Note: Descriptions are shown in the official language in which they were submitted.
l~S1286
1 The present invention relates generally to television
communication systems and, more particularly, concerns a method
and apparatus for scrambling subscription television programs so
that an unauthorized receiver will not receive an intelligible
television program, which method and apparatus can not be circum-
vented by channel tunning adjustments of an unauthorized receiver.
As used herein, the term "subscription television system"
is intended to encompass all television communication systems in
which a television program signal, which is recoverable by a
general group of television receivers, is intended to be recovered
by only specific authorized ones of the receivers. The descriptive
term "subscription television" is intended to be similarly broadly
interpreted.
; In conventional television broadcasting and cable
systems, each transmitter station is assigned a channel having a
predefined frequency bandwidth. The signal produced by each
station includes an audio carrier signal and a video carrier
signal disposed near opposite ends of the assigned channel and
modulated, respectively, to carry the audio and video portions of
the program. By convention (in the United States~, commercial
television channels are 6 megahertz wide and the video and audio
carriers are disposed, respectively, at 1.25 megahertz and 5.75
megahertz above the lower boundary of the channel. Also by conven-
tion, video information is amplitude-modulated onto the video
carrier and audio information is frequency-modulated onto the
audio carrier.
In subscription television systems (for example, in modern
cable television systems), it is customary to withhold a program
from an unauthorized receiver or to provide the program to the re-
ceiver while conditioning the program signal so that it is unin-
telligible unless received by an authorized subscriber. Various
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~l~5~286
methods have been suggested for scrambling a television signal to
render it unintelligible, including the injection into the televi- -
sion signal of radio frequency (RF) spoiling signals or "tones" to
interfere with the reception of the signal.
In our Canadian Patent 1,122,695, Garodnick et al., April
27, 1982, we disclose a television signal scrambling method and ap-
paratus in which an RF spoiling signal, serving as an interfering
carrier, is provided intermediate the audio and video carriers and
is modulated with a substitute television signal. At an unautho-
rized receiver, the interferring carrier blocks reception of the
subscription television signal and the substitute program is re-
ceived. At an authorized receiver, a band elimination filter or
other frequency eliminating device is provided ahead of the re-
ceiver and is tuned to the interferring carrier, thereby extract-
ing it before the signal reaches the receiver. As a result, the
subscription program is received at an authorized receiver without
interference from the interferring carrier.
In practice, it has been found that a television receiv-
er may be sufficiently detuned (i.e. tuned away from its nominal
frequency setting for the channel to be received) to place the
interfering carrier outside of the reception band of the receiver.
Although such detuning also shifts the audio carrier out of the
audio reception band, it also results in sufficient attenuation
of the interferring carrier to permit reception of the video
portion of the subscription program because the video is not
shifted out of the reception band. As a consequence, it would be
possible to detune two television receivers in opposite senses so
that one receives the audio portion of the subscription program
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~15~;286
1 and the other recieves the video portion of the subscription
program, while both receivers attenuate the interfering carrier
sufficiently to eliminate its effect o~ picture reception. In --
using one receiver for audio and one for video (black and white
only, as will be explained below), the entire subscription program
can be reconstructed and viewed by an unauthorized party.
Broadly, it is an object of this invention to prevent
scrambling of a subscription television program to be circum-
vented by detuning one or more conventional television receivers.
More specifically, it is within the contemplation of the present
invention to provide a method and apparatus for conditioning a
scrambled subscription television program signal to prevent its
reception by unauthorized receivers by detuning the same, while
avoiding any perceptible degadation of the subscription tele-
vision program received at an authorized receiver.
It is another object of this invention to provide a
method and apparatus for scrambling or blocking both the audio
and video portions of a subscription television signal while
permitting the introduction of a replacement program signal
having audio and/or video components, which method and apparatus
can not be circumvented by detuning one or more authorized
receivers and which produce no appreciable degradation of the
subscription television program received at an authorized
receiver.
It is a further object of this invention to provide
a method and apparatus of the type described which can be
employed in existing subscription television systems with a
minimum of modifications.
It is also an object of this invention to provide
apparatus of the type described which is efficient, convenient
and reliable in use, yet relatively inexpensive and simple in
construction.
