Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
2695
1 Ti~e presellt invention relates generally to television
communicatioll 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 also permit the substitution of
replacement program materi.al for the subscription television
program.
As used herein, the term "subscription television system"
is intended to encompass all television communication systems in which
a television program signal, ~Ihich 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 televisi.on broadcasting, each transmitter
station is assigned a channel having a predeEined 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 oE the program. By convention (in the
United States), commercial televi.sion 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 convention, video i.norrnati.on is ampli.tude-modulated onto
the video carrier and audio information i.s frequency-modulated onto
the audio carrier.
In subscri.ption t.elevi.sion systems (for example, in modern
cable televisi.on systems), it is customary to withhold a program
from an unauthorized receiver or to provide the program, but to
condition the program signal so that it is unintelligible unless
received by an authorized subscriber. Various methods have been
~'
~ . .
6~5
suggested for scrambling a television signal to render it
unintelligible, including the injection into the television
signal of radio frequency (RF) spoiling signals or "tones" to
interfere with the reception of the signal.
In a technical paper entitled "Thick Film Technology
for Pay TV Security: The T.E.S.T. System", which paper was
presented by B. Becht at the 1976 Annual National Cable
Television Association Convention, it is suggested that a
modulated RF spoiling signal at a fre~uency intermediate the
video and audio carrier signals can be added to a
subscription television signal to scramble it. The spoiling
signal is amplitude-modulated with tones selected to
interfere with the video and audio information detected by a
conventional television receiver. Normal reception of the
subscription television signal (i.e., unscrambling) is
accomplished by extracting the amplitude modulated spoiling
signal ahead of the receiver.
It has also been suggested that, in addition to
spoiling tones, an auxiliary carrier signal, which is
modulated with promotional audio information, be in]ected
into the RF television signal in order to provide a
replacement audio signal while blocking reception of the
regular television program. For example, U.S. Patent
No. 3,202,758, issued to G. Brownstein on August 24, 1965,
suggests that a spoiling tone at a frequency below the
video carrier be inserted into the RF television signal
~122695
together with an auxiliary carrier signal modulated with
substitute audio information. The spoiling tone is provided
at a frequency that will cause maximum interference with the
video information and the auxiliary carrier signal is inserted
at 4.5 MHz above the spoiling tone. At a television
receiver, the spoiling tone and auxiliary carrier interact
to provide a replica of the substitute information at their
difference frequency of 4.5 MHz, which corresponds to the
intermediate frequency for sound detection in an "intercarrier"
type system. A major shortcoming of this method of
introducing substitute audio is that
. .
,
' '
~1~2695
1 it requires both a spoilins tone and a modulated auxiliary carrier.
Althougll methods and apparatus have been available for
blocking the unauthorized reception oE television programs whilè
permitting the insertion of a substitute audio or promotional
message, such methods and apparatus have a number of disadvantages
undesirable in equipment of this type. As in the aforementioned
Brownstein patent, a typical arrangement has required the insertion of
interfering signals at, at least, two different frequencies in order
to block the reception of both the video and audio portions of the
program. This proves disadvantageous because, at an authorized
receiver, a relatively complex and expensive~ sharply tuned, band-
elimination filter must be provided to remove each of the spoiling
tones. Where an auxiliary audio-modulated carrier signal is injected
together with at least one spoiling tone, further filtering is
lS required at an authorized receiver to prevent interference with the
regular television program~ In addition, prior art scrambling
arrangements have required that spoiling tones be injected in close
proximity to the video and/or audio carriers, in order effectively
to block the reception of the television signal at an unauthorized
receiver. At an authorized receiver, the band-elimination filters
which remove the spoiling tones may also remove some important
portions of the television signal and thereby reduce the signal-to-
noise ratio at the receiver.
Broadly, it is an object of this invention to block
the unauthorized reception of a subscription television program
while substituting promotional or other replacement material for
the television program. More speciEically, it is within the
contemplation of the preserlt invention to provide a method and
apparatus for scrambling or blocking both the audio and video
portions of a subscription television signal without the requirement
~ .
,. . , ., .. ~
~1;2Z695
1 of employing a plurality of spoiling signals at different RF carrier
frequencies and for permitting the introduction of a replacement
program signal which may have either or both audio and video
components.
It is another object of this invention to provide a
method and apparatus for scrambling a subscription television
signal with spoiling signals which permit ready and economical
removal of the spoiling signals at an authorized receiver without
introducing an excessive reduction in signal-to-noise ratio at the
receiver.
It is a further object of this invention to provide
a method and apparatus for blocking unauthorized reception of
subscription television signals while permitting the introduction
of a substitute television signal, which method and apparatus can
be employed in existing subscription television systems with a
minimum of modifications.
It is also an object of this invention to provide an
apparatus for blocking unauthorized reception of a subscription
television signal while permitting the introduction of a
substitute television signal, which apparatus is efficient,
convenient and reliable in use, yet relatively inexpensive and
simple in construction.
In accordance with one aspect of t:he present invention,
a single RF spoiling signal, serving as an interfering carrier, is
provided intermediate the audio and video carriers and is modulated
with a substitute television program signal. The frequency of
the interfering carrier is selected so that beat frequencies (i.e.,
sum and difference frequencies of this carrier with the other
carriers) which result from the non-linear characteristics of a
conventional television receiver, or harmonics of the beat frequen-
~i;22695
cies, which are similarly produced, will block reception
of the subscription television signal. The non-linear
receiver characteristics will also produce replicas of the
interfering carrier modulation in the bands of the
detected audio and video signals in the receiver, and these
replicas replace the blocked subscription television program.
At an authorized receiver, there is provided a band
elimination filter, or other frequency eliminating device such
as a phase-locked loop, which is tuned to the interference
carrier and is placed ahead of the receiver. As a result, the
interference carrier is removed prior to being applied to the
receiver so that the subscription television program can be
received in the usual manner.
In an illustrative embodiment demonstrating objects
and features of the present invention, the interfering carrier
signal is half way between the audio and video carriers, is
fre~uency-modulated with substitute audio information, and is
amplitude-modulated with substitute video information. At an
unauthorized receiver, the subscription television program is
blocked by the interaction in the receiver of the interfering
signal with the audio and video carriers of the program
signal, as explained above, and the substitute program, which
may for example be promotional material, is received. At
an authorized receiver, a filter is provided to remove the
modulated interfering carrier signal and the subscription
television program is received in the conventional manner.
..
: . ~
lZZ695
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 description of a presently preferred, but nonetheless
illustrative, embodiment in accordance with the present
invention, with reference being had to the accompanying
drawings in which:
.. . .
~lZ2695
1 Iig. 1 is a Lrequency plot sho~ing the spectrum of a
composite radio ~requency (RF) elevision signal modified in
accordance with the present invention;
Fig. 2 is a functional block diagram illustrating the
manner of modulating an interfering carrier with substitute program
information and combining the modulated interfering carrier 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 the RF preemphasis network incorporatecl in tlle preferred
embodiment of Fiy. 2; and
Fig. 4 is a functional block diagram illustrating the
equipment that is incorporated at an authorized receiver to
facilitate removal of the interfering carrier and the replacement
program signal in order to permit reception of the regular
subscription television program signal.
It has been found that, when an interference carrier
signal is injected into a conventional television signal at any of
a predefined set of frequencies internlediate the video and audio
carrier signals, the interfering carrier signal will not only block
normal reception of the conventional t:elevision signal, but, if
appropriately modulated, can provide a replacerllent program signal
for the conventional television sicJIlal~ This pnenomellon is believed
to res~lt from t~le non~ lear rcsponse o~ a conventional television
receiver to tne various carrier signals ~i.e. audio, video and
interfering carrier signals) present in the composite RF television
signal. A receiver i~aving a non-linear respollse produces beat
frequencies between various ~requency com~onents present in the
several RF signals applied to the television receiver and also
3~ produces harmonics of the beat frequencies. i~oreover, experimen-
__
~lZZ695
tation has sho~n that blocking o[ the normal television signal and
the substitution of a replacement signal will occur ~hen the frequency
difference between the audio and video carrier signals of the normal
television signal (i.e., 4.5 Mllz) 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 conventional "intercarrier" audio
detection system). ~ihen an interfering carrier signal of any of the
above-defined frequencies is appropriately fre~uency-modulated with
replacement audio signals, these audio signals will replace the
regular audio program signals and ~ill be reproduced by the receiver.
Substit~tion of replacement vicieo program material can be obtained
with the same interLering carrier frequencies by appropriately
amplitude-modulating the interfering carrier by the replacement
video program signals.
~eferring now to the details o~ the drawings, and in
particular to l~ig. 1, there is shown the frequency spectrum of a
composite RF television signal, indicatec] generally by the numeral
10, as modified in accordance with the E~resent invention. The
television signal is included in a channel ranging in frequency from
a lower bounding frequency fL to an u~)~?(?r boulldincJ frccluency ~1,
with a difference between fll and fL of G megahertz. For
example, if the signal 10 is beiny transmitted over channel 4~ fL
and fH are equal to 66 megahertz and 72 megahertz, respectively.
By convention (in the United States), a video carrier signal 12 is
provided at 1.25 meyahertz above l:~ alld an audio carrier signal 14
is provided at 5.75 megahertz above fL. rhus, for a channel 4
signal, the video and audio carrier signals are at 67.25 and 71.75
~ .. .
llZZ695
l megahertz, respectively. In addition to the conventional components
just 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. 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. ~30wever, the principles of the invention apply
equally well if fO is spaced from the video carrier by a frequency
which is an interger submultiple of 4.5 megahertz (preferably a low
submultiple), as previously explained.
As is customary in commercial television broadcasting,
the video carrier 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 waveEorm 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 scllematically 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
of Fig. 1, the interfering carrier is amplitude-modulated with
video information and frequency-mcdulated with audio information.
The spectrum of the video information, which is represented by
. , ... _ .. _, . .
~lZ2695
1 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 comp~site signal 10 repre-
sented in ~ig. 1, is applied to a conventional television receiver,
the non-linear characteristics of the receiver produce replicas of
the various rnodulateci carriers at frequencies equal to the sums and
~- i differences of the various carriers and harmonics trhereof. In the
absence of the interfering carrier (i.e., with a conventional
television signal), the modulated video and audio carriers are
non-linearly combined to provide a repli~ca of the audio carrier
modulation on an intermediate ca~rier 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 videosignal modulation of this interfering carrier is provided
in a narrow frequency band about 0 Ml-lz 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 interfc[ing 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 cause
the normal audio signal to be suppresseci as noise. It has been
found that a modulated interfering carrier replica which is 6dU
above the modulated audio carrier replica is sufficient to
capture the FM channel.
--10--
_,,,__ . .. , _ .
- ~lZZ69S
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~
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 conventional 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-
-- 11 ~
_, . . . . ~
.. ' ,'' ' ' ~ .
,
~22695
modulate a carrier signal of frequency fO, which issupplied to modulator 32 from precision oscillator 36
via lead 38. The fre~uency-modulated carrier from
modulator 32 is provided via lead 40 as the carrier input
to a conventional amplitude-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
~2;2695
1 44 is desiyneci ~o have an upprr cutof ~rcquency substant;.ally
lower than is common in the vidco siynal of conventi.onal television
systems, so that the spectrum for the amplitude-modulation of
the interfering carrier will have a relatively narrow bandwidth, as
previously exr~lained. The relationship between the bandwidth of
filter 44 and other cornponents in the system is discussed more fully
below. In the preferred ernbodiment, filter 44 has a bandwidth of
about 30 K~lz. By virtue of the portion of the arrangement of Fig.
2 described thus ~ar, a carrier of frequency ~O is produced on
.10 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 televisi.on program signal is provide~d 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 vicini.ty 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, and RF
combiner 56 combines the normal program si.gnal and the rnodulated
interfering carri.er to produce, on lead 64, the composite signal 10
depicted in Fig. 1. In operation, vari.able attenuator 50 is conveni-
ently adjusted to control the magnitude of the interfering carrierrelative to the video carrier so as to insure that the replacement
si.gnal car~turtes tht~ video and audio portions of a television receiver.
An externall~ al~pli.etl remot- tin/off si.ynal 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 siynal in
-12-
~lZZ695
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, as depicted in Fig. 1, is
produced on lead 64. With the remote on/off signal in the "off"
condition, RF switch 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.
In operation, 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"
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.
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
Eo and has a 3db bandwidth, ~ F, approximately equal to .001 fO.
Networks for achieving such gain characteristics are well-known
in the prior art.
-13-
~ ..
l~Z269S
1 When composite television signal 10, as depicted in
Fig. 1, is received at an authorized receiver, it is necessary
that the inter~ering carrier and its modulation be removed from
the composite signal be~ore the signal is introduced into the
receiver. Referring now to Fig. 4, it will be observed that the
interferiny carrier and its modulation are eEfectively removed by
inserting a filter 7~ ahead of an authorized receiver, so that the
composite RF signal 10 ~rom the transmitter is conditioned by the
filter 70 prior to being app~ied to the authorized receiver.
In a system with permanent subscribers, each authorized receiver
would have a filter 70 permanently connected:ahead of the receiver.
~nauthorized 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 autho~ize 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
receives the scrambled signal and replacement program.
- The operation o 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 in~ormation receiver 76 is provided in
parallel with switcll 72 to extract thc control siynal from the
composite ~F 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 tle associated receiver is
.
-14-
.. ~
- \
~ 95
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 characteristic 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 or filter 70 in the immediate vicinity of the
frequency fO substantially exceeds the maximum gain of
preemphasis network 60, but the "side" portions of the
characteristics of filter 70 and network 60 are substantially
identical. As a result the attenuation of filter 70 is only
- 15 -
, . .
~12Z695
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.
.
. .
~1~2695
1 The attenuation characteristic of low-pass filter 44 is
intimately related to the attenuation characteris~ic 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. ~lodulation 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 designed 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 ~Odb and filter 70 and low-pass filter 44 are
designed so that all components of the video-modulated interferiny
carrier are attenuated by at least 55 db.
~O 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
is not limited to conventional television systems which amplitude-
modulate video information on a video carrier and frequency-modulate
audio inEormation on a a~dio carrier. Clearly, interfering carrier
16 could be modulated in any form which would be recognized by the
video and audio sections of a receiver in a television system in
which scrambling and signal subs-titution are to be achieved.
-16-