Scrambling and descrambling of video signals using horizontal line combinations

A plurality of video lines in a raster scan video image are scrambled by delaying a first video line in time equal to the horizontal line scan time for the raster scan video field to produce a delayed first video line. The delayed first video line is inverted to produce an inverted delayed first video line and one of the delayed first video line, the inverted delayed first video line and a null signal is added to a second horizontal video line to produce a horizontal line of a scrambled video signal. The scrambled horizontal lines of video information are descrambled by delaying a first scrambled horizontal video line in time equal to the horizontal line scan time for the raster scan video field to produce a delayed first scrambled video line. The delayed first scrambled video line is inverted to produce an inverted delayed first scrambled video line and one of the delayed first scrambled video line, the inverted delayed first scrambled video line and a null signal is added to a second scrambled horizontal video line and to a feedback signal to produce a descrambled horizontal video line.

TECHNICAL FIELD 
The present invention relates generally to encoding and decoding of video 
information, and more particularly to a method and apparatus which secures 
the transmission of video signals so that only authorized persons can view 
or have access to the associated video information. 
BACKGROUND ART 
Various systems have been proposed and are in use for scrambling video 
signals, such as television signals, to secure transmission of the video 
information therein. Generally, these systems include a scrambler which 
alters the video signals in a predetermined manner before the video 
signals are propagated through a video channel associated with, for 
example, an RF, microwave, cable or other video communications system. At 
the receiving end of the video system, a descrambler reverses the 
scrambling process to assemble the scrambled video signals into their 
original order, sequence or state so that they can be suitably displayed 
for viewing on a television screen. Of course, if a receiver does not 
contain a suitable descrambler, the displayed video signals will be 
generally unintelligible and the information therein will be masked. 
Thompson, et al., U.S. Pat. No. 4,716,588, discloses a multimode video 
signal scrambling system controlled by a central computer facility which 
scrambles each field of a video signal using one or more of several 
techniques including adding extraneous and confusing synchronizing (sync) 
pulses onto the video signal, compressing the amplitude of the video 
signal, alternatively inverting horizontal lines of the video signal in 
time to reverse the sequence thereof, removing horizontal sync pulses from 
the video signal, adding false leading edge sync pulses to horizontal 
lines of a video signal and shifting the amplitude of the active video 
information within a video signal in a predetermined direction so that a 
television receiver does not register or lock onto the active video 
information. 
Cheung, U.S. Pat. No. 4,527,195, also discloses an apparatus for encoding 
and decoding video signals using multiple scrambling techniques including 
developing a scrambled video signal having horizontal sync pulses 
pseudorandomly placed on some of the horizontal lines of the video signal. 
This apparatus also pseudorandomly reverses the polarity of certain 
selected lines of video within each video field and shifts portions of 
video signals containing luminance and chrominance information in 
amplitude to mask such information. 
Robbins, U.S. Pat. No. 4,628,358, discloses a video scrambling system which 
inverts video information within a video signal about a predetermined 
inversion level during selected lines of the video signal thereby to 
encrypt or scramble the video signal. This system also reverses the 
polarity of the selected audio portions of a video signal using a keyed 
pseudorandom generator. 
Field, et al., U.S. Pat. No. 4,600,942, discloses an apparatus for 
scrambling a television signal which stores particular segments of each 
horizontal line of a video signal in an analog storage device and 
retrieves these horizontal line segments in a cyclic manner to re-arrange 
the horizontal line and, thereby, scramble the television signal. The 
retrieval of the information from the storage devices is carried out on a 
selective basis in accordance with a pseudorandom signal that is 
independently generated at each of the encoding and decoding stations. 
Ryan, U.S. Pat. No. 4,916,736, discloses a video scrambling system which 
shifts active video information in time with respect to the horizontal 
sync pulse and the color burst signal within each horizontal line of the 
video signal. This system includes an analog-to-digital (A/D) converter 
which digitizes and stores the video signal in a memory and a 
digital-to-analog (D/A) converter which reads the video information out of 
the memory for transmission through a communications channel. A controller 
controls the sequence in which the information is read out of the memory 
and shifts the digitized active video information in time with respect to 
the horizontal sync pulse and the color burst signal. 