115~Z86
In accordance with one aspect of the present invention, a sub-
scription television signal which has been conditioned for reception by
authorized receivers only is provided with a security carrier signal at a
frequency which is outside the reception band of the television receiver for
the selected channel and which is also sufficiently remote from the reception
band to avoid interference with the program when th~ receiver is correctly
tuned to the channel. However, the security carrier is sufficiently close in
frequency to the reception band to be drawn into it if an attempt is made to
detune the television receiver. This permits the conditioned television
signal to be received at a properly tuned receiver (either in scrambled or
unscrambled form depending on whether the receiver is unauthorized or
authorized). ~owever, the security carrier interferes with or blocks
reception of the television signal, should an attempt be made to circumvent
scrambling by detuning.
In accordance with another aspect of the present invention, the
security carrier is alternately modulated with a plurality of different low
frequency signals. As a consequence, when the television receiver is detuned
and the security carrier is drawn into the reception band, the receiver dis-
plays a moving interference signal which is exremely annoying to a viewer.
In accordance with the present invention there is provided in a
television communication system transmitting normal program information by
means of a radio frequncy signal disposed in a channel occupying a pre-
determined frequency band and including separate frequency-spaced audio and
video carrier signals, said audio and video carrier signals being modulated
respectively, with the audio and video portions of said normal program in-
formation, the method of preventing reception of said normal program at a
receiver with a predefined reception band when said receiver is detuned sub-
stantially from the nominal adjustment for receiving said channel, comprising
the steps of: producing a security carrier signal together with said trans-
mitted noxmal pxogram infoxmation and at a frequency outside said predetermined
~SlZ86
frequency band and sufficiently removed therefrom in frequency to avoid any
appreciable interference with reception when said receiver is in said
nominal adjustment, the frequency of said security carrier being selected
to permit reception of adjacent channels when said receiver is detuned sub-
stantially from said nominal adjustment; and linearly combining said security
carrier signal with said radio frequency signal.
In accordance with the present invention there is further provided
in a television communication system transmitting normal program information
by means of a radio frequency signal disposed in a channel occupying a pre-
determined frequency band and including separate frequency-spaced audio and
video carrier signals, said audio and video carrier signals being modulated,
respectively, with the audio and video portions of said normal program
information, apparatus for preventing reception of said normal program at a
receiver with a predefined reception band when said receiver is detuned sub-
stantially from nominal adjustment for receiving said channel, said apparatus
comprising: a source of a security carrier signal at a frequency outside said
predetermined frequency band and sufficiently removed therefrom in frequency
to avoid any appreciable interference with reception when said receiver is in
said nominal adjustment, the frequency of said security carrier being selected
to permit reception of adjacent channels when said receiver is detuned sub-
stantially from said nominal adjustment; and a combiner for combining said
security carrier signal with said radio frequency signal.
In accordance with the present invention there is further provided
a transmitter for use in a television communication system for normal program
material incorporating audio and video information, said transmitter producing
a composite radio frequency signal in an assigned channel occupying a pre
determined frequency band, said composite radio frequency signal comprising:
an audio carrier signal at a frequency in said band, said audio carrier
signal being modulated with said audio information; a video carrier signal at
~ different frequency in said band, said video carrier signal being modulated
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with said video information; and a security carrier signal at a frequency
outside said predetermined fre~uency band and sufficiently removed therefrom
in frequency to avoid any appreciable interference with reception in a
receiver which is nominally adjusted to receive said channel, said security
carrier frequency being selected to prevent receipt of said normal program
when said receiveris detuned from the nominal adjustment while permitting
receipt of an adjacent channel.
In accordance with the present invention, there is further provided
in a television communication system transmitting normal program information
by means of a radio frequency signal disposed in a channel occupying a
predetermined frequency band in a multichannel signal including an additional
channel adjacent to said band, said radio frequency signal including
separate frequency-spaced audio and video carrier signals, said audio and
video carrier signals being modulated, respectively, with the audio and
video portions of said normal program information, the method of preventing
reception of said normal program at a receiver with a predefined reception
band when said receiver is detuned substantially from the nominal adjustment
for receiving said channel, comprising the step of amplitude modulating the
audio carrier of the additional channel.
