U.S. patent number RE35,078 [Application Number 08/137,582] was granted by the patent office on 1995-10-31 for method and apparatus for encrypting and decrypting time domain signals.
This patent grant is currently assigned to Macrovision Corporation. Invention is credited to John O. Ryan.
United States Patent |
RE35,078 |
Ryan |
October 31, 1995 |
Method and apparatus for encrypting and decrypting time domain
signals
Abstract
A technique for encrypting and decrypting information signals
normally arranged as a succession of lines of active information,
with each line having a line timing reference, such as color video
information signals. The active video portion is time shifted with
respect to the horizontal sync portion of the corresponding line
using a predetermined slowly varying time shifting function. The
time shifting information is conveyed to the decryption site by
encoding the instantaneous value of the time shifting wave form for
the beginning of each field in the vertical blanking portion of
that field. To provide a reasonable maximum time shifting range,
portions of the trailing edge of the active video in the preceding
line and portions of the leading edge of the active video in the
current line are discarded. During decryption, the original line
timing and color burst signals are discarded and new signals are
generated which are time displaced from the active video portion by
the original amount before encryption.
Inventors: |
Ryan; John O. (Cupertino,
CA) |
Assignee: |
Macrovision Corporation
(Mountain View, CA)
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Family
ID: |
23596059 |
Appl.
No.: |
08/137,582 |
Filed: |
October 15, 1993 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
403514 |
Sep 6, 1989 |
05058157 |
Oct 15, 1991 |
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Current U.S.
Class: |
380/218; 380/35;
380/240 |
Current CPC
Class: |
H04N
7/1693 (20130101) |
Current International
Class: |
H04N
7/169 (20060101); H04N 007/167 () |
Field of
Search: |
;380/10,11,15,20,35 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Swann; Tod R.
Attorney, Agent or Firm: Brill; Gerow D.
Claims
What is claimed is:
1. A method of decrypting previously encrypted information signals
to permit use thereof, said encrypted information signals
comprising encrypted versions of original information signals
arranged as a succession of lines of active information, each line
having a line timing reference, said encrypted signals having been
produced by time shifting at least some of the lines of the
original information signals with respect to the line timing
reference in a predetermined manner .[.with the amount of time
shifting in each line in the succession being different than that
of the previous line.]., said decrypting method comprising the
steps of:
(a) providing an indication of the amount of time shifting
performed on a given line; and
(b) restoring the original time relationship for each line between
the line timing reference and the line of information using the
indication of the time shifting by generating deliberately
temporally misaligned line timing reference for each line and
combining said deliberately temporally misaligned line timing
reference with the line of information.
2. The method of claim 1 wherein said encrypted information signals
are video information signals containing line synchronization
portions which were not time shifted during the encryption process;
and wherein said step (b) of restoring includes the step of
deleting line synchronization portions which have been received and
inserting a new line synchronization portion.
3. The method of claim 1 wherein said encrypted information signals
are video information signals containing color reference signal
portions which were not time shifted during the encrypting process;
and wherein said step (b) of restoring includes the step of
deleting color reference signal portions which have been received,
generating a new reference color signal portion and combining said
new color signal portion with the line of information.
4. The method of claim 1 wherein said encrypted information signals
are video information signals containing a plurality of lines of
non-active video which were not time shifted during the encryption
process; and wherein said step (b) of restoring includes the step
of leaving the non-active video lines undisturbed.
5. The method of claim 1, wherein the step of restoring leaves a
time base error between the decrypted information signal and a
predetermined timing reference, said time base error being
correctable by a television receiver.
6. A system for decrypting previously encrypted information signals
to permit the use thereof, said encrypted information signals
comprising encrypted versions of original information signals
arranged as a succession of lines of active information, each line
having a line timing reference,- said encrypted signals having been
produced by time shifting at least some of the lines of the
original information signals with respect to the line timing
reference in a predetermined manner .[.with the amount of time
shifting in each line in the succession being different than that
of the previous line.]., said system comprising:
means for providing an indication of the amount of time shifting
performed on a given line; and
means for restoring the original time relationship between the line
timing reference and the line of information by using the
indication of the amount of time shifting, said restoring means
including means for generating a deliberately temporally misaligned
line timing reference for each line and means for combining said
deliberately temporally misalignd line timing reference with the
line of information.
7. The system of claim 6, wherein the means for restoring leaves a
time base error between the decrypted information signal and a
predetermined time reference, said time base error being
correctable by a television receiver.
8. The system of claim 6, wherein the line timing reference for
each line is temporally misaligned by the amount of timeshifting in
the encrypted version of that line.
9. A system for encrypting information signals to prevent
unauthorized use thereof, said system receiving original
information signals arranged as a succession of lines of active
information, each line having a line timing reference,
comprising:
means for timeshifting the active information signal in each line
relative to the line timing reference of that line, wherein the
active information signal in each line is timeshifted by a
predetermined .[.about different from the amount of timeshifting of
the previous line in the succession of lines.].; and
means for inserting into the information signals an indication of
the mount of time shifting;
wherein a maximum amount of timeshifting of one line relative to
any other line in the succession of lines is a predetermined about,
and wherein the encrypted information signals are decryptable by a
receiver, and a rate of change in the amount of time shifting
between successive line is within a capture range of
synchronization circuitry in the receiver.
