block diagrams of compensation 9¦ networks using double negative feedback for use in the system of 10l the invention~ aB shown with Figs. 9(a) and 9(b).
11' Figs. 12~a) and (b) are graphs illustrating total frequency 12,~ response using double compensation according to the invention.
I3l Detailed description of the invention l4~ The system of the present invention provides a more fully l5~l secure T.V. transmission system and is illustated ~raphically in 16'¦ Fig. 1 which represents the band of a standard T.V. channel of 6 ~7 1i MegaHertz (MHz). V~deo carrier 10 is 1.25 MHz from the lower 18 ii band edge lndicated as zero. The audio and color carrier l9 I indicated as 12 and 14 are respectively 4.5 MHz and 3.58 MHz 20, above the video carrier. "
21 li The T.V. scramble system disclosed and described in the 22 ll above mentioned Patent, produces scrambling by inserting an 23¦¦ interfering carrier 16 typically halfway between video 2~ and audio carriers 10 and 1'2 at 2.25 MHz above the video ~ carrier. A second option is an offset possibility indicated 26 i at 18 at approximately 1.5 MHz away from video carrier 10. Both of those frequency slots are related in a simple mathematically way to both video and audio carriers; therefore an interfering i carrier at tho~e fr~uensies will disrupt.b~th picture ~nd.
Il sound.
31 ¦I The approximate frequency response of a typical decoder used in the system of the above referenced patent is indicated Il , 12 !I' lZ7625fl l ¦ in dotted lines at 20 and 22. This decoder is produced using 2 ¦ coils and capacitors in a filter network to produce a band 3 ¦ rejection filter having a substantial bandwidth. The purpose of 4 ¦ the decoder is to reduce the interfering carrier as much as 5 ¦ possible, while at the same time minimally affecting the video 6 ¦ carrier.
7 ¦ As shown, the band re~ection of the filters produced with 8 ¦ discrete clrcuit components is relatively poor because the 3 dB
9 ¦ and 10 dB bandwidths are relatively wide. So the major 10¦ disadvantage of the system is that the interfering carrier must 11¦ be kept far from the video carrier to be able to successfully 12¦ remove the interfering carrier to restore the signal.

13¦ The result is that amateurs, experimenters and technicians 14¦ with a mlnimum kn~wledge can build filters with discrete 15¦ components that will remove the interfering carrier, allowing 16¦ easy pirating of the T.V. transmissions. Further, since the 17 bandwidth 20 and 22 re~ection filters are relatively wide, as 18 ¦ much as several MHz at the 3 dB point to a few hundred kHz at 19 ¦ the re~ectlon band~, serious degradations in the quality of 20 ¦ the picture can result. As a consequence subscribers to a Pay 21 ¦ T.V. Channel would get degraded picture quarity compared to 22 ¦ standard television transmissions.

23 ¦ The second problem mentioned above is that a relatively 24 ¦ crude filter could remove a sufficient portion of the 25 ¦ interfering carrier to give a pirate a watchable video and 26 ! usable audio. While the typical quality obtained by the pirate 27 was poor, he did not have to pay the usual charge. Because 28 there was typically only one code, the incentive to pirate was 29 even greate Offset carrier 18, being even closer to the video 31 carrier was plagued with even more video degradation because 32 decoders tend to take out a substantial portion of the carrier iZ76~8t3 1¦¦ in addition to a substantial amount of the low frequency video 2~ components, resulting in very poor pictures.
3 ,i A further disadvantage of the system is that filters 4 ¦¦ consisting of many discrete parts are expensive to manufacture.
5 ¦¦ Thus the previous system had many serious drawbacks and ¦ technological compromises had to be accepted in order for the 7 il system to be practical and to be accepted by the operator and 8 1 the subscriber.
91~ The present system was developed to eliminate most or all 101, of the shortcomings of prior art systems and will provide a ~ system having greater or "hard" security. "Hard" secur~ty is 121 defined as security which i5 difficult to defeat with 13i conventional means. The system of the invention will also 14i eliminate most of the degradatlon of the picture and sound 15'1 occuring in previous systems while at the same time making the 16 ¦ decoders, for use by subscrlbers, lower in cost.
17¦¦ This is achieved by using very sophisticated, solid state 181 technology in the construction of the decoders which will 19¦ achieve deep and very narrow notches in the band re~ection 20' filters. Those filters wil~ permit the placement of the 211 interfering carriers extremely close to the video carrier.
221 It is intended that the scrambling carriers be placed into 231 bands 24 and 26 which are approximately 300 kHz wide above and 24 ! below video carrier 10. Those bands extend from 27 to 28.
251 The distinction between the prior art and the present 26 invention is illustrated in Fig. 1. Prior art systems place an 27 1i interfering carrier in the band ranging from 29 through 30 which !¦ is the middle part of the band extending from video carrier 10 29!1 to sound carrier 12. The present invention, by u4il~2ing solid-state technology and compensating techniques, to be discussed in ¦ the following, plans to use a 600 kHz band extending from 27 to li .