In accordance with the present invention, there is further provided
in a television communication system transmitting normal program information
by means of a radio frequency signal disposed in a channel occupying a
predetermined frequency band in a multichannel signal including an
additional channel adjacent to and lower in frequency than said band, said
radio frequency signal including separate frequency-spaced audio and video
carrier signals, said audio and video carrier signals being modulated,
respectively, with the audio and video portions of said normal program
information, apparatus for preventing reception of said normal program at
a receiver with a predefined reception band when said receiver is detuned
~ubsta~tiallY from the nominal adjustment for receiving said channel,
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~iS~2~6
comprising: means for extracting the audio carrier of the additional
channel from the multichannel signal, means for amplitude modulating the
extracted audio carrier of the additional channel, and means for reinse~ting
the amplitude modulated audio carrier into the multichannel signal.
In illustrative embodiments demonstrating objects on features of
the present invention, a security carrier signal is provided in a subscription
television signal at the frequency of the audio carrier of the lower adjacent
channel and is alternately amplitude modulated with bursts from first and
second interference frequencies at a third or keying frequency. The
television signal also includes an interfering carrier signal intermediate
the audio and video carriers in the channel and the interfering carrier is
modulated with a substitute television program. When a receiver is properly
tuned to the channel, the security carrier has no effect on reception
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.
~l~S1286
1 and either the subscription or ~ubstitute program is received,
depending on whether the receiver is authorized or not. Should
an attempt be made to receive the subscription program in an
unauthorized receiver by, detuning the receiver, the security
carrier interferes with or blocks video reception in the detuned
receiver and the modulation as that carrier produces a moving visual
pattern which is extremely annoying to a viewer.
The foregoing brief description, as well as further
objects, features and advantages of the present invention, will
be more completely understood from the following detailed descrip-
tion of a presently preferred, but nonetheless illustrative, embodi-
ment in accordance with the present invention, with reference being
had to the accompanying drawings in which:
Fig. 1 is a frequency plot showing the spectrum of a
composite radio frequency (RF) television signal modified in
accordance with the present invention on which the frequency
response curve of the video portion of a properly tuned receiver
is superimposed in broken lines;
Fig. 2 is a functional block diagram illustrating the
manner of modulating an interfering carrier with substitute program
information, modulating a security carrier with a signal of time-
varying frequency and combining the modulated interfering and
security carriers with a standard television signal, in accordance
with a preferred embodiment of the present invention;
Fig. 3 is a frequency plot illustrating the preferred
spectrum of an RF preemphasis network incorporated in the preferred
embodiment of Fig. 2;
Fig. 4 is a functional block diagram illustrating the
equipment that is incorporated at an authorized receiver to facili-
tate removal of the interfering carrier and the replacement program
signal in order to permit reception of the regular subscription
television program signal.
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1 Fig. 5 is a waveform diagram useful in describing the
manner of generating the modulated security carrier~
Fig. 6 is a circuit diagram illustrating how the modu-
lating signal for the security carrier may be generated; and
Fig. 7 is a functional block diagram showing how the
block diagram of Fig. 2 is modified for operation with a multi-
channel signal with adjacent channels occupied, such as is found,
for example, in cable television systems.
In our aforementioned patent, we disclosed that when an
interference carrier signal is injected into a conventional television
signal at any of a predefined set of frequencies intermediate the
video and audio carrier signals, the interfering carrier signal will
not only block normal reception of the conventional television
signal, but, if appropriately modulated, can provide a replacement
program signal for the conventional television signal. This phenome-
non is believed to result from the non-linear response of a conven-
tional television receiver to the various carrier signals (i.e.
audio, video and interfering carrier signals) present in the com-
posite RF television signal. A receiver having a non-linear response
produces beat frequencies between various frequency components
present in the several RF signals applied to the television receiver
and also produces harmonics of the beat frequencies. Moreover,
experimentation has shown that blocking of the normal television
signal and the substitution of a replacement signal will occur when
the frequency difference between the audio and video carrier signals
of the normal television signal (i.e., 4.5 MHz) is an integral
multiple (preferably a low multiple) of the frequency difference
between the interfering carrier signal and either the video or audio
carrier signal (i.e., when the frequency difference between the
interfering carrier signal and one of the other carrier signals is
an integral submultiple of the intermediate frequency for the
115~286
1 conventional "intercarrier" audio detection system). When an
interfering carrier signal of any of the above-defined frequencies
is appropriately frequency-modulated with replacement audio signals,
these audio sig~als will replace the regular audio program signals
and will be reproduced by the receiver. Substitution of replacement
video program material can be obtained with the same interfering
carrier frequencies by appropriately amplitude-modulating the
interfering carrier by the replacement video program signals.
As will be explained below, it has been found that a
conventional television receiver, by being intentionally detuned,
could be operated to receive either the audio or video portions of
the normal or subscription program, thereby circumventing, at least
partially, the intended scrambling effect of the interfering carrier
signal.
Referring now to the details of the drawings, and in
particular to Fig. 1, there is shown the frequency spectrum of
a composite RF television signal, indicated generally by the numeral
10, as modified in accordance with the present invention. Superim-
posed on the composite signal 10 is a wave shape 26 (shown in broken
lines~ representing the frequency response curve of the video
portion of a conventional television receiver which is properly
tuned to the channel to be received. The television signal is
included in a channel ranging in frequency from a lower bounding
frequency fL to an upper bounding frequency fH, with a differ-
ence between fH and fL of 6 megahertz. For example, if thesignal 10 is being transmitted over channel 4~ fL and fH are
equal to 66 megahertz and 72 megahertz, respectively. By convention
(in the United Statec), a video carrier signal 12 is provided at
1.25 megahertz above fL and an audio carrier signal 14 is provided
at 5.75 megahertz above fL. Thus, for a channel 4 signal, the
video and audio carrier signals are at 67.25 and 71.75 megahertz,
respectively. In addition to the conventional components just
`` llS~l286
1 described, the television signal 10 also includes an interfering
carrier signal 16 at a frequency fO, which is intermediate the
video and audio carrier frequencies and a security carrier signal 25
at a frequency fs~ which is outside the frequency range of the
channel being received. For purposes of illustration, fO is shown
in Fig. 1 as being half way between the video and audio carrier
signals, or at a frequency of 69.5 megahertz for a channel 4 signal,
and fS is shown at the frequency of the audio carrier of the next
lower channel (i.e. channel 3 audio at 65.75 Mhz). However, the
principles of the invention apply equally well if fO is spaced
from the video carrier by a frequency which is an i~terger submul-
tiple of 4.5 megahertz (preferably a low submultiple), as previously
explained, and when fS is not precisely at the frequency of the
lower adjacent channel audio carrier.
As is customary in commercial television broadcasting,
the video ca`rrier 12 is amplitude-modulated with video program
information and the audio carrier 14 is frequency-modulated with
audio program information. For illustrative purposes, the frequency
spectrum of the modulated video carrier is schematically represented
by waveform 18. It should be noted that the waveform 18 is preempha-
sized to produce a peak 18' in the vicinity of the frequency fO of
the interfering carrier 16. The preemphasis compensates for the
attenuation introduced in the vicinity of the frequency fO by
the band-elimination filter which is provided at an authorized
receiver to remove the interfering carrier signal, as described
below. The spectrum of the modulated audio carrier signal 14 is not
shown, but is represented schematically by the double-headed
horizontal arrow 20 provided on the spectral line corresponding to
the audio carrier 14.
Interfering carrier 16 is modulated with replacement
information for the normal television program. In the embodiment
~lS~ 8~
1 of Fig. 1, the interfering carrier is amplitude-modulated with
video information and frequency-modulated with audio information.
The spectrum of the video information, which is represented by
the partial waveform 22, is restricted to a narrow bandwidth of the
order of magnitude of a conventional band-elimination filter so that
it may be readily removed without appreciably degrading the normal
video material. The spectrum of the audio portion of the substitute
signal is not shown, but is represented schematically by the double-
headed horizontal arrow 24 on interfering carrier 16.
It is believed that, when the composite signal 10 repre-
sented in Fig. 1, is applied to a conventional, properly tuned
television receiver, the security carrier signal is substantially
attenuated owing to the notch 28 in the frequency response curve 26
of the video portion of the receiver, so that the security carrier
has no effect on picture reception. The non-linear characteristics
of the receiver produce replicas of the various modulated carriers
at frequencies equal to the sums and differences of the various
carriers and harmonics thereof. In the absence of tbe interfering
carrier (i.e., with a conventional television signal), the modulated
video and audio carriers are non-linearly combined to provide a
replica of the audio carrier modulation on an inteemediate carrier
of 4.5 megahertz (i.e., the difference between the audio and video
carrier frequencies), and to provide a replica of the video carrier
modulation ranging from 0 to 4 MHz. When the interfering carrier is
present, a replica of the video signal modulation of this interfer-
ing carrier is provided in a narrow frequency band about 0 MHz and,
in modified form, at 4.5 megahertz (i.e., the second harmonic of the
difference between the interfering and video or audio carrier
frequencies). The replicas of the modulated interfering carrier and
audio carrier signals which are both at 4.5 megahertz compete for
the FM channel, but the modulated interfering carrier replica,
having a greater energy content, will capture the channel and will
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1~5~;~86
1 cause the normal audio signal to be suppressed as noise. It has
been found that a modulated interfering carrier replica which is 6dB
above the modulated audio carrier replica is sufficient to capture
the FM channel.
In a similar manner, the video and interfering carrier
signal replicas at 0 MHz compete for the video channel.
The interfering carrier amplitude modulation obliterates the normal
video signal, probably, by virtue of two effects. First of all, the
additional energy provided by the interfering carrier causes the
automatic gain control of the receiver to drop the gain of the
receiver down to a level where the reception of the normal video
signal is unsatisfactory. In addition, the horizontal synchronizing
pulses in the interfering carrier video signal (which are slightly
off the standard sync pulse rate) tend to lock the synchronization
circuits of the receiver to the interfering signal, so that the
normal video signal appears to be out of synchronization. Thus,
when the video portion of the interfering carrier captures the
receiver, the normal video signal is not discernible, but merely
appears as low-level background noise.
In an attempt to avoid the scrambling effect of the
interfering carrier, for example on the video portion of the
normal program, the receiver would be detuned so as to shift
wave shape 26 to the left. In the absence of a security carrier,
a conventional television receiver is capable of being detuned
sufficiently to make the righthand one of notches 28 coincide with
frequency fO, so that the interfering carrier is substantially
attenuated by the notch and is prevented from interfering with
picture reception. Detuning also removes the color carrier
(not shown), which is intermediate the interfering and audio
carriers, and prevents reception of sound. However, sound
~151286
1 could be received on a second receiver by detuning that receiver to
shift wave shape 26 to the right. Thus, an unauthorized party could
receive a complete subscription program including sound and black-and-
white picture by embodying two receivers.
Providing the security carrier at frequency fS prevents
unauthorized reception of the subscription program. Should a
receiver be detuned 50 as to shift waveshape 26 to the left with
respect to the carrier, the security carrier passes out of the
lefthand one of notches 28 into the reception band and is received
together with the modulated video carrier. When the righthand
notch is shifted to fO, the security carrier is completely
unattenuated and, by enhancing the total energy in the received
signal, causes the automatic gain control of the receiver to
reduce the gain to a level at which the reception of the normal
video signal is unsatisfactory. It has been found that a security
carrier which has a peak amplitude as much as 12dB below the video
carrier, will provide a sufficient blocking effect. In addition,
the security carrier may be amplitude modulated with a video
interference signal (described in detail below) which produces a
moving pattern on the receiver picture that is extremely annoying
to a viewer.
Referring now to Fig. 2, there is shown an arrangement
for producing the composite RF signal 10 illustrated in Fig. 1.
A replacement program, including separate audio and video signals,
is provided from a conventional source. As is common in con-
ventional television transmitters, the replacement audio signal
is supplied to an audio preemphasis network 30 prior to being
utilized for modulating a carrier. The preemphasized audio signal
is applied to a conventional frequency modulator 32 via lead 34 and
is used to frequency-modulate a carrier signal of frequency fO,
which is supplied to modulator 32 from precision oscillator 36
via lead 38. The frequency-modulated carrier from modulator 32 is
provided via lead 40 as the carrier input to a conventional amplitude-
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1 modulator 42. The replacement video signal serves as the modulating
signal for amplitude-modulator 42 and is coupled thereto through
a conventional low pass filter 44 and lead 46. Low pass filter
44 is designed to have an upper cutoff frequency substantially
lower than is common in the video signal of conventional television
systems, so that the spectrum for the amplitude-modulation of
the interfering carrier will have a relatively narrow bandwidth, as
previously explained. The relationship between the bandwidth of
filter 44 and other components in the system is discussed more fully
below. In the preferred embodiment, filter 44 has a half-power
bandwidth of about 30 KHz. By virtue of the portion of the arrange-
ment of Fig. 2 described thus far, a carrier of frequency fO is
produced on lead 48, which carrier is frequency-modulated with
replacement audio information and is amplitude-modulated with
replacement video information.
The output signal from amplitude modulator 42 is coupled
to a conventional variable attenuator 50 via lead 48, and is provided
therefrom to an RF combiner 56 via lead 52 and RF switch 54. The
normal television program signal is provided in radio frequency form
(for example from a televison transmitter), and is coupled via lead
58 to RF preemphasis network 60, in order to produce a sharp peak in
the normal program signal in the vicinity of the interfering carrier
frequency fO. The preemphasized normal program signal is coupled
from RF preemphasis network 60 to RF combiner 56 via lead 62. In
operation, variable attenuator 50 is conveniently adjusted to
control the magnitude of the interfering carrier relative to the
video carrier so as to insure that the replacement signal captures
the video and audio portions of a television receiver.
An externally applied remote on/off signal is provided
on lead 66 and is coupled as a control signal to RF preemphasis
network 60 and RF switch 54. With the remote on/off signal in
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~l15~;~86
1 its "on" condition, the RF switch is rendered conductive and the
normal program signal is passed through RF preemphasis network 60,
so that the composite output signal 10 produced on lead 64 includes
an interference carrier modulated with substitute program informa-
tion. With the remote on/off signal in the "off n condition, RFswitch 54 is open and the normal program signal is made to bypass RF
preemphasis network 60, so that the normal program signal appears on
lead 64 and the interfering carrier is absent.
Security carrier 25 is generated by means of a conventional
oscillator 80 and is provided to a conventional amplitude modulator
82 via lead 84. In addition, a video interference signal from a
generator 86 is provided to modulator 82 via lead 88 and is utilized
to amplitude modulate the security carrier. The modulated security
carrier is coupled to RF combiner 56 via lead 90, and RF combiner 56
combines the normal program signal, the modulated interfering carrier
and the modulated security carrier to produce, on lead 64, the
composite signal 10 depicted in Fig. 1.
In operation, the security carrier is always present, but
has no perceptible effect on operation when tuning is proper. The
remote on/off signal is normally maintained in its "on" condition,
so that the normal program is scrambled at an unauthorized receiver
and the replacement program is received instead. It might be
desirable, for example, to make the replacement program a promotional
presentation intended to entice a non-subscriber into subscribing to
the normal program. As an added enticement, at some time during the
replacement program, the remote on/off signal could temporarily be
placed in its "off n condition, thereby permitting the non-subscriber
to see a short segment of the normal subscription program which is
being transmitted. At the end of this segment, the remote on/off
signal returns to its ~on" condition, whereupon the signal received
at the non-subscriber's receiver is once more scrambled. As explained
above, when tuning is improper the security carrier acts to interfere
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~l~SJ~ 8~;
1 with picture reception.
Fig. 3 illustrates a preferred gain versus frequency
characteristic for RF preemphasis network 60 of Fig. 2. At frequen-
cies substantially below and above the interfering carrier frequency
fO, preemphasis network 60 has a substantially constant gain, such
as 3.5 dB, which compensates for losses due to combiner 56. The gain
characteristic achieves a sharp peak, such as 30db at the frequency
fO and has a 3db bandwidth, /\ F, approximately equal to .001 fO.
Networks for achieving such gain characteristics are well-known
in the prior art.
When composite television signal 10, as depicted in
Fig. 1, is received at an authorized receiver, it is necessary
that the interfering carrier and its modulation be removed from
the composite signal before the signal is introduced into the
receiver. Referring now to Fig. 4, it will be observed that the
interfering carrier and its modulation are effectively removed by
inserting a filter 70 ahead of an authorized receiver, so that the
composite RF signal 10 from the transmitter is conditioned by the
filter 70 prior to being applied to the authorized receiver.
In a system with permanent subscribers, each authorized receiver
would have a filter 70 permanently connected ahead of the receiver.
Unauthorized receivers would not have such a filter, and would
therefore receive a scrambled signal, together with the replacement
program. It may be desirable, however, to authorize reception of
subscription programs on a program-by-program basis. In such
instances, the composite RF signal is coupled to the filter 70
through a switch 72 which is controlled by a signal applied to it
via a lead 74, as will be more fully explained hereinafter. The
switch has two positions, designated as 1 and 2 in Fig. 4. In
position 1, the composite RF signal is coupled to the authorized
receiver through filter 70 as explained above, but in position 2 the
composite signal is bypassed around filter 70, so that the receiver
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.
~l15~36
1 receives the scrambled signal and replacement program.
The operation of switch 72 is conveniently controlled by
means of an auxiliary control signal which is injected in the
composite RF signal at a carrier frequency which is out of the video
and audio spectra. A control information receiver 76 is provided in
parallel with switch 72 to extract the control signal from the
composite RF signal, and the signal extracted by receiver 76 is
applied to a control information decoder 78. Decoder 78 interprets
the control signal to determine whether the associated receiver is
authorized to receive the particular program being transmitted.
Decoder 78 produces, on lead 74, a signal which controls the position
of switch 72 in accordance with whether the receiver is or is not
authorized to receive the signal being transmitted. Devices such as
switch 72, receiver 76 and decoder 78 are well-known in the art
and have been used in the manner described herein.
Filter 70 is a sharply tuned band-elimination filter,
commonly known as a "notch~ filter. Preferably, filter 70 conforms
generally to the frequency characteristic of Fig. 3 but is the
complement thereof (i.e. the preferred characteristic for filter 70
is obtained by considering the vertical axis of the characteristic
of Fig. 3 to be attenuation instead of gain) and includes a much
sharper peak at the frequency fO. When the frequency character-
istic of filter 70 is closely matched to the frequency characteristic
of RF preemphasis network 60 of Fig. 2, the interfering carrier 16
and its modulation are not only effectively removed, but the removal
is achieved with minimal distortion of the original video signal
spectrum. Thus, the preemphasis network 60 provides gain which
compensates for attenuation introduced by filter 70 when producing
normal programs. In the preferred embodiment, the attenuation of
filter 70 in the immediate vicinity of the frequency fO substan-
tially exceeds the maximum gain of preemphasis network 60, but the
"side" portions of the characteristics of filter 70 and network 60
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~15~;~136
1 are substantially identical. As a result the attenuation of filter
70 is only compensated at the side portions. This has the effect
of making the attenuation characteristic of filter 70 appear much
steeper to the normal television signal (i.e. only the uncompensated
portion affects the normal television signal). This helps assure
that the scrambling and signal substituting arrangement of the
invention does not appreciably degrade the normal subscription
television signal. Notch filters which satisfy the requirements of
filter 70 are well-known in the art and are commercially available.
The attenuation characteristic of low-pass filter 44 is
intimately related to the attenuation characteristic of filter 70.
The general purpose of low-pass filter 44 is to limit the spectral
content of the replacement video signal so that, when the inter-
fering carrier is modulated with the replacement video signal and
bandlimited by filter 44, no appreciable energy is produced outside
of the attenuation "notch" of filter 70. Modulation theory teaches
that the spectrum of the video modulated interfering carrier on
either side of the carrier frequency will be a replica of the
spectrum of the bandlimited replacement video signal. Consequently,
low-pass filter 44 should be désigned so that it produces no
appreciable energy beyond a frequency equal to half the width of the
"notch~ in filter 70. The minimum bandwidth of low-pass filter 44 is
selected in accordance with the requirement that the filter should
not appreciably attenuate the 15 KHz horizontal synchronization
signal. In the preferred embodiment, filter 70 has a maximum
attenuation of about 80dB and filter 70 and low-pass filter 44 are
designed so that all components of the video-modulated interfering
carrier are attenuated by at least 55 dB.
The production of the modulated security carrier will be
described with the aid of the waveform chart of Fig. 5. The
reference character for each waveform is indicated at the points
where that waveform appears in Figs. 2 and 6. Waveform fs is the
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~S128~;
1 output of oscillator 80 (i.e. the security carrier signal) and
appears on lead 84. Waveform A, which appears on lead 88, is the
video interference signal which is used to amplitude modulate
the security carrier. Waveform A is produced by generator 86
by alternately transmitting sinusoids fa and fb at a periodic
rate determined by the CONTROL waveform.
In the preferred embodiment, fa and fb are 4 Hz
and 12 Hz sinusoids, respectively, and the CONTROL waveform is a
lHz square wave. Although this combination of signals has been
found to produce an extremely effective and annoying interference
pattern, other combinations will also work well. Alternately,
the security carrier may be modulated by a single frequency or
may be unmodulated. Waveform B is the security carrier amplitude
modulated by waveform A and appears on lead 90. It has been
found that a good interference pattern can be produced when the
modulation extends 6-10 dB below the peak amplitude of the
security carrier signal.
Fig. 6 is a circuit schematic diagram for control signal
generator 86, which illustrates how waveform A may be generated.
Waveforms fa and fb are each applied as an input to one of
analog gates 92,94~ The CONTROL waveform is applied as the control
input to gate 92 via lead 96 and as the control input to gate 94
through an inverter 98. The output of each of gates 92,94 is
applied to lead 88. In operation, each of gates 92,94 becomes
transmissive when its control input is high, thereby passing its
input signal (waveform fa or fb to lead 88. When the control
input to the gate is low, it blocks transmission of its input
signal. It will therefore be appreciated that gates 92, 94
transmit on alternate half-cycles of the CONTROL waveform, so
that waveform A includes alternate bursts from the waveforms
fa and fb.
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~ ~`~
5~86
1 In broadcast television it is the usual practice not to
utilize channels in a common area channels which are adjacent in
frequency. However, in cable television systems, for example,
channels which are adjacent in frequency are utilized and a multi-
channel RF signal is usually proviZed. It then becomes possible to
utilize ~he audio carrier signal of a lower adjacent channel as the
security carrier for the channel being tuned in. Fig. 7 is a
functional block diagram which can be used in combination with the
diagram of Fig. 2 to provide scrambling of a television program
while utilizing the lower adjacent audio carrier as a security
carrier. The multichannel RF signal is applied to a channel filter
100 (which passes only the signal of the selected channel), a lower
audio bandpass filter 102, and an elimination filter 104 (which
transmits the multichannel signal while blocking the selected
channel and the lower adjacent audio). The output of filter 102 is
applied to RF preemphasis filter 60 (Fig. 2) as the normal program
(RF) and the output of filter 104 is applied to modulator 82 (via
lead 84') as the interfering carrier signal. The arrangement of
Fig. 2 produces (on lead 64) the composite signal 10 for the selected
channel, and this signal is combined with the output of elimination
filter 104, in RF combiner 106, to produce a modified multichannel
signal on lead 108. This modified signal is like the original
multichannel signal except that the selected channel has a scrambled
signal in which the lower adjacent audio carrier serves as a modulated
security carrier. It has been found that the audio carrier can
be substantially amplitude modulated withDut causing a perceptible
adverse effect on lower adjacent channel sound reception.
Although specific embodiments of the invention have been
disclosed for illustrative purposes, it will be appreciated by those
skilled in the art that many additions, modifications and substitutions
are possible without departing from the scope and spirit of the inven-
tion as defined in the accompanying claims. For example, the invention
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~5~;~86
1 is not limited to conventional television systems which amplitude-
modulate video information on a video carrier and frequency-modulate
audio information on a audio carrier. Clearly, interfering carrier
16 and security carrier 25 could be modulated in any form which
would be recognized by the video and audio sections of a receiver in
a given television system in which scrambling, signal substitution
and security against circumvention by detuning are to be achieved.
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