Although many methods of scrambling a video signal are known, these methods 
usually require complicated timing and control techniques and use 
expensive circuitry. Furthermore, none of these methods arithmetically 
combines video information from separate horizontal lines of a video 
signal to produce a scrambled video signal. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a video scrambling and 
descrambling technique provides a high degree of masking of the video 
information contained in a raster scan video field using arithmetic 
combinations of portions of separate horizontal lines of a video signal. 
According to one aspect of the present invention, a method of scrambling a 
video signal having first and second components, such as first and second 
horizontal lines, includes the steps of delaying the first component in 
time to produce a delayed first component and combining the second 
component with the delayed first component to produce a scrambled video 
signal. 
The delayed first component, which may comprise the active video portion of 
a first horizontal line of the video signal, may be added to the second 
component, which may comprise the active video portion of a second 
horizontal line of the video signal. Likewise, one of the first delayed 
component and the second component may be subtracted from the other of the 
first delayed component and the second component to produce the scrambled 
video signal. More particularly, a delayed first horizontal line may be 
inverted to produce an inverted delayed first horizontal line and either 
the delayed first horizontal line or the inverted delayed first horizontal 
line may be added to a second horizontal line to produce the scrambled 
video signal. Preferably, the first horizontal line is delayed an amount 
of time equal to the horizontal line scan time of the raster scan video 
field. A scrambling signal may be added onto the first and second 
components in order to provide a further level of scrambling to the 
scrambled video signal. Also, scrambling information indicative of the way 
in which the video components were combined may be encoded onto the 
scrambled video signal prior to transmission of the scrambled video signal 
through a channel. 
According to another aspect of the present invention, a video signal 
descrambling technique descrambles a scrambled video signal having first 
and second scrambled components, such as first and second scrambled 
horizontal lines, to produce a portion of a descrambled video signal 
having first and second components, such as first and second descrambled 
horizontal lines. This technique includes delaying the first scrambled 
component in time, combining the delayed first scrambled component with 
the second scrambled component and a feedback signal to produce the first 
component of the descrambled video signal. The second component of the 
descrambled video signal is delayed in time to produce the feedback 
signal. 
Preferably, the first scrambled component, i.e., the first scrambled 
horizontal line, is delayed an amount of time equal to the horizontal line 
scan time of the raster scan video field, and the second component of the 
descrambled video signal, i.e., the second horizontal line of the 
descrambled video signal, is delayed an amount of time equal to twice the 
horizontal line scan time of the raster scan video field. 
The second scrambled horizontal line and the delayed first scrambled 
horizontal line may be added together or subtracted from one another by 
identifying one of the second scrambled horizontal line and the delayed 
first scrambled horizontal line as a first signal and identifying the 
other of the second scrambled horizontal line and the delayed first 
scrambled horizontal line as a second signal. Thereafter, the first signal 
may be inverted and one of the first signal and the inverted first signal 
may be added to the second signal. Furthermore, the delayed second 
horizontal line of the descrambled video signal may be inverted and one of 
the delayed second horizontal line of the descrambled video signal and the 
inverted delayed second horizontal line of the descrambled video signal 
may be chosen as the feedback signal. 
The technique for descrambling a scrambled video signal may also include 
removing a scrambling signal from one or both of the first and second 
horizontal lines of the descrambled video signal. Scrambling information 
indicative of the manner in which the scrambled video signal was generated 
may also be obtained from the scrambled video signal and used to control 
the process of combining the delayed first horizontal line, the second 
horizontal line and the feedback signal. 
According to another aspect of the present invention, a method and 
apparatus for scrambling a plurality of video lines in a raster scan video 
image for a video field includes delaying a first video line an amount of 
time equal to the horizontal line scan time for the raster scan video 
field to produce a delayed first video line and inverting the delayed 
first video line to produce an inverted delayed first video line. The 
method and apparatus also selects one of (a) the delayed first video line, 
or (b) the inverted delayed first video line, or (c) a null signal, such 
as an electrical ground signal, and adds a second video line to the 
selected one of (a), (b) or (c). This process is repeated for each of the 
plurality of video lines to produce the scrambled video signal. 