ll lZ76Z8~3 1¦ This band is shown on an enlarged scale in Fig. 2. line ~a) 2 ¦ of Fig. 2 shows the spectrum around T.V video carrier 40 of a 31 typical T.V. scene. As shown, the spectrum intensity of a 4 typical scene increases and decreases periodically with periodicity 48 of 15,734Hz or "H" Hz where H stands for the 6 horizontal frequency. Typically the spectral intensity is high 7 for harmonic multiples of the horizontal frequency and low in 8 between those harmonics.
9 ~he first sideband harmonics of the horizontal frequency "H" Hz above and below video carrier 40 are indicated at 41 and 11 43 respectively. The upper and lower sideband's second 12 harmonics are respectively indicated at 42 and 44. The envelope 13 indicated at peaks is 45; typically the envelope decreases for 14 higher values of the harmonic.
In order to show detail only a few harmonics have been 16 ¦ shown between lines 49, a few higher harmonics have been shown 17 ¦ schematically. It should be pointed out that the spectrum will 18~ strongly depend on the partlcular scene and Fig. 2 ls only to 19 $ndlcate the general nature of the spectrum.
Line (b) of Fig. 2 shows the preferred locations of 21 scrambling signals, which ls between the harmonics where the 22 spectral intensity is typlcally 1QW. A specific location of 23 1.SH Hz above video carrier 40 i8 shown at 46 but any of thè
24 locations in proximlty to (n~O.S)*H Hz would be acceptable.
The integer n ranges from -20 to ~19 to correspond to a band 26 ¦ of approximately 600 kHz around the video carrier. If the 27 decoder 3 dB bandwidth of the filter is narrower than 15,734Hz 28 (H) not much video information will be lost, because the 23 spectral lnt~nsity ls low ~ere the filter is eff~ctive.
In some older cable T.V. distribution systems 31 lntermodulation is somewhat high, typically due to incorrect 32 channel levels or amplifiers with limited power handling ll 12762~fl ~1 capabilities. ~ypically second and third order distortion 21 products are the most important type of intermodulation 3 products present. If in a transmission system strong frequency 4 components with frequencies f ,f ,f ....f are present second order intermodulation will create components with frequencies 61 f + f and f - f .
1 j k j k 71 Third order distortion will cause components with 81 frequencies f ~f~ f . One form of such an component is 2f - f 9¦ where f is the scrambling carrier and f is the video 10¦ carrier. It can easily be shown that if the scrambling carrier 11¦ is located a frequency distance a (delta) above ~or below) the 12¦ video carrier, the spurious component 2f - f will fall 2 13 ! above (or below) that carrier. Although those spurious 14 components are expected to be small, their presence can reduce the video quality.
16 Low amplitude signals which fall close to an harmonic of 17¦ the horizontal frequency are about 20 dB more obvious to the 18¦ viewer than signals which fall in-between those harmonics. If 191 the scrambling signal is located at a halfharmonic as indicated 20¦ at 46 ln scale b of in Fig. 2 the third order intermodulation 21¦ product 2f - f will fall on an harmonic. The scrambllng signal 22 wlll be filtered out in the decoder but the intermbdulation 23 signal will not be filtered. Its presence may reduce the 24 overall picture quality.
In those lsolated cases the scrambling frequency allocation 26 I indicated in line (c) of Fig. 2 may be preferred. The 27j scrambling carriers and therefore the center frequencies of the 28 decoder filters are then optimally located 0.25*H Hz from the .9 ~armonlcs of the ho~zon~al freq~ency H and on~ specif~c location 50 is indicated in at 50. In general the scrambling 31 carriers will be in the vicinity of (0.25+m/2)*H above the 32 video carrier where m is an integer ranging from -40 to +39 i276288 1I corresponding to a band of 600 kHz centered around the vldeo 2,! carrier 40.
3 1I With this allocation the intermodulation products 2f - f 4 il will now fall at a half harmonic of the horizontal frequency and 5'1 will be 20 dB less obvious to the viewer. In the frequency 6,1 allocation for the scrambling carrier shown in line (c) of Fig.2 i 71 the scrambling carriers are about 4 kHz away from the spectral 81 peaks around the hori~ontal frequency and the narrow filters 911 in the decoder will not cause much distortion ~y destroying 10 1 important video information.
~ Fig. 3 s~ows a block diagram for cable T.V. distribution 12,1 system with Pay Channels according to this lnvention. Encoder 62¦
13'1 will inject one or more scrambling carriers into the RF signal 14 1 produced by the Channel Signal Processor 60 which passes through ~ a Compensation Circuit 61 (to be discussed hereinafter). This 16 i signal is transmitted to distribution amplifiers 63 through 66 ~7,1 which will service a number of subscribers with receivers 76 18!1 through 84. Each subscriber who subscribes for premium pay 19ll programs will have a variable decoder 68 through 72. In a 201 variable,decoder, decoder ,requencies can be easily changed by 21ji removing and changing small filter modules.~A module with a 22¦1 specific code could be used for continuous programming on a 231¦ Premium Pay channel which for instance could show movies. The ¦ decoder would have one or more slots for "Pay-per-View"
Il programs. The subscriber will simply pay a small fee for a ,I particular coded filter module which he will insert into the ¦
1I decoder to be able to receive a specific program.
28 1¦ The desired frequency response for a filter in ,1 the decode. is shown in Fig. 4~a). Fig. 4(a) shows ~he reponse 30 ¦ for a scrambling carrier located as shown in line ~b) of Fig. 2.

31 I The response 90 is for a band elimination filter having a 321 rejection band 91 with a required minimum width ranging from 500 Il . I
;, ' . ' 1276Z~
¦I Hz to 400~ Hz depending on the center frequency of the filter I and the specific technology used for making the filter. The 3 31 ds points 92 and 93 should be such that the 3 ds bandwidth will 4 range between 5 and 20 kHz, again depending on center frequency and filter construction, 6 Fig. 4(b) shows the desired frequency response for a 71 scrambling carrier located as shown in line (c) of Fig. 2.
8~ Response 94 shows the frequency response for a scrambling 9¦ carrier located 0.25*H Hz above an harmonic of the horizontal frequency; the response 95 shown dotted is the response for a 11 filter 0.25*H Hz below an harmonic of the horizontal 12 frequency.
13 Scrambling carriers not located at the center 14 between harmonics require deeper decoder filters with an attenuation of approximately 50 dB. The 3 dB point 96 should 161 preferably fall above the near harmonic of horizontal 98 with 17 the frequency nH such that the video information around the 18 harmonics is not too heavily attenuated. The 3 dB point 97 of 19 response 95 should preferably be below the harmonic 99 with the frequency ~n~1)H for the same reason.
21 For background information Fig. 5 show~ some typical 22¦ elements which will be used in the Solid State, High 231 Technology filters.
241 A Quartz resonator is fabricated on a Quartz substrate 251 100 as shown in Fig. 5(a). The frequency of the resonator ~s 26 1l determined by thickness 105 which has to be accurate to a few 27 parts in a million. If the resonator operates in the third 28 overtone, the thickness will be about 0.004 inch for a T.V.
29 channe-~ 2 fil~er.~ ~he technology to acc~mplish these tasks i~
available at low cost in the Quartz filter industry. Electrodes 31 101 and 102 are deposited on either side of the substrate with 32 electrically attached bonding areas 103 and 104 provided to ~ 2~762~3 l connect electrical wires to the terminals on the package.
2 Two resonators 107 and 108 on a substrate 105 are sho~n ln 3 Fig~ 5~b). sy carefully controlling the electrode material and 4 mass, the acoustic energy will be confined to the electrode segment of the substrate and the mutual resonator coupling can 6 be made very small. Two ~or more) decoupled resonators can be 7 made on a single substrate with this technology which is known 8 in the profession as Monolithic Technology.
9 A Surface Acoustic Wave (SAW) device is shown in Fig. 5~c).
Here the thickness of the substrate 108 is immaterial and the 11 substrate can be relatively thick and sturdy. The device 12 contains transducer 111 made of one or more interdigitated 131 metallic fingers enclosed by two arrays of reflectors 109, 110.
~4l Reflectors 109, 110 are either made by thin metallic electrodes or by etching grooves by means of plasma etching or other solid 16 state manufacturing techniques. Typically all the patterns are 17 made by photo lithographic processes, which makes it possible to 18 fabricate the devices at low cost. The linewidth for a high VHF
191 filter is of the order of three microns. Therefore the 201 equipment to fabricate those devices is very costly and the 21 ¦ operators are extensively trained. Therefore it is near 22 ¦ impossible for even skilled electronic technicians to make 23 I devices of this type. A SAW device with two transducers 112 and 24 113 is shown in Fig. 5~d) which allows deep notch filters to be made without the use of inverting transformers.
26~ Several filter geometries to make band rejection filters 27¦ are shown in Figs. 6(a) through lf). Fig. 6(a) is the 28 electrical equivalent circuit of a Quartz resonator of the type 29 shown in Fi~. 5(a) cr a~SAW reso~ator as shown in Fi~. 5~-). It contains series resonant circuit 121, resonant at the resonator 31 frequency, with series resistor 120. Parallel capacitor 122 32 represents the capacity between the electrodes. The circuit ' 12~6~88 l 1 exhibits a series resonant behavior at the series resonant 2 ¦ freqùency followed by a parallel resonance at a somewhat higher 3 ¦ frequency. The parallel resonance comes about by interaction of 4 1 series resonant circuit 121 with parallel capacitance 122.
5 l One particular re~ection band filter configuration is shown 6 1 in Fig. 6(b). The filter is connected to source 130 by a source 7 ¦ reslstor 123 and to load resistor 124 and conslst of three 8 1 resonators 125, 126, and 127 coupled by two inductors 12~ and 9 ¦ 129. The filter exhibits an attenuation band at the parallel 10 resonance frequency of the resonators band.
11 The structure shown in Fig. 6(c) i8 lnverted compared to 12 Fig. 6(b). The filter ls connected between a source 131 with 13 lts source reslstor 132 and load 133. Again the filter shown 14 here has three resonators 134, 135 and 136 coupled by two lnductor~ 137 and 138. The re~ection band of this fllter occurs 16 ¦ at the series resonance frequency of the resonators.
17 ¦ A balanced band rejection filter between source 140 with 18 ! lts source resistor 141 and a load resistor 145 is shown in Fig.
19 1 6(d). It contalns balancing transformer 142 with a mid-tap, a 20 1 resonator 143 and balanclng resistor 144. By setting balancing 21 1 reslstor 144 equal to the reslstance of resonator 143 at 22 l~ either serles or parallel resonance the output signal can be 23 j made zero at the partlcular resonance frequency. The advantage 24 ,¦ of a fllter of this type is that at the resonance frequency 25 ~I the attenuation is infinite.
26, A different form of a balanced filter is shown in Fig.
27 l 6~e). Here the filter is ln-between source 150 with its source 28 il reslstor 151 and load resistor 155. The filter contains 29 1! inverting transformer 154, resonator 152 and balancing resistor 30 !1 153. Balancing resistor 153 can agaln be adjusted to the 31 '! reslstance of resonator 152 at either the parallel or series 32 1I resonance. The SAW resonator of Fig. 5(d) can replace resonator !l , ~ .

l 152 and inverting transformer 154 as shown in Fig. 6~f). In Fig 2l 6(f) source 156 is connected through balancing resistor 158 to 3 ¦ load resistor 159. Two terminals of ~n 180 degree SAW resonator 4 160, one from each transducer, are connected to either side of balancing resistor 158 while the two other transducer terminals 6 are grounded. The balancing resistor can be adjusted to the 7 resistance at either the series or parallel resonance of the SAW
8 resonator to obtain a bandstop filter.
9 A filter arrangement proven to be very effective bandstop filters with deep notches is shown in Fig. 7. It 11 basically consists of two cascaded filter circuits, as shown 12 in Fig. 6(e), havlng mutua} coupling resistor 173. In the 13 filter shown source 170 is coupled through source resistor 171 14 to a series arrangement of balancing resistors 172, 173 and 174 to load resistor 177. The filter contains two resonators 175 and ~ 176 which are connected through inverting transformers 178 and 17 179.
18 If initially coupling resistor 173 is set to zero, the 19 filter degenerates into a set of two cascaded bandstop filters.
20¦ By means of balanclng resistors 172 and 174 each filter can be 21 ad~usted to have a very deep re~ection band. Those bands can be 22 at different frequencies depending on the resonant frèquencies 23 of the resonators. The re~ection bands can be made to move 24 towards each other by increasing the value of coupl~ng resistor 173 together with decreasing the value of resistors 172 and 26~ 174.
27 1 This procedure can be used to obtain a large number of 28 ¦ different frequency responses from one specific set of ~, I r~s^nator~ It can also b~ used to utilize a large number of 30 ~ devices, which have a frequency spread of e.g. 0.01%, for the 31 ¦ fabrication of filters in which the re~ectlon band has to be 32 I exactly at the same frequency.

~276Z8~3 1 An example of a frequency response of a ladder filter 2 with four resonators is shown in the graphs of Figs. 8(a) and 3 (b), constructed similar to the filter shown in Fi~. 6(c).
4 The center frequency of the filter is 83.25 MHz while the 3 dB
bandwidth is about 20kHz. Curve 190 is the amplitude response 6 while curve 191 is the phase response of the filter.
7 The 3 dB bandwidth of this filter is relatively large and 8 all spectral componen~s of the modulated video signal which fall 9 within this band will be attenuated and phase shifted. This can affect the video quality in an adverse way. By means of pre-11 compensation at the transmitter the overall amplitude and phase 12 response of the system can be considerably improved, resulting 13 in a nearly perfect decoded picture.
~41 One lmplementation of pre-compensation is shown in Fig.
151 9(a). Signal processor 20~ sends a signal to compensating 16 network 201 which sends a signal to summing circuit 202 where 17 scrambling signals generated by encoder 203 are added. (The 18 blocks are similar to the blocks 60, 61 and 62 in Fig. 3).
19 Fig. 9~b) shows one lmplementation of compensating circuit 201.
21 Compensation in Fig. 9~b) is obtained through negative 22 feedback. The signal from Channel Signal Processor 200 (Fig.
23 8~a)) is applied to input 207 of the Compensation Network. .
24 Input 207 is connected to one of the summing inputs~of summing network 206 while the other summing input 209 is the output of 261 filter 205 in the feedback loop. The output of summing 27 ¦ network 206 is connected to the input of amplifier 204 which has 28 ¦ an open loop galn of~ . The output of amplifier 204 is 2~ 1 connected to the input of filter 205, which is identical to 30 ¦ the filter in the decoder which has to be compensated for. The 31¦ output of the amplifier 204 is also connected to output cable 321 208 of the compensation network.

lZ76Z~38 1 1 In an feedback amplifier with an open loop gain ~ (mu) and 2 ¦ a feedback transfer function ~ (beta) the closed loop gain ~ is 31 given by ~ c 4 ~ c = --------------- (1) 61 In the limiting case ~ 1 the closed loop gain becomes 71 ~
81 c = ~--- (2) 9 If decoder bandstop filter F is used in the feedback loop, the transfer function of the closed loop gain becomes the 11¦ inverse of the response of the bandpass filter. If the filter 12¦ is a bandstop filter, the closed loop transfer function will 13¦ resemble a bandpass network. If the phase of the filter has a 14¦ negative slope, the phase bf the closed loop transfer function 15¦ will have a positive slope.
16¦ Amplitude response 220 and phase response 221 of the 17¦ compensation network in series with decoder bandstop filter as 18 shown in Figs. 10(a) and ~b). This overall corrected response 191 glves a direct measure of the video quality to be expected.
If the compensation would be 100% effective amplitude 21 response 220 would be flat and phase response 221 would be 22 linear.
23 Comparing Figs. 10(a) and (b) with Figs. 8 (a) and (b) 24 shows that the compensation produces considerable i~provement.
251 The 3 dB bandwidth has been reduced from about 18 to 6 kBz and 26j the phase excursion has been reduced accordingly.
271 It can be seen from formula ~1) that in the ideal case 28¦ ~ equals infinity ~ -cxO1 and complete compensation ~ takes place. In ~ractice the amount of allowable gain is limited to prevent oscillations at some frequency 31 where the phase excursion is 180 degrees. To be able to make 32 further improvements the circuit shown in Fig. 11 can be ~Z76Z88 11¦ used. By using double feedback a near perfect response can be 2¦¦ obtained.
3 I The block diagram of Fig. 11 (a) shown is essentially the 4 ¦ same as that of Fig. 9(a). Channel signal generator 230 is 5 ¦ connected to the input of compensation network 231. The 6 ¦ output of compensation network 231 is connected to sum~ing 7 ¦ network 232 in which the scrambling signals from encoder 8 ¦ generator 233 are added.
91 The compensation network, using double feedback is shown 10¦ in Fig. 11 (b). The left loop containing amplifier 234, filter 11¦ 235 and the summing network 236 is identical to the network 12 ¦ shown ln Fig. 9(B). The signal output at 238 is therefore a 13 ¦ first filter correction applied to the input signal at 237.
14¦ The second loop to the right of the first loop makes a 15 ¦ secondary correction. It contains a tertiary loop which is a 16¦ copy of the first loop with amplifier 244 , summing network 243 17¦ and filter 245. This loop is connected in series with filter 18¦ 242 which again is a replica of the decoder bandstop Pilter. The 19l response between the input of 242 and output line 246 is 20 ¦ therefore the overall corrected response, identical to the one 21 ¦ shown in Figs. 10(a) and (b). By placing those networks with 22 ¦ the overall corrected response in the feedback loop of 23 ¦ amplifier 240 the network between input 238 and output 241 24 ¦ will tend to correct the before mentioned oYerall corrected 251 response. The network between input 237 and output 241 will 26, generate therefore a double corrected response for the decoder 271 filter.
28¦ The filter response of the decoder band stop filter in L9 ¦ serl~s with.the double compensation is s~own ln FigO12 where 301 curve 250 is the overall amplitude response and curve 251 the 31¦ overall phase response. Comparing Figs. 12(a) and ~b) with 32~ Figs. 10~a) and ~b) shows that double compensation ma~es a l 2~

~ 276Z8~3 1¦ considerable improvement. The 3 dB bandwidth is now reduced to 2 ¦ 2.5 kHz and the amplitude and phase excursions have become 3 ¦ quite small.
4 1 For the compensation to be effective it is essential that 5 ¦ the response of the filters is very similar.
6 ¦ As described, the compensation has been obtained by using 7 negative feedback on the R.F. (radio frequency) channels. It is 8 also possible to compensate the video signals for distortion 9 caused by the decoder filters.
This invention is not to be limited by the embodiment 11 shown in the drawings and described in the description, which is 12 given by way of example and not of limitation, but ony in 13 accordance with the scope of the appended claims.

191 ~

222 _. ~

24 , 2 I ;

Claims (63)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A television signal transmission security system comprising:
television signal generating means generating a television signal having a video carrier and an audio carrier;
encoder means for injecting at least one interfering signal into said television signal, said at least one interfering signal being injected into said television signal about midway between harmonics of the horizontal sweep frequency of said video carrier where spectral intensity of said harmonics is not substantial and in a frequency band including said video carrier and on either side or both sides of said video carrier;
transmission means for transmitting said television signal with said interfering signal to a plurality of television receivers; and decoder means having notch filter means with at least one notch at each television receiver receiving and removing said at least one interfering signal, each of said at least one notch of said notch filter means having a bandwidth at 3db down of less than about the frequency bandwidth between adjacent harmonics of said horizontal sweep frequency wherein said notch filter means does not remove a significant amount of video sideband information, allowing said television signal to be viewed at said plurality of television receivers without substantial degradation.

2. The system according to claim 1 in which said frequency band including said video carrier is approximately 300 kHz on either side of said video carrier; said at least one interfering signal being injected at a frequency of at least approximately 6kHZ
from said video carrier.

3. The system according to claim 1 in which said interfering signal is injected at a frequency offset from a harmonic of said horizontal sweep frequency of said video carrier which is in the range of 0.25 to 0.75 of the frequency bandwidth between adjacent harmonics of said horizontal sweep frequency.

4. The system according to claim 3 in which one or more interfering signals are injected at a frequency that is offset one half the bandwidth between adjacent frequency components due to horizontal frequencies of said video carrier and harmonics thereof.

5. The system according to claim 1 in which said narrow band notch filter means is a monolithic piezoelectric crystal filter.

6. The system according to claim 5 in which said narrow band notch filter means is a bulk crystal filter.

7. The system according to claim 5 in which said narrow band notch filter means is a surface acoustic wave filter.

8. The system according to claim 1 in which said narrow band notch filter means produces at least a 40 dB notch having a maximum bandwidth at 3db down of approximately 20 kHz.

9. The system according to claim 8 in which said narrow band notch filter means has a minimum bandwidth at 40 db down of approximately 500Hz.

10. The system according to claim 6 in which said narrow band notch filter is an impedance coupled notch filter means.

11. The system according to claim 7 in which said narrow band notch filter means is an impedance coupled notch filter means.

12. The system according to claim 6 in which said narrow band notch filter means is comprised of impedance cross-coupled filter piezoelectric crystals.

13. The system according to claim 7 in which said narrow band notch filter means is comprised of impedance cross-coupled filter piezoelectric crystals.

14. The system according to claim 1 in which said means for injecting at least one interfering signal injects at least two interfering signals, one of said interfering signals being injected below the video carrier, the other of said interfering signals being injected above the video carrier.

15. The system according to claim 1 in which said means for injecting at least one interfering signal injects at least two interfering signals, said two interfering signals being injected at spaced apart frequencies either above or below said video carrier.

16. The system according to claim 1 in which said means for injecting at least one interfering signal injects three or more interfering signals, at least one of said three or more interfering signals being injected on the opposite side of said video carrier from the other interfering signals.

17. The system according to claim 1 including pre-compensation means for pre-compensating for distortion caused by said narrow band notch filter means; said pre-compensation means being at said television signal generating means.

18. The system according to claim 1 wherein said narrow band notch filter means is an interchangeable notch filter means whereby selected interfering signals may be changed at preselected intervals.

19. The system according to claim 1 in which said offset frequency is approximately 0.25 of the frequency bandwidth between adjacent frequency components due to horizontal frequencies of said video carrier and harmonies thereof.

20. The system according to claim 1 in which said offset frequency is approximately 0.75 of the frequency bandwidth between adjacent frequency components due to horizontal frequencies of said video carrier and harmonies thereof.

21. The system according to claim 17 in which said pre-compensation means comprises:
feedback means for feeding back a portion of said television signal being transmitted;
summing means for summing said feedback signal with said generated television signal;
amplifying means receiving and amplifying the output of said summing means.

22. The system according to claim 21 in which said feedback means includes notch filter means substantially identical with said notch filter means at said receiver.

23. The system according to claim 22 including a second feedback means having a narrow band notch filter substantially identical with said notch filter means at said receiver.

24. The system according to claim 23 wherein said second feedback means includes a feedback network comprised of summing means; narrow band notch filter means substantially identical to said narrow band notch filter means at said receiver; and amplifier means.

25. The system according to claim 24 including network summing means summing the output of said feedback network with the output of a first feedback means.

26. A method of securing television transmission comprising:
generating a television signal having a video carrier and an audio carrier;
injecting at least one interfering signal into said television signal, said at least one interfering signal being injected into said television signal about midway between harmonics of the horizontal sweep frequency of said video carrier where spectral intensity of said harmonics is not substantial and in a frequency band including said video carrier on either side or both sides of said video carrier;
transmitting said television signal with said at least one interfering signal to a plurality of television receivers;
removing said at least one interfering signal at said plurality of television receivers with decoder means having notch filter means with at least one notch, each of said at least one notch of said notch filter means having a bandwidth at 3db down of less than about the frequency bandwidth between adjacent harmonics of said horizontal sweep frequency so that said notch filter means does not remove a significant amount of video sideband information, allowing said television signal to be viewed at said plurality of television receivers without substantial degradation.

27. The method according to claim 26 in which said at least one interfering signal is injected in a frequency band including said video carrier of approximately 300kHz on either side of said video carrier, said at least one interfering signal being injected at a frequency of at least approximately 6kHz from said video carrier.

28. The method according to claim 27 in which said at least one interfering signal is injected in said frequency band including said video carrier at a frequency which is an intermediate frequency component of horizontal frequencies of said video carrier or harmonics thereof.

29. The method according to claim 26 in which said at least one interfering signal is two interfering signals, one of said interfering signals being injected above said video carrier, the other of said interfering signals being injected below said video carrier.

30. The method according to claim 26 in which said at least one interfering signal is two spaced apart interfering signals injected on the same side of and either above or below said video carrier.

31. The method according to claim 28 in which said at least one interfering signal is three or more interfering signals with at least one of said interfering signals being injected in said frequency band including said video carrier on the opposite side of said video carrier from the other interfering signals.

32. The method according to claim 26 including inserting pre-compensation signal means into said transmitted television signal to compensate for distortion introduced when said interfering signal is removed by said narrow band notch filter.

33. The method according to claim 32 in which said at least interfering signal is two spaced apart interfering signals injected in the frequency band including said video carrier on the same side of and either above or below said video carrier.

34. A television signal transmission security system comprising:
television signal generating means generating a television signal having a video carrier and an audio carrier;
encoder means for injecting multiple interfering signals into said television signal, each of said multiple interfering signals being injected into said television signal about midway between harmonics of the horizontal sweep frequency of said video carrier where spectral intensity of said harmonics is not substantial and in a frequency band including said video carrier and on either side of both sides of said video carrier;

transmission means for transmitting said television signal with said interfering signal to a plurality of television receivers; and decoder means having notch filter means with multiple notches at each television receiver receiving and removing said multiple interfering signals, each of said multiple notches of said notch filter means having a bandwidth at 3db down of less than about the frequency bandwidth between adjacent harmonics of said horizontal sweep frequency wherein said notch filter means does not remove a significant amount of video sideband information, allowing said television signal to be viewed at said plurality of television receivers without substantial degradation.

35. The system according to claim 34 in which said multiple interfering signals comprise at least two interfering signals injected into said television signal, at least one of said interfering signals being injected in said television signal below said video carrier.

36. The system according to claim 35 including at least one interfering signal injected at a frequency which is outside the bandwidth of said television signal; said decoding means including notch filter means for removing said interfering signals injected outside the bandwidth of said television signal.

37. The system according to claim 34 in which said frequency band including said video carrier is from approximately 6kHz to approximately 300kHz away from and on either side of said video carrier.

38. The system according to claim 34 in which said interfering signal offset is one half the bandwidth between adjacent frequency components due to horizontal frequencies of said video carrier and harmonics thereof.

39. The system according to claim 34 in which said notch filter means is a monolithic piezoelectric crystal filter.

40. The system according to claim 39 in which said notch filter means is a bulk crystal filter.

41. The system according to claim 39 in which said notch filter means is a surface acoustic wave filter.

42. The system according to claim 34 in which said notch filter means produces at least a 40 dB notch having a maximum bandwidth at 3db down of approximately 20kHz.

43. The system according to claim 42 in which said notch filter means has a minimum bandwidth at the 40db point of approximately 500 Hz.

44. The system according to claim 40 in which said notch filter is an impedance coupled notch filter means.

45. The system according to claim 44 in which said notch filter means is an impedance coupled notch filter means.

46. The system according to claim 40 in which said notch filter means is comprised of impedance cross-coupled filter piezoelectric crystals.

47. The system according to claim 41 in which said notch filter means is comprised of impedance cross-coupled filter piezoelectric crystals.

48. The system according to claim 34 in which said means for injecting at least one interfering signal injects at least two interfering signals, one of said interfering signals being in said frequency band below the video carrier, the other of said interfering signals being in the band above the video carrier.

49. The system according to claim 34 including pre-compensation means for pre-compensating for distortion caused by said notch filter means; said pre-compensation means being at said television signal generating means.

50. The system according to claim 35 wherein said notch filter means is an interchangeable notch filter means whereby selected interfering signals may be changed at preselected intervals.

51. The system according to claim 34 in which said offset frequency is approximately 0.25 of the frequency bandwidth between adjacent frequency components due to horizontal frequencies of said video carrier and harmonics thereof.

52. The system according to claim 34 in which said offset frequency is approximately 0.75 of the frequency bandwidth between adjacent frequency components due to horizontal frequencies of said video carrier and harmonies thereof.

53. The system according to claim 49 in which said pre-compensation means comprises:
feedback means for feeding back a portion of said television signal being transmitted:
summing means for summing said feedback signal with said generating television signal;
amplifying means receiving and amplifying the output of said summing means.

54. The system according to claim 53 in which said feedback means includes notch filter means substantially identical with said notch filter means at said receiver.

55. The system according to claim 54 including a second feedback means having a notch filter substantially identical with said notch filter means at said receiver.

56. The system according to claim 55 wherein said second feedback means includes a feedback network comprised of summing means;
filter means substantially identical to said notch filter means at said receiver; and amplifier means.

57. The system according to claim 56 including network summing means summing the output of said feedback network with the output of a first feedback means.

58. A method of securing television transmissions comprising:
generating a television signal having a video carrier and an audio carrier;
injecting multiple interfering signals into said television signal; said multiple interfering signals being injected into said television signal about midway between harmonics of the horizontal sweep frequency of said video carrier where spectral intensity of said harmonics is not substantial and in a frequency band including said video carrier and on either side or both sides of said video carrier;

transmitting said television signal with said multiple interfering signals to a plurality of television receivers;
removing said multiple interfering signals at said plurality of television receivers with decoder means having notch filter means with multiple notches, each of said multiple notches of said notch filter means having a bandwidth at 3db down of less than about the frequency bandwidth between adjacent harmonics of said horizontal sweep frequency wherein said notch filter means does not remove a significant amount of video sideband information, allowing said television signal to be viewed at said plurality of television receivers without substantial degradation.

59. The method according to claim 58 in which said multiple interfering signals are injected in said frequency band including said video carrier of from 6 kHz to approximately about 300 kHz away from and on either side of said video carrier.

60. The method according to claim 59 in which said multiple interfering signals are injected in said television signal at frequencies which are intermediate frequency components of horizontal frequencies of said video carrier or harmonics thereof.

61. The method according to claim 58 in which said multiple interfering signals are at least two interfering signals, at least one of said two interfering signals being injected in said television signal below said video carrier.

62. The method according to claim 60 in which said multiple interfering signals are at least three interfering signals with at least one of said interfering signals being injected in a frequency band outside the bandwidth of said television signal.

63. The method according to claim 58 including inserting a pre-compensation signal into said transmitted television signal to compensate for distortion introduced when said interfering signal is removed by said notch filter means.