10. The system of claim 9, wherein the indication of the amount of
time shifting is encrypted into data of predetermined length.
11. The system of claim 9, wherein the original information signals
are NTSC standard television signals, and the predetermined maximum
amount of timeshifting is 4 microseconds.
12. The system of claim 9, wherein the indication of the amount of
timeshifting is inserted into an unused line in the vertical
blanking interval in encrypted form.
13. The system of claim 9, wherein the indication is inserted into
a portion of one line other than the active information
portion.
14. The system of claim 9, wherein the amount of time-shifting of
successive lines varies as a sinusoidal function.
15. The system of claim 9, wherein the amount of time-shifting of
successive lines varies at a rate of no more than about 20 cycles
per second.
16. A system for decrypting previously encrypted video signals to
permit the use thereof, said encrypted video signals including time
shifting of the active video portions of at least some of the lines
of the video signals with respect to the synchronization portion of
the lines in a predetermined manner .[.with the amount of time
shifting in each line varying from that of the previous line.].,
comprising:
means for receiving the video signal and extracting therefrom one
predetermined line from a video field;
means for receiving a decrypting key and the extracted line and
providing therefrom time shift data relative to the television
signal;
means for processing the time shift data;
means for generating synchronization and color burst signals in
response to the processed time shift data for each line; and
means for receiving the video signals and also receiving the
generated synchronization and color burst signals, and for removing
original synchronization and color burst signals present in the
video signals and substituting the generated synchronization and
color burst signals at locations which are temporally misaligned by
the amount of timeshifting in each line in the video signals as
determined by an output of the means for processing for each line
which has been time shifted.
17. A system for decrypting previously encrypted information
signals to permit the use thereof, said encrypted information
signals comprising encrypted versions of original information
signals arranged as a succession of lines of active information,
each line having a line timing reference, said encrypted signals
having been produced by time shifting at least some of the lines of
the original information signals with respect to the line timing
reference in a predetermined manner .[.with the amount of time
shifting in each line in the succession being different than that
of the previous line.]., said system comprising:
means for providing an indication of the amount of time shifting
performed on a given line; and
means for restoring the original time relationship between the line
timing reference and the line of information by using the
indication of the amount of time shifting, said resorting means
including means for generating a temporally misaligned line timing
reference for each line which is temporally misaligned by the
amount of timeshifting in the encrypted version of that line, and
means for combining said temporally misaligned line timing
reference with the line of information.
18. A method for encrypting original information signals to prevent
unauthorized use thereof, said original information signals being
arranged as a succession of lines of active information, each line
having a line timing reference, the method comprising the steps
of:
timeshifting the active information signal in each line relative to
the line timing reference of that line, wherein the active
information signal in each line is timeshifted by a predetermined
.[.about different form the amount of timeshifting of the previous
line in the succession of lines.]. .Iadd.amount.Iaddend.; and
inserting into the information signal an indication of the amount
of time shifting;
wherein a maximum amount of timeshifting of one line relative to
any other line in the succession of lines is a predetermined
amount, and wherein the encrypted information signals are
decryptable by a receiver, and a rate of change in the amount of
time shifting between successive lines is within a capture range of
synchronization circuitry in the receiver.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
This invention is related to the invention disclosed and claimed in
copending commonly assigned U.S. Patent Application Ser. No.
203,676, filed June 7, 1988 and U.S. Pat. No. 4,916,736 for "Method
and Apparatus for Encrypting and Decrypting Time Domain Signals",
the disclosure of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
This invention relates to signal processing of time domain
electronic signals, such as video information signals. More
particularly, the invention relates to techniques for encrypting
and decrypting such signals to prevent unauthorized use
thereof.
Many techniques have been devised for encrypting and decrypting
time domain information signals. The purpose for such techniques is
always the same: viz., to prevent unauthorized use of the signals.
In the case of video type information signals, the unauthorized use
to be prevented is normally the visual display of the information
signals for their entertainment or instructional value. Such
signals are vulnerable to unauthorized use in a number of ways. For
example, if the video signals are being broadcast over a satellite
or microwave link, unauthorized users attempt to intercept the
signals, and view same without paying for the subscription service.
In an effort to defeat such unauthorized uses of broadcast video
information, several specific signal scrambling techniques have
been successfully used.
Another means of conveying video information from one location to
another is through the medium of video tape. For example, it is
quite common for motion picture studios to send master videotapes
of movies around the world. If the videotapes get stolen or "lost"
in transit, a clear opportunity for piracy exists. It is therefore
desirable to be able to scramble the video signal prior to
recording it on videotape so that the tape can only be utilized by
a user having a descrambler and appropriate codes. Such a
scrambling system must have two important characteristics--it must
be very secure and it must be compatible with the record/replay
electronics of preferably all professional and consumer grade video
recorders.
There are many known ways for scrambling video signals. Two simple
techniques are sync suppression and sync inversion, each of which
can, however, be readily defeated by using elementary video signal
processing techniques and in any case cannot be recorded. Another
technique is termed pseudo-random video-level invention, which is
relatively difficult to defeat but which suffers from the
disadvantage of a severe loss of picture quality due to
non-linearities in the record/playback process. Still another
technique is line-order interchange, also known as line shuffling,
in which the order of the lines in the raster scanned picture is
shuffled. As an example, instead of transmitting the lines
sequentially as line number 1, line number 2, line number 3, . . .
etc., the information might be transmitted as line number 182, line
number 99, line number 4 . . . , etc. Such a system can be made
very secure (i.e., very difficult to defeat), but it cannot be used
in any videotape format employing the color-under principle which
relies upon line adjacency to obtain correct color rendition upon
reproduction.
Still another technique is pseudo-random line rotation in which
some of the lines of the picture selected in random fashion are
transmitted in inverse temporal order (i.e., right to left), while
the remainder are transmitted in the normal fashion (i.e., left to
right). Yet another technique is termed line segmentation with
pseudo-randomly chosen break points, in which each line is broken
into two randomly chosen segments and the segments are sequentially
transmitted with the right hand segment being transmitted first,
followed by the left hand segment. Both of these video signal
scrambling methods give rise to severe color contamination between
the left and right hand sides of the picture when employed on any
format which uses color-under recording.
In addition to the above disadvantages, the last three noted
techniques suffer from the further disadvantage that the processing
is incompatible with the drop out compensation signal processing
employed in most video recorder devices. While, in principle, these
three methods could be used for video signal processing formats
which do not employ color-under recording, such as professional
type B and type C one inch formats, such a use would require
special drop out compensation circuitry in which drop out sensing
and correction are controlled by the descrambling system. This
would require special modification of playback equipment, which
adds undesired cost and complexity to an encryption/decryption
system.
None of the above-described video scrambling techniques fully meets
the desired requirements for a video scrambling system in which (1)
the scrambled video can be recorded and subsequently replayed on
any video tape format--professional or consumer--and be descrambled
on replay, with negligible loss of picture quality; (2) the
scrambling technique is virtually impossible to defeat by any
unauthorized user; and (3) the scrambled video is unaffected by
passage through the various kinds of processing equipment used in
television production facilities, satellite links and cable
networks.
In the above-referenced copending U.S. Patent Application Ser. No.
203,676, now U.S. Pat. No. 4,916,736 a method and apparatus are
disclosed which provides a highly secure video type information
signal encryption and decryption technique which is compatible with
all video tape formats and transmission systems and is free of
picture impairments caused by the interaction of the scrambling
algorithm and the chrominance consecutive--line averaging--system
used in color-heterodyne recording. According to the method
disclosed therein, video type information signals are encrypted by
individually time shifting the active video portion of at least
some of the lines of the video signals with respect to the line
timing reference (horizontal sync in an NTSC encoded system) and
providing an indication of the time shifting performed in order to
enable subsequent decryption. For color video information signals,
time shifting is inhibited during the horizontal sync signal
portion and the color reference signal portion. Similarly, the
non-active video portions of a field or frame of information (i.e.,
the vertical blanking portions) are not time shifted. Decryption of
the encrypted signals is accomplished by using a process which is
the inverse of the encryption process. For optimum results, and in
order to ensure compatibility between the encryption method and
other conventional signal processing techniques (in particular the
color-heterdodyne system of video cassette recorders) the amount of
time shift between adjacent lines is preferably limited to .+-.N
subcarrier cycles where N is a whole number (preferably 0 or 1). In
addition, the maximum aggregate time shift of the active video is
limited so that the active video does not overlap either the color
burst or the horizontal sync reference portions of the individual
lines.
While the above encryption/decryption technique is highly
effective, optimal implementation requires digital video circuitry
at the decryption site (i.e., the t.v. monitor or receiver) of some
complexity, which adds substantial cost to the total system.
SUMMARY OF THE INVENTION
The invention comprises a method and apparatus for providing a
highly secure video type information encryption and decryption
technique which is compatible with all video tape formats and
transmission systems, is free of picture impairments caused by the
interaction of the scrambling algorithm and the chrominance
consecutive line averaging systems used in color-heterodyne
recording, and which can be implemented at substantially lower cost
than the system previously described.
From a method standpoint, the invention includes the encryption of
information signals normally arranged as a succession of lines of
active information, each line having a line timing reference, the
method comprising the basic steps of individually time shifting the
active information portion of at least some of the lines of the
signals with respect to the line timing reference portion, and
providing an indication of the time shifting performed in the time
shifting step in order to enable subsequent decryption. For
information signals which are video information signals containing
line sync signal portions and color reference signal portions, the
time shifting is performed on the active video with respect to both
these timing portions of the individual lines. The non-active
portions of a field or frame of information, i.e., the vertical
blanking portions, are not time shifted.
The type of time shifting performed may comprise any one of a
number of slowly varying functions, such as a sinusoidal wave form
or a linearly changing ramp signal. The rate of change in the
signal should be relatively slow when compared to the line rate of
the input signals to be processed. For video type signals, a
sinusoidal wave form having a frequency of no more than about 20 Hz
is suitable, while for a linearly varying ramp signals a slew rate
of comparable rate is suitable. The absolute amount of time
shifting performed is preferably limited to a maximum value which,
in the case of NTSC video signals, does not exceed a total of 4
microseconds (.+-.2 microseconds in each direction).
The instantaneous value of the time shifting wave form function at
the beginning of each field is conveyed along with the field
information, typically during the vertical blanking interval. For
example, with respect to a sinusoidal time shifting function the
starting amplitude of the wave form during a given field is
transmitted during the vertical blanking interval as a single byte
of information which, when combined with a separately provided
authorization key, enables a descrambling circuit to synthesize the
scrambling wave form function. Decryption is the inverse of the
encryption process and is performed by restoring the original
timing relationship between the horizontal sync (and color burst)
and the active video portion of the corresponding line. This is
done by generating new line timing reference signals (horizontal
sync and color burst) which bear the same timing relationship to
the active video portion as the original line timing reference
signals before encryption. The resulting descrambled signals still
contain time base errors, but the errors are within the capture or
correction range of the follow-on television monitor/receiver.
The invention can be implemented using mostly conventional analog
circuits for the descrambling device, which makes the descrambling
devices economical to manufacture and easy to repair.
For a fuller understanding of the nature and advantages of the
inventions, reference should be made to the ensuing detailed
description taken in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram illustrating one line of video type
information to which the invention applies;
FIGS. 2A-2C are schematic diagrams illustrating the visual effect
of a sinusoidal time shifting on three successive fields of
information;
FIGS. 3A-3B are schematic diagrams illustrating the scrambled and
descrambled signals;
FIG. 4 is a block diagram of a scrambler unit;
FIG. 5 is a more detailed block diagram illustrating portions of
the video input processor, video output processor and sync/timing
generator of the FIG. 3 scrambler unit;
FIG. 6 is a series of wave form diagrams illustrating selected wave
forms from the FIG. 4 block diagram;
FIG. 7 is a block diagram of a portion of the controller 34 used to
generate the time shifting wave form;
FIG. 8 is a block diagram illustrating a descrambler unit;
FIG. 9 is a series of wave form diagrams illustrating selected wave
forms from the FIG. 6 block diagram; and
FIG. 10 is a block diagram illustrating the descrambler wave form
synthesizer.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The underlying principle of the invention can be best understood
with reference to FIGS. 1, 2A-2C and 3A-3B. FIG. 1 illustrates one
line of NTSC video information, with the active video portion of
the line compressed along the horizontal scale. As seen in this
Figure, one line of active video, which extends between the leading
edge of the horizontal sync signals of adjacent lines, includes a
color burst reference signal portion followed by active video. The
leading edge of the horizontal sync pulse precisely defines the
beginning of the line and serves as a line timing reference.
According to the invention, the active video portion of a line is
time shifted with respect to the active video portion of other
lines in a predetermined manner. For example, the normal position
of the active video is illustrated in FIG. 1. During encryption,
this position is time shifted in either an advance direction (i.e.,
closer to the horizontal sync portion of the given line) or in a
delay direction (i.e., toward the horizontal sync portion of the
next succeeding line). In order to preserve most of the active
video in each line, maximum limits are placed on the relative and
total amount of time shifting in the advance and delay directions.
In the preferred embodiment, for NTSC video this amount is .+-.2
microseconds (a total of 4 microseconds).
The manner in which the time shifting is performed is determined in
advance and must be relatively slow with respect to the line rate
of the information signals in order to permit the signals to be
properly processed after descrambling, as described more fully
below. Many different types of wave form functions may be used to
control the amount and direction of time shifting. Examples of such
wave forms are a sinusoidal wave form, rectangular waves, ramps,
and low frequency random or pseudo random noise signals. Other
appropriate time shifting functions will occur to those skilled in
the art. It has been empirically determined that a practical
maximum rate on the time varying wave form used to control the time
shifting is about 20 Hz for presently equipped television monitors
and receivers equipped for NTSC signal processing.
FIGS. 2A-2C illustrate in schematic form the visual effectiveness
of the invention on an image when a relatively slowly varying
sinusoidal time shifting is performed on the active video portions
of the video information signals. In these Figures, the rectangular
outline illustrates the entire field of the raster (including the
non-viewable portions of each line), and the vertical dotted lines
represent the normal position of the beginning of the viewable
portion of each line. The curved solid lines illustrate the manner
in which the image is distorted during three successive fields
using a slowly varying sinusoidal time shifting wave form. This
level of distortion is sufficient to remove the entertainment value
from a picture. It should be understood that the magnitude of the
time shifting illustrated in FIGS. 2A-C is greatly exaggerated for
illustrative purposes.
FIGS. 3A and 3B illustrate the manner in which the scrambled or
encrypted signals are decrypted or descrambled at the reception
site. With reference to FIG. 3A, three successive lines of NTSC
video are shown which have been time shifted successively by
increasing mounts. As with the FIG. 1 diagram, the active video
portions of each of the lines in FIGS. 3A. and 3B are only
fractionally illustrated. The topmost line represents a line N
having had no time shifting between the active video portion, and
the time between the beginning of the horizontal sync portion and
the active video portion is designated as t.sub.1. The next line,
line N+1, has undergone time shifting in the delay direction so
that the time between the beginning of the horizontal sync portion
and the beginning of active video portion is t.sub.2, greater than
t.sub.1. Line N+2 has undergone even more time shifting in the
delay direction by an amount labelled t.sub.3 greater than t.sub.2.
These three successive lines could represent lines from the upper
portion of the raster image schematically depicted in FIG. 2A. It
is important to note that the line timing reference part of each of
the lines N, N+1 and N+2 are all temporally aligned: the leading
edge of the horizontal sync portion of each line is exactly aligned
with the leading edge of the horizontal sync portion of the other
lines. The same is true of the location of the color burst
portions. The active video portions, however, are deliberately
mis-aligned in lines N+1 and N+2 with respect to line N.
FIG. 3B illustrates the signals for the same three lines after
descrambling or decrypting. As can be seen in this figure, the
leading edges of the horizontal sync portions of the three lines
are no longer precisely aligned, but are rather staggered: however,
the distance between the leading edge of the horizontal sync
portion and the beginning of active video is the same for all three
lines, viz., the value t.sub.1. Similarly, the color burst portions
of the three lines are no longer temporally aligned, but are rather
staggered in the same fashion as the horizontal sync portions. The
relative positioning of the active video portion of the three lines
remains the same.
Although the descrambled signals are still relatively mis-aligned,
the precise timing relationship t.sub.1 between the leading edge of
horizontal sync and the beginning of active video ensures that each
line of information, when processed by the follow-on television
receiver or monitor, can be properly displayed, provided that the
timing error in a given line does not exceed the capture range of
the television receiver or monitor synchronization circuitry. It
has been empirically determined that synchronization of each line
of video can be assured provided that the time shifting function
used to initially encrypt the signals does not vary at a rate
greater than about 20 Hz for NTSC encoded video. While other
maximum frequency limits may apply to other television information
signal encoding system (such as PAL or SECAM), the general rule is
that the time shifting applied to the original signals during
encryption must be relatively slowly varying compared to the line
rate. Stated differently, the time error introduced into the
signals as a result of the scrambling/descrambling process must not
fall outside the capture range of the synchronization circuitry in
the follow-on television receiver or monitor.
FIG. 4 is a block diagram of a scrambler system capable of
providing the encryption described above. As seen in this Figure,
input video to be encrypted is coupled to an input terminal 11 of a
video input processor 12. Processor 12 functions to normalize the
incoming video signal relative to gain, DC offset and bandwidth,
and provides a stable low impedance buffer unit for the video
appearing on output terminal 13. In addition, the incoming vertical
and horizonal sync portions are separated from the input video by
processor unit 12 and supplied as an input to a sync/timing
generator and phase locked loop unit 15, which is illustrated in
greater detail in FIG. 5.
The signals output from processor unit 12 appearing on output
terminal 13 are coupled to a conventional NTSC decoder and
anti-alias filter unit 16 in which the luminance component Y and
chrominance quadrature components I, Q are separated for parallel
processing in the digital domain. The Y output of unit 16 is
coupled to an analog-to-digital converter 18 in which the luminance
is converted from analog to digital form at a preselected clock
rate by means of an input sample clock signal supplied on clock
input line 19. The output of converter unit 18 is coupled to an
input portion of a dual ported luminance memory unit 20. Memory
unit 20 is configured as a memory in which a word is written from
the A/D converter 18 into the memory every memory cycle and a word
is read from the memory unit 20 to a digital-to-analog converter
unit 22 every memory cycle. The storage capacity of luminance
memory unit 20 should be at least equal to the number of multi-bit
characters (bytes) required to store one complete line of luminance
information at the selected clock rate. Read/write control signals
and multi-bit address signals are supplied to the luminance memory
unit 20 from a memory controller unit 24. The output of luminance
memory unit 20 is coupled to the input of a digital-to-analog
converter 22, in which the multi-bit digital words output from
memory unit 20 are converted into analog samples at the clock rate
by clock signals supplied from unit 15 on clock input line 23. The
output of converter unit 22 is coupled to the input of an NTSC
encoder and low pass filter unit 25 in which the luminance signal
is combined with the I and Q chrominance components and
renormalized with respect to bandwidth and DC offset. The I, Q
chrominance quadrature components are processed in an essentially
identical manner to that already described for the luminance
component Y in units 18', 20', and 22', which function in the same
manner as units 18, 20 and 22.
Sync timing unit 15 is used to generate the input clock signals
used to provide the sample clock for A/D converter unit 18, the
read and write clock signals from memory unit 20, and the clock
signals for D/A converter unit 22. Preferably, unit 15 is comprised
of a discrete phase detector, a number of sampling gates, an error
amplifier and a crystal clock oscillator.
The above described units are coupled to a user interface device
32, such as a keyboard terminal, via a controller unit 34 and a
plurality of control registers 36. The controller 34 includes the
circuitry shown in FIG. 7 for generating the time shifting wave
form used to time shift the signals undergoing encryption.
Controller 34 also generates an encrypted byte of information
containing information required by the descrambler to generate the
same time shifting wave form. This byte is encrypted using any
suitable encryption technique and the result is inserted into one
of the unused lines of the vertical blanking interval.
FIG. 5 illustrates key portions of the video input processor 12,
video output processor 26 and the sync/timing generator 15 of FIG.
4. As seen in this Figure, the video present on input terminal 11
is coupled to a sync separator 31 in which the horizontal and
vertical sync portions are detected. The vertical sync pulses
output from the sync separator 31 trigger a vertical pulse
generator 32, which is coupled as a reset signal to the input of a
divide by 525 counter 33 functioning as a line counter. The
horizontal sync pulses output from sync separator 31 are used to
drive a horizontal phase locked loop 35, which generates on a first
output line 36 a clock signal for counter 33 having a frequency
which is twice the line frequency (31.5 Khz). Horizontal phase
locked loop 35 also generates at the line rate a pulse which is 2
microseconds wider than the normal horizontal blanking pulse (See
FIG. 6, wave form 6C), and this pause is coupled via output line 37
to the input of a pair of monostable multivibrator circuits 40, 41.
Multi-vibrator circuit 40 generates a 2.0 microsecond wide pulse
(wave form 6D) triggered on the rising edge of the input signal.
Multivibrator circuit 41 generates a 2.0 microsecond wide pulse
(wave form 6E) triggered on the falling edge of the input signal
thereto. The output signals from multivibrator units 40, 41 are
passed through an OR gate 45, the output of which (wave form 6F) is
coupled as one input to an AND gate 47. The other input to AND gate
47 is a vertical sync gate signal output from a logic state
detector unit (preferably a PROM) 50. The vertical sync gate signal
output from unit 50 is a disabling signal for AND gate 47 which has
a duration of nine horizontal lines and which functions to disable
the output of AND gate 47 throughout the vertical sync interval.
The output of AND gate 47 is coupled to a blanking switch 52 and
serves to extend the blanking interval by two microseconds on each
side of the normal blanking time. Although this results in the loss
of some active video on the trailing edge of previous line and the
leading edge of active video in the current line, this loss is not
significant. The output of blanking switch 52 (wave form 6H) is
coupled to the input of the NTSC decoder 16 described above with
reference to FIG. 4, and also to the input of a burst gate circuit
55. Burst gate circuit 55 is operated by the control output signal
from a monostable multivibrator unit 57 (wave form 6I), which is a
3.5 microsecond wide pulse beginning at the trailing edge of
horizontal sync and which is used to gate the burst portion of the
incoming video signal to a phase locked subcarrier oscillator
circuit 59. Oscillator circuit 59 generates a subcarrier signal at
eight times the nominal subcarrier frequency, and the output from
oscillator circuit 59 is used as the clock signal for the A/D units
18, 18', D/A converter units 22, 22', memory controller unit 24,
line counter 30 and any other circuits requiring a synchronized
clock. The output of oscillator circuit 59 is also coupled to the
input of a divide by eight tuned circuit 61, the output of which
provides 3.58 Mhz subcarrier which is phased locked to the incoming
color burst. This subcarrier (wave form 6J) is coupled to the NTSC
decoder and encoder circuits 16, 25.
The output of multivibrator circuit 57 is also coupled to a back
porch clamp circuit 63 and is used to enable the clamp during color
burst time.
The output of the multivibrator circuit 40 is coupled as an
activating input to a monostable multivibrator circuit 65 triggered
by the falling edge of the input signal and which generates a
horizontal blanking pulse of normal length (11 microseconds; wave
form 6G). The output of multivibrator circuit 65 is coupled via an
OR gate 66 to the control input of a video switch circuit 68. The
other input provided to switch circuit 68 via OR gate 66 is a
vertical blanking gate signal generated by logic state detector
unit 50. The vertical blanking gate signal is an enabling signal
having a duration of 21 lines and occurring during the vertical
blanking interval of each field.
The purpose of video switch circuit 68 is to alternate between two
versions of the time shifted video: one passing through a video
inverter circuit 70 and one bypassing the video inverter circuit
70. The video from NTSC encoder unit 25 (FIG. 4) is coupled to the
input of a back porch clamp circuit 72 which is also controlled by
the output of the multivibrator circuit 57. The output of the back
porch clamp circuit 72 is coupled to the two input terminals of
switch 68: the video is coupled directly to HI terminal 74 and
through video inverter circuit 70 to LO terminal 75.
The output of switch 68 (wave forms 6L, 6N) is coupled through a
video amplifier 78 and serves as the video output for follow-on use
(typically either for broadcast or recording on tape).
As noted above, the time shifting is performed in the digital
domain on the luminance and chrominance quadrature components in a
synchronous fashion. After the time shifting has been effected, the
digital signals are transformed to the analog domain and are
recombined in the encoder circuit 25. The time shifted video is
then inverted during the active video portions by means of the
inverter circuit 70 and the switch 68 to produce the time shifted,
inverted video scrambled signals shown in wave forms 6L and 6N of
FIG. 6. In particular, wave form 6K illustrates the result of time
shifting the active line video in the advance direction. Wave form
6L illustrates the result of passing this time shifted signal
through the video inverter circuit 70 during active video time.
Similarly, wave form 6M illustrates the result of time shifting the
active line video in the delay direction, while wave form 6N
illustrates the result of passing this time shifted signal through
the video inverter circuit 70 to invert the active video
portions.
With reference to FIG. 7, the portion of the controller 34 used to
generate the time shifting wave form includes a low frequency noise
signal generator 101 capable of generating any appropriate
relatively low frequency wave form to be used to define the time
shifting function. As noted above, this may comprise a sinusoidal
wave form, a ramp, a rectangular pulse or a random noise wave form.
Such devices are well known in the art and will not be further
described. The signal generated by the low frequency noise signal
generator 101 is coupled to an analog-to-digital converter 103
which digitizes the amplitude of the signal output from the
generator 101 at the rate of one sample per field of information.
The sampling is controlled by a signal on controller input terminal
104. This control signal is obtained from the logic state detector
unit 50 (FIG. 5) and, in the preferred embodiment, comprises a
pulse generated during one of the lines occurring during the
vertical blanking interval, such as line 21. The sample output from
the analog-to-digital converter 103 is coupled via a 20 Hz low pass
filter 105 to the input of a second analog-to-digital converter
106. Analog-to-digital converter 106 is clocked by the clock pulses
generated at the horizontal sync rate (such as wave form 6B, FIG.
6). The output of the analog-to-digital converter 106 is coupled to
the memory controller 24 via the control registers 36 and is used
to control the actual magnitude of the time shifting performed on
the active video portion of each line.
The output of analog-to-digital converter 103 is also coupled via a
gate circuit 108, which is enabled during line 21, to an encryptor
110 which provides the encryption noted above for the time shift
byte signal. The output of encryptor 110 is added to the video
signal via video amplifier 78 (FIG. 5). Thus, the encrypted byte of
time shifting wave form amplitude information at the beginning of
each field is transmitted to each descrambler device along with the
scrambled video signals and other timing signals.
FIG. 8 illustrates the descrambler unit used to decrypt the signals
received after encryption by the process described above. The
descrambler shares many units in common with the scrambler shown in
FIG. 5 and identical reference numerals have been employed for such
units. The purpose of the descrambler shown in FIG. 8 is to restore
the original time relationships between the horizontal sync and
color burst portion of each line of video and the active video
portion of that line. This operation will now be described, in
conjunction with the wave form diagrams shown in FIG. 9.
Incoming video on input terminal 11 is coupled to the input of sync
separator 31 and to the input of the back porch clamp circuit 63.
The output of the sync separator 31 is coupled to the input of the
vertical pulse generator 32, the input of the horizontal phase lock
loop 35, to the input of the 3.5 microsecond monostable
multivibrator 57 used to drive the back porch clamp circuit 63, and
to the input of a descrambling wave form synthesizer circuit 80. A
second input to the synthesizer circuit 80 is the authorization key
signal generated by the user (i.e., the subscriber) used to decrypt
the incoming bytes of time shifting wave form data encrypted by
controller 34. This key is separately communicated to the
subscriber in any secure mode, i.e., electronically, by mail, via
telephone or the like. The other input to synthesizer 80 is the
video input 11 which contains the information regarding the
instantaneous value of the time shifting wave form at the beginning
of a field of information. As described more fully below with
reference to FIG. 10, the synthesizer circuit 80 regenerates the
original scrambling wave form for input to voltage comparator unit
82, this voltage varying during the field in accordance with the
nature of the time shifting wave form. For example, if a sinusoidal
time shifting wave form was employed during encryption, the same
sinusoidal wave form is needed during decryption to generate the
varying voltage reference for each line of a given field. At the
beginning of the next field, a new byte of time shifting wave form
information is provided during vertical blanking in the received
video, and this information is coupled to the synthesizer circuit
80.
The horizontal phase locked loop circuit 35 generates a first
output pulse (wave form 9H) having a width equal to 6.0
microseconds but advanced in phase with respect to horizontal sync
by a predetermined amount (1.5 microseconds in the preferred
embodiment). This signal is supplied via output lead 37 to the
input of a 1.5 microsecond monostable multivibrator circuit 83 and
also to the input of a ramp generator circuit 85. Ramp generator
circuit 85 generates a ramp voltage (wave form 9I) at a linear
rate, and this ramp voltage is coupled to the other input of
voltage comparator 82. When the level of the ramp voltage output by
generator 85 matches that of the reference voltage from synthesizer
circuit 80, the voltage comparator 82 generates an output signal
which is used to initiate a nine microsecond monostable
multivibrator circuit 87 and a 4.7 microsecond monostable
multivibrator circuit 88. The multivibrator circuit 88 generates
the new repositioned horizontal sync pulse to be added to the video
(wave form 9L) and this sync pulse is coupled to one input of a
horizontal sync and burst generator 89. The output of multivibrator
circuit 88 is also coupled to the input of a 0.9 microsecond
monostable multivibrator circuit 90, and the output of circuit 90
(wave form 9M) is coupled to the input of a monostable
multivibrator circuit 92. The output of circuit 92 (wave form 9N)
is coupled to the input of a burst generator circuit 94, which
gates a burst signal at the subcarrier frequency from oscillator
circuit 59 to the other input of the horizontal sync and burst
mixer circuit 89. The output of mixer circuit 89, which comprises
horizontal sync and color burst properly re-timed with respect to
the active information portion of that line, is coupled to input
terminal 74 of video switch circuit 68".
The output of the multivibrator circuit 83 (wave form 9J) is
coupled to the input of monostable multivibrator circuit 65, which
generates an 11 microsecond wide pulse (wave form 9K) defining the
normal horizontal blanking interval. This signal is passed through
an OR gate 66 along with the output of multivibrator circuit 87 to
the first input of an AND gate 95. The other input to AND gate 95
is the vertical sync gate signal generated by logic state detector
50, which serves to disable the AND gate 95 from passing the
control signal to video switch 68" during the nine lines of
vertical sync within each field. Thus, video switch 68" can be
switched from the normally closed terminal 75 to terminal 74
whenever the output pulse signals from multivibrator circuit 65 or
87 are active (wave forms 9K, 9R). Whenever switch 68" is connected
to terminal 74, the output of the horizontal sync and burst mixer
circuit 89 is coupled to the video output amplifier 78. Otherwise,
the active video output from video switch 68' is coupled to the
video output amplifier 78.
Video switch 68' is provided with two video inputs: the direct
video passed through back porch clamp 63 (wave form 9P) or an
inverted version (wave form 9Q) provided by inverter 70. The state
of switch 68' is controlled by the vertical blanking gate signal
output from logic state detector 50. Whenever this signal is
active, the switch 68' is connected to terminal 74 and the direct
video passes through. At all other times, the inverted version from
inverter circuit 70 is coupled via terminal 75 to the output of
switch 68'.
In operation, the incoming horizontal sync and color burst are
discarded by the descrambler circuit of FIG. 8 and new horizontal
sync and color burstt are generated in the proper timing
relationship with respect to the active video portion of the
incoming line. The new sync and burst provided by the mixer circuit
89 are coupled via terminal 74 through switch 68" as the new sync
and color burst portion of the reconstituted video signal. During
active video time, the input video is inverted by means of inverter
70 and coupled to the video output amplifier 78 to reconstitute the
entire line. During the last 12 lines of the vertical blanking
portion of a field, the active video line is passed directly
through the first and second video switch circuits 68', 68". During
the vertical sync gate portion of a given field, the AND gate 95 is
disabled to prevent the synthesized horizontal sync and burst from
being added to the incoming video signal.
FIG. 10 illustrates the subunits comprising the descrambling wave
form synthesizer 80. As seen in this Figure, the incoming video is
coupled to the input of a data extractor 112 which detects the
information in line 21 of the field specifying the amplitude of the
time shifting wave form at the beginning of that field. Since this
data is in encrypted form, it is coupled to the input of a
decryptor 114 along with the authorization key provided by the
subscriber/user by any suitable means, e.g., a keyboard. The
decrypted digital amplitude value is coupled from decryptor 114 to
the input of a 20 Hz low pass filter, which replicates or recovers
the slowly varying time shifting wave form. The output of the 20 Hz
low pass filter 116 is coupled to the voltage comparator 82 as the
reference wave form voltage.
As will now be apparent, the invention provides a completely secure
technique for encrypting and decrypting video type signals, which
is fully compatible with all video tape formats and transmission
systems and which causes at least enough picture concealment to
remove all entertainment value from a program. In addition, since
nearly all of the circuit elements and subunits in the descrambler
are conventional off the shelf circuits and components, the
scrambler unit is relatively inexpensive to manufacture and
repair.
While the above provides a full and complete description of the
preferred embodiment of the invention, various modifications,
alternate constructions and equivalents will occur to those skilled
in the art. For example, while the limitation on the combined
maximum advance and delay time shifting has been specified as .+-.2
microseconds, other values can be selected. In general, the greater
the magnitude of the combined maximum advance and delay employed
the more active information is lost from the trailing portion of
the preceding line and the leading portion of the line being time
shifted. Therefore, the above descriptions and illustrations should
not be construed as limiting the scope of the invention, which is
defined by the appended claims.
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