According to a still further aspect of the present invention, a method and 
apparatus for descrambling a plurality of scrambled video lines in a 
raster scan video image for a video field delays a first scrambled video 
line an amount of time equal to the horizontal line scan time for the 
raster scan video field to produce a delayed first scrambled video line 
and inverts the delayed first scrambled video line to produce an inverted 
delayed first scrambled video line. The method and apparatus also selects 
one of (a) the delayed first scrambled video line, or (b) the inverted 
delayed first scrambled video line, or (c) a null signal, such as an 
electrical ground signal, and adds a second scrambled video line to the 
selected one of (a), (b) , or (c) and to a feedback signal to produce a 
descrambled horizontal video line. The feedback signal is developed from a 
previously descrambled horizontal video line by delaying the previously 
descrambled horizontal video line, inverting the delayed descrambled 
horizontal video line, and choosing one of (a) the delayed descrambled 
video line, or (b) the inverted delayed descrambled horizontal video line, 
or (c) a null signal, such as an electrical ground signal, as the feedback 
signal. The apparatus repeats this process for each of the plurality of 
scrambled horizontal video lines in the video field. 
According to a still further aspect of the present invention, a method of 
transmitting a video signal having first and second original components to 
develop a reproduced video signal having first and second reproduced 
components includes the steps of delaying the first original component of 
the video signal in time to produce a delayed first original component and 
combining the second original component of the video signal with the 
delayed first original component to produce a portion of a scrambled video 
signal. The method also includes the steps of transmitting the scrambled 
video signal having first and second scrambled components and receiving 
the transmitted scrambled video signal. Upon receiving the transmitted 
scrambled video signal, the method delays the first scrambled component of 
the scrambled video signal in time and combines the delayed first 
scrambled component, the second scrambled component and a feedback signal 
to develop the first reproduced component of the reproduced video signal. 
The method also includes the step of delaying the second reproduced 
component of the reproduced video signal to produce the feedback signal.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1, a typical raster scan frame of a television video 
signal, which may be developed and transmitted according to any standard 
including, for example, Electronics Industries Association (EIA) standard 
RS-250-B or Network Transmission Committee standard NTC-7, is illustrated. 
Typically, each frame of a television video signal includes 525 horizontal 
lines of raster scan video information (marked in FIG. 1 as L1, L2, etc.) 
and is divided into two video fields, 20 and 22, each of which includes 
2621/2 horizontal lines of video information. Each of the fields 20 and 22 
includes a vertical blanking interval (VBI) 24 and 26, respectively, and a 
video portion 28 and 30, respectively. Each of the VBI's 24 and 26 
includes twenty-one horizontal lines of flat field (black) video 
information which are not displayed on the television receiver. Typically, 
one or more of the first ten horizontal lines of each of the VBI's 24 and 
26 include standardized sets of equalizing pulses and/or vertical 
synchronizing pulses which are used by a television to synchronize the 
television beam with the incoming video signal. The horizontal lines of 
the VBI's 24 and 26 may also include closed caption information, audio 
information, copyright information, scrambling codes and/or any other 
desired information. 
Each of the video portions 28 and 30 includes 2411/2 horizontal lines of 
raster scan video information having active video signals which are 
displayed consecutively downwardly on a television screen such that the 
first horizontal line of video information within each of the video 
portions 28 and 30 is displayed at the top of the television screen and 
the last horizontal line within each of the video portions 28 and 30 is 
displayed at the bottom of the television screen. Furthermore, the 
horizontal lines of the video portions 28 and 30 are interleaved with each 
other so that, for example, horizontal line L285 is displayed directly 
below horizonal line L22 and directly above horizontal line L23. 
Furthermore, as indicated in FIG. 1, the horizontal lines of the video 
portion 30 are offset from the horizontal lines of the video portion 28 by 
one-half of a horizontal line. As a result, the active video information 
of the first field 20 begins at the top left-hand corner of the television 
screen while the active video information of the second field 22 begins at 
the top center portion of the television screen. 
Referring now to FIG. 2, each horizontal line of video information within 
the video portions 28 and 30 includes a plurality of components including 
a front porch 40, a horizontal sync pulse 42, a breezeway 44 and a back 
porch 46, all of which comprise horizontal synchronizing signals. 
Furthermore, each horizontal line within the video portions 28 and 30 
includes a color burst signal 48 and an active video signal 50 containing 
active video information which is displayed on a television screen. 
Typically, each horizontal line of a video signal is approximately 63.6 
microseconds long. As a result, the horizontal line scan time of the 
raster scan video field is approximately 63.6 microseconds in duration. 
Referring now to FIG. 3, a scrambling unit 60 scrambles a television video 
signal by alternatively (1) adding one or more portions of one horizontal 
line of video information to one or more portions of a second horizontal 
line of video information, or (2) subtracting the one or more portions of 
one horizontal line of video information from one or more portions of a 
second horizontal line of video information, or (3) passing a horizontal 
line of video information unaltered to produce a horizontal line of a 
scrambled video signal. 
In accordance therewith, an original video signal is provided to a 
scrambling controller 62 which controls the operation of the scrambling 
unit 60. The scrambling controller 62 decodes the incoming video signal to 
determine the exact portion of the video signal present at the input of 
the scrambling controller 62. The scrambling controller 62 may include a 
horizontal and vertical sync detector (not shown) which detects the 
horizontal and vertical sync pulses present in each frame and/or each 
horizontal line of the incoming video signal. The scrambling controller 62 
may also include a microprocessor and/or a timing device (not shown) which 
resets at the beginning of each frame and/or horizontal line of the 
incoming video signal. The microprocessor determines which portion of the 
frame and/or horizontal line of the video signal is present at the input 
of the scrambling controller 62 at any particular time by, for example, 
comparing the output of the timing device with markers stored in memory 
which indicate which portion of the video signal corresponds to each 
particular timer output. The microprocessor uses this comparison to 
control the operation of the scrambling unit 60 as described hereinafter. 
The original video signal is also delivered to a delay line 64 and to a 
summer 66. The delay line 64, which may comprise any analog or digital 
delay line, delays the original video signal by one horizontal video line 
scan time, i.e., 63.6 microseconds, to produce a delayed horizontal line 
of video information. The delayed horizontal line of video information is 
delivered to a first input 68 of a switch 70 and is also inverted by an 
inverter 72 to develop an inverted delayed horizontal line of video 
information which is delivered to a second input 74 of the switch 70. A 
third input 76 of the switch 70 is connected to ground so that a null 
signal, i.e., a ground or zero reference voltage signal, appears at the 
switch input 76. 
The switch 70 is controlled by the scrambling controller 62 to connect one 
of (1) the delayed horizontal line of video information present at the 
switch input 68, (2) the inverted delayed horizontal line of video 
information present at the switch input 74, or (3) the null signal present 
at the switch input 76, to the summer 66. The scrambling controller 62 may 
operate the switch 70 in any desired manner, including, for example, 
according to a predetermined sequence or a random or pseudorandom code 
generated by the scrambling controller 62. 
The summer 66 adds the horizontal line of video information present at the 
input of the scrambling unit 60 to the signal passed by the switch 70 to 
develop a scrambled video signal on an output line 67 comprising one of 
(1) the addition of the horizontal line of video information present at 
the input of the scrambling controller 62 and the previous horizontal line 
of video information (when the switch 70 is connected to the switch input 
68), (2) the subtraction of the previous horizontal line of video 
information from the horizontal line of video information present at the 
input of the scrambling controller 62 (when the switch 70 is connected to 
the switch input 74), or (3) the original video signal present at the 
input of the scrambling controller 62 (when the switch 70 is connected to 
the switch input 76). 
An amplitude adjusting circuit 76 adjusts the amplitude of the scrambled 
video signal developed by the summer 66 to add a further level of 
scrambling thereto and to provide a scrambled video signal having an 
amplitude within the limits of the standard under which the scrambled 
video signal is to be transmitted through a channel. As indicated in FIG. 
3, the amplitude adjusting circuit 76 and, therefore, the particular level 
of amplitude adjusting performed on the scrambled video signal, is 
controlled by the scrambling controller 62. 
A summer 78 then adds scrambling information developed by the scrambling 
controller 62 and indicative of, for example, the operation of the switch 
70 and/or the amplitude adjusting circuit 76, to the amplitude adjