Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
The present invention relates to a method and system
for scrambling transmitted information and, more particularly,
to a method and system for scrambling and unscrambling the
transmissions of information services such as subscription
television to prevent the unauthorized use of such services.
The secure transmission of information has become
extremely important, particularly in the fleld of television
transmission via cable, satellite and other media. Numerous
encoding or scrambling techniques have been developed and some
are now in use, particularly in broadcast systems where there
is no control over who receives the signals and the signals
must be encoded to prevent unauthorized use of the received
signals.
One known broadcast subscription television system
transmits a video signal that has been amplitude modulated
by a sine wave signal in such a way that the blanking and
synchronizing levels cannot be recognized by a normal television
receiver. Specifically, the video signal is modulated in amplitude
by a sine wave synchronized with the horizontal line rate of
the video. The sine wave amplitude modulates the synchronizing
signals and the video signal so that they are at the same level
other than that which the television receiver expects. Thus,
the television receiver synchronizes on something other than
the normal synchronizing signals. The resulting video
display on the normal television receiver is unintelligible or
at least very annoying to a viewer.
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While this method is relatively inexpensive and
effective, it can be easily defeated and cannot be made secure.
The sine wave i5 at a fixed rate and its amplitude modulation
effects can be relatively easily eliminated at the receiver by
unauthorized individuals. Since there is no way to vary the
scrambling, a fixed clrcuit arrangement which, in effect, revers~s
the amplitude modulationl defeats the securi~y of the system.
Other approaches to scrambling have proven more secure
but are typically more expensive due to their complexity. In
addition, certain other scrambling methods have affected
picture quality in an unacceptable manner, and attempts
to improve pictur~ quality have resulted in the lowering of the
level of security.
OBJECTS AND SUMMARY OF THE INVENTION
It is ~ccordingly an object of the present invention
to provide a novel method and system for scrambling information
signals for secure transmission thereof in a relatively inexpensive
and distortion free manner.
It is a further object of the present invention to
provide a novel television signal scrambling method and system
in which the scrambling is very effective, security is relatively
high, and cost i5 relatively low.
It is yet a further object of the present invention to
provide a novel method and system for scrambling both the audio
and video portions of a televislon sig~al for broadcast subscription
television services such as direct broadcast satellite (DBS) or
subscription television (STV) in which the scrambling is very
effective, and the scrambled signal can be unscrambled with
relatively low cost equipment and without noticable distortion.
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It is a more specific object of the present invention
to provide a novel method and system for scrambling information
signals for secure transmission thereof through the use of one or more
scrambling or jamming signalswhich may be varied in frequency at
the transmission end and yet effectively eliminated at the receiving
end.
It is yet another object of the present invention to
provide a novel television signal scrambling system and method
particularly suitable for broadcast television systems in which
one or more scrambling or jamming signals effectively scramble both
video and audio portions of the television signal and the jamming
signal or signals may be varied in a manner which provides relatively
high security.
In accordance with the invention, an informa~ion signal
having a regular, repeating information rate, for example, a
television or other video signal, is summed with one or more jamming
signals each having a frequency which is a function of the
information rate of the information signal plus an error quantity
within a predetermined bandwidth. The sum of the information
signal and the jamming signal is transmitted as the scrambled
information signal.
The scrambled information signal is received, and a jamming
signal (or signals) is locally generated at the receiver. The
locally generated jaming signal is identical in frequency and 180
out of phase with the jamming signal in ~he scrambled information
signal. The locally generated jamming signal is summed
with the received scrambled information signal and cancels the
jamming signal from the scrambled signal.
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In a preferred embodiment of the invention, the
information signal is a television video signal. The jamming
signal frequency is a function of a synchronization rate of the
video signal and an error quantity within the pass band of the phase
error circuitry within a normal television receiver synchronizing
circuit. The video signal is summed with the jamming signal
and the thus scrambled video signal is transmitted in a
conventional manner as in normal broadcast television or DBS.
To prevent over-modulation in a television transmission
system, the video signal may be preemphasized to lower the
amplitudes of signals in the lower end of the video frequency
spectrum, particularly the synchronizing pulses, prior to
summing with the jamming signal. Of course, in some information
transmission systems that may not be necessary.
In scrambling a television signal, the jamming frequency
preferably is selected as a function of the horizontal frequency
of the video signal plus an error value which is within the
pass band of the error amplifier of the horizontal synchronization
phase lock circuitry of a normal television receiver. The
amplitude of the jamming signal is relatively low, but it has
a serious effect on the horizontal phase lock circuitry in the
television receiver because it creates an error voltage in that
circuitry that results in horizontal displacement of the start
of scan lines both left and right of their normal starting position.
The scrambling effect is thus very significant. However, because
the jamming signal is preferably relatively low in amplitude~
e.g., less than about 20~ of the video signal amplitude,
linearity errors in transmission, typically about 1% in a
television system, introduce an insignificant error when the
jamming signal is eliminated. For example, a 100 milivolt
jamming signal undergoing a 1% linearity error may produce
artifacts of about 1 millivol-t peak-to-peak after elimination
of the jamming signal by the unscrambling circuit. ~ven
with some amplification, this value of error artifacts is
insignificant in relation to normal peak-to-peak video which
may be on the order of 700 millivolts. Thus, the error artifacts
do not create any visible problem in the quality of the unscrambled
video signal.
BRIEF DESCRIPTION OF ~HE DRAWINGS
The foregoing and other objects and advantages of the
present invention will become more apparent to one skilled in the
art to which the invention pertains from the following detailed
description when read in conjunction with the appended drawings
in which:
Figure 1 is a functional block diagram of one
embodiment of a scrambler or encoder in accordance with the
present invention;
Figure 2 is a functional block diagram of an
unscrambler or decoder for unscrambling the signals transmitted
by the encoder illustrated in Figure :;
Figure 3 illustrates an alternative embodiment of the
scrambler or encoder of Figure 1 in which security is increased
by modifying the jamming signal during the scrambling process; and,
Figure 4 is a functional block diagram illustrating
an alternative embodiment of the decoder or unscrambler of
Figure 2 suitable for unscrambling the signals produced by
the encoder of Figure 3.
Figure 5 is a functional block diagram of an-other
embodiment of a decoder in accordance with the present invention; and
Figures 6 and 7 are functional block diagrams of
another embodiment of an encoder and decoder for scrambling
and unscrambling television video signals without altering the
television sync signals.
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ETAILED DESCRIPTION
Figures l through 4 illustrate a scrambling and
unscrambling portion of a television system in which viewers
pay for services to which they subscribe. For example, the
system illustrated in the drawings may be used in a direct
broadcast satellite (DBS) system to scramble television video
and audio signals so that they may not be used by unauthorized
persons.
It should be noted that the scrambling and unscrambling
portions of the system of the present invention may be combined
with other features provided in DBS and STV systems, such as
addressing schemes, pay-per-view, teletext, etc. Particular
implementations of the present invention with existing information
transmission systems and known features of such systems will be
apparent to one skilled in the art and are therefore not
specifically disclosed herein in detail. Moreover, details
of known information transmission systems such as modulation and
broadcast schemes are not set forth herein since the use of
the present scrambling and unscrambling techniques wlth such
systems is relatively straightforward.
One embodiment of the present invention in a medium
level security, television transmission environment is
illustrated in Figures l and 2. Referring now to Figure l,
wherein a scrambler or encoder 10 in accordance with the invention
is illustrated, the scrambler receives unscrambled video and audio
signals from respective sources 12 and 14 as in any normal
television transmission system. Similarly, the scrambled
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output signals are provided to conventional transmission circuits
such as the illustrated rnodulators 16 and 18 and the transmitter
20. The scrambler 10 is thus interposed between the normal
video and audio sources and the normal transmitting equipment
in a conventional transmission system such as DBS, cable or STV,
and it will be appreciated that the input and output equipment
with which the scrambler is used need not be described further.
With continued reference to Figure 1, the scrambler
10 includes a conventional preemphasis circuit 22 through which
`the video signal from the video source 12 is routed. The Olltput
signal from the preemphasis circuit 22 is applied to a conventional
summing circuit 24 which receives a second signal from a conven-
tional signal generating circuit designated the jamming signal
generator 26. The output signal from the summing circuit 24
is the scrambled video signal and is applied to the modulator
16.
The audio signal from the audio source 14 is applied
to a conventional double balanced mixer 28 together with a jamming
signal from the jamming signal generator 26. The output
signal from the balanced mixer 28 is applied through a
conventional low pass filter 30 to the modulator 18 as the
scrambled audio signal.
In operation, the unscrambled video signal is
preemphasized by the preemphasis circuit 22 in accordance with
a function which attenuates the low frequency components of the
video signal and has a flat response to the high frequency
components. Thus, the synchronizing or sync signals (e.g.,
horizontal sync) and other low frequency components are
attenuated.
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The preemphasized video si~nal is summed with the jamming
signal by the summer 24. Since the video si~nal has been
preemphasized, those low frequency components which are normally
the highest amplitude are at a lower than normal level, and
the summed jamming signal does not raise the video signal to
a level that would drive the modulator beyond its normal
operating range (e.g., the modualtor is not overdriven into
a non-linear range). A preemphasis on the order of 6 db. has
been found to be acceptable for this purpose.
The jamming signal has a frequency Fj related to the
frequency of a video synchronizing signal plus an error quantity,
i.e., Fj = AFh + B when A is an integer, Fh is the frequency of
the video synchronizing signal and B is the error quantity. In
accordance with the preferred embodiment, that synchronizing
signal is the horizontal snyc signal FH and the error quantity
is a quantity within the pass band of the horizontal
synchronizing circuit in a normal television receiver. In
particular, the normal television receiver includes a phase
locked loop which generates the horizontal synchronizing signal
for the television set in response to the received video signal.
That phase locked loop contains an error amplifier which will
respond to frequency errors within a pass band of about 2500 Hz.
Thus, the error quantity B is within that pass band and preferably
a multiple of the television field rate.
In the embodiment illustrated in Figure 1, the jamming
signal frequency Fj is given by AFH + B where A is an integer which
remains constant. The error ~uantity B is preferably a function
of the field rate of the video signal which, in NTSC systems, is
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about 59.94 Hz. As will be appreciated by one skilled in the
art, the effect of the error quantity B on a normal television
receiver is to cause a displacement of the starting points of
the horizontal lines. With an error quantity s equal to the
field rate, one displacement cycle, both positive and negative,
will occur over the face of the television screen. If B is
equal to twice the field rate, then two displacement cycles
will occur and so no. Very effective scrambling can be
achieved using an error quantity B equal to about three to
five times the field rate.
The audio signal is scrambled by mixing it with a
signal having some predetermined relationship to the jamming
signal and transmitting the lower sidebald of the product of
the mixer. More specifically, the mixer 28 will output the
original input signals and their sum and difference. The
difference or lower sideband is the audio signal on a suppressed
carrier which is, in effect, the audio signal inverted about
some frequency determined by the frequency of the jamming signal.
For example if the signal applied to the mixer 28 is
Fj/D, where D is equal to A, then the audio signal will vary
in the normal baseband audio range (0-15 KHz) but the low
frequency audio will appear at the upper end of the range and
vice-versa. In a system where Fj is related to FH which is 15,730
Hz, the audio signal thus varies from 15.73 KHz to zero inversely
with the audio signal frequency. A 300 Hz audio signal would thus
be scrambled and appear as a 15.43 KHz signal in the output of
the low pass filter 30.
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Figure 2 illustrates a decoder or unscrambler for use
at a subscriber location to unscramble the scrambled video and
audio signals transmitted in accordance with the embodiment of
Figure 1. Referring now to Figure 2, the scrambled audio and
video signals are received and demodulated by a suitable conventional
receiver front end 32. The nature of the receiver front end will
vary, depending upon the nature of the transmitted signal. For
example, a conventional satellite receiver may be used in a
DBS system to provide the scrambled video and audio signals
SVID and SAUD at the baseband level. A receiver suitable for
conventional television frequencies may be used in an STV
broadcast system. Other types of receivers may be used in
cable systems.
The scrambled video signal SVID iS supplied to a
conventional deemphasis circuit 34 which deemphasizes the high
~requency components of the received signal. The deemphasis
circuit 34 is matched to the preemphasis circuit 22 of Figure 1
in the sense that it has a frequency crossover corresponding
to that of the preemphasis circuit and it deemphasizes or attenuates
the high frequencies by an amount comparable to the preemphasis
of the low frequencies by the preemphasis circuit 22. For example,
if the preemphasis circuit attenuates the low frequency components
of the video signal by 6 db, the deemphasis circuit preferably
attenuates the high frequency components of the received video
signal by 6 db.
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The output signal from the deemphasis circuit is
supplied through a conventional amplifier 36 to a conventional
summing circuit 38. The deemphasis circuit output signal is
also supplied to a phase locked loop circuit 40, the output
signal of which is applied to the summing circuit 38 and to a
conventional divider 42. The output signal from the divider 42
is applied to one input terminal of a conventional double
balance mixer 44.
The baseband scrambled audio signal SAUD is supplied
to the other input terminal of the balanced mixer 44. The
output signal from the balanced mixer 44 is applied through a
low pass filter 46 to conventional modulators 48. Similarly,
the output signal from the summer 38 is applied to the
modulators 48. The output signal from the modulators 48 is
supplied to the antenna terminals of a television set.
In operation, the receiver front end 32 receives the
transmitted scrambled television signal and provides baseband
video and audio signals SVID and SAUD. The deemphasis circuit
34 deemphasizes the high frequency components of the scrambled
daseband video signal and the amplifier 36 amplifies the signal
from the deemphasis circuit sufficiently to compensate for the
preemphasis and deemphasis at the transmitting and receiving end.
For example, with a 6 db preemphasis and deemphasis, the
amplifier 36 may amplify the signal from the deemphasis circuit
by a factor of 2.
The phase locked loop circuit 40 locks onto the
jamming signal Fj in the scrambled video signal and generates an
output signal having the same frequency as the jamming signal.
Sufficient phase shift is built into the phase locked loop 40
to provide a replica of the jamming signal frequency phase
shifted by 180 relative to the jamming signal in the incomming
scrambled video. The summation of the scrambled video signal
with this phase shifted replica of the jamming signal by the
summing circuit 38 therefore eliminates the jamming signal
from the scrambled video signal and produces a video output
signal VID in its unscrambled form. The modulators 48 merely
modulate an appropriate carrier with the video signal so that
the output signal from the modulators 48 is compatible with a
no~mal television receiver.
The output signal from the phase locked loop circuit 40
is also divided by the constant D by the divider 42 to produce
the signal Fj/D which is mixed with tne audio signal SAUD in
the double balanced mixer 44. The lower sideband is passed by
the low pass filter 46 and thus the output signal from the low
pass filter 46 is the unscrambled audio signal AUD. This
unscrambled signal is also applied to the modulators 48 to
produce a normal television signal compatible with a
television receiver.
- It will be appreciated that there may be some distortion
of the jamming signal in the transmission process due to
nonlinearities of the transmitter. As we previously mentioned,
however, the relatively low amplitude of the jamming signal
ensures that artifacts remaining after summing the locally
generated jamming signal with the scrambled baseband video will be
on the order of 1 milli~olt peak-to-peak and will be insignificant.
Morever, to match the amplitude of the locally generated
jamming signal to that contained in the received baseband signal,
suitable automatic gain control circuitry ~now chown) may be used
to control the amplitude of the jamming signal locally generated
by the phase locked loop 40.
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Figures 3 and ~ illustrate an embodiment of the
invention in which the security level of the scrambling is raised
by altering the frequency of the jamming signal Fj on a
periodic basis. In the Figures 3 and 4, in which like numerical
designations are used to indicate elements previously described
in connection with Figures 1 and 2.
In the Figure 3 embodiment, the scrambler or encoder
includes a scramble controller 50 which changes the frequency
of the jamming signal. The controller 50 may be, for example,
a psuedo-random number generator which, depending upon the value
of the generatednumber, determines the value of the constant A and/or
the error quantity s in the previously noted equation for
Fj.
In the embodiment of Figure 3, the scramble controller
50 alters the value of constant A. A control signal from
the scramble controller 50 is encrypted by a conventional
encryption circuit 52 and is inserted in the transmitted
video (or audio) signal by a suitable code insertion circuit
54. For example, the encrypted code may be inserted in the
vertical or horizontal interval so that the decoder or
unscrambler will be provided with some indication of how the
jamming signal is being varied at the transmitting station.
Encryption of codes and insertion thereof into video signals is
described in the prior art, for example, in U.S. Patent No.
4,398,216.
In addition or as an alternative to the frequency
"hopping" of the jamming signal as indicated above, the jamming
signal may be comprised of two or more jamming signals at
different frequencies which may remain constant or may vary in
response to the controller 50 outpu~ signal. Of course, if
multiple jamming signals of predetermined, unvarying frequencies
are used, there is no need to send a code with the television
signal as illustrated in Figure 3. The decoder or unscrambler
would merely be adapted to produce replicas of each jamming
signal for summation with the baseband scrambled video, e.g.,
through the use of multiple phase locked loops or multiple
dividers to produce the various jamming frequenciesO On the
other hand, if the multiple jamming signals are changed in
frequency, the encrypted codes would be necessary to inform
the decoder of the nature of the frequency changes so that
jamming signals of appropriate frequencies may be locally
generated.
The scrambled television signals from the encoder of
Figure 3 are received by the receiver 32 of Figure 4 and
converted to ~aseband as previously described. A suitable
code detector 56 detects the transmitted codes which contain
the jamming freguency information. These detected codes are
used to control the frequency of the output signals applied
to the summer 38 and the double balanced mixer 44.
More specifically, the phase locked loop 40 may
include a conventional phase detector 58, error amplifier 60,
loop ilter 62 and voltage controlled oscillator (VCO) 64.
In addition, a conventional divider 66 may be provided to divide
the VCO output signal by an appropriate value related to the
frequency of the jamming signal. This divider 66 may be
controlled in any suitable conventional manner to produce
an output signal of a proper frequency.
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A conventional synchronous syrnmetrical counter 68 further
divides the VCO output signal by a constant (e g., by 4) and
produces an output signal of the proper phase to cancel the
jamming signal in the scrambled video signal SVID. As was
previously mentioned, the desired phase relationship is 180
out of phase with the transmitted jamrning signal. Conventional
band pass filters 70 and 72, preferably delay equalized,
are provided both in the feedback circuit to the phase detector
58 and the output circuit to the summer 38.
The operation of the decoder of Figure 4 is identical
with that described in connection with Figure 2 except that
the frequency of the locally generated jamming signal may be
varied as a function of the encrypted code transmitted in the
television signal. That code is detected by the code
detector 56 and controls the frequency of the signal from the
dividers 66 and 42.
It should be noted that a potentiometer 74 is shown
in the output circuit of the phase locked loop 40 to permit
manual control of the amplitude of the locally generated jamming
signal applied to the summer 38. Such a control may be
automated as was previously mentioned. In addition, a suitable
conventional amplifier 76 is illustrated in the audio signal
path in the event that amplification of the unscrambled audio
signal AUD is necessary or desirable before modulation.
Figure 5 illustrates an embodiment of the video
channel of a decoder or unscrambler in which command signals are
inserted in the television program signal to permit control over
the programming available to individual subscribers. For example,
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category control or tiering signals identifying individual
programs or groups of programs may be transmitted in the program
signal and detected by a suitable code detector 80 as described
in greater detail in U.S. Patent No. 4,225,284.
The detected code may be compared by a conventional
comparator 8~ with a code stored in a category or tiering
controller 84. That stored code may be, for example, a code
indicative of a service for which the subscriber has paid or a
code indicative of a category or prograrnming the subscriber
wished to view. If the comparison is positive indicating that
the subscriber is authorized to use the incoming service and/or
has selected it, the comparator 82 causes a conventional
multivibrator or other gating signal generator 84 to produce a
gating signal which enables a gate 86 and passes the locally
generated jamming signal to the summer 38. Thus, the video
signal is unscrambled as long as the gate 86 remains enabled.
It may be desirable to avoid modifying the synchronization
signals in the video signal and merely add sufficient jamming
to ensure effective scrambling. One embodiment of an encoder
and a decoder to accomplish that objective is illustrated in
Figures 6 and 7.
Referring now to Figure 6, the unscrambled video signal
is supplied to the preemphasis circuit 22 in the encoder, to a
bypass gate 90, and to a sync detector 92. The output signal
from the gate 90 is applied to the summer 24, and the output
signal from the sync detector 92 is supplied to the control input
terminal of the gate 90 and through an inverter 94 to the control
input terminal of a gate 96. The jamming signal from the jamming
signal generator 26 is gated through the gate 96 to the summer
24 which provides the scrambled video signal SVID.
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In operation, the synchronizing slgnals in the
video are detected by the detector 92 and the sync signals are
bypassed around the preemphasis circuit 22 to avoid attenuation.
Of course, in a system in which deemphasis is unnecessary, e.g.,
in a satellite or FM system, the bypass ga~e 90 will be unnecessary.
The sync detector 92 also disables the gate 96 during
the synchronization period. Accordingly, the jamming signal
is not applied to the summer 24 during the sync period and the
synchronization signal appears unaltered in the scrambled
video signal SVID.
In the Figure 6 embodiment, the jamming signal is
preferably comprised of multiple frequencies to ensure effective
scrambling. Moreover, the video signal VID is preferably
inverted, although it is preferable not to invert the
synchronizing signals if they are used, for example, to trigger
parts of the system, for recording purposes or the like.
Referring now to Figure 7, the decoder for the signal
scrambled in accordance with the embodiment of Figure 6
removes the jamming signals as in the previous embodiments.
However, the synchronizing signals are detected by a sync
detector 98, and the outpu~ signal from the phase locked loop 40
is gated to the summer 38 by a suitable gate 100.
The gate lOO is normally enabled and i5 inhibited by
the output signal from the sync detectox 98. Accordingly,
the locally generated jamming signal is blocked from reaching
the summer during the synchronization period. Thus, the
sync signals pass through the decoder unaltered and appear
in their original form in the unscrambled video signal VID.
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The principles, preferred embodiments and modes of
operation of the present invention have been described in the
foregoing specification. The invention which is intended to be
protected herein, however, is not to be construed as limited
to the particular forms disclosed, since these are to be
regarded as illustrative rather than restrictive. Moreover,
variations and changes may ~e made by those skilled in the
art without departing from the spirit of the present invention.
WHAT IS CLAIMED I S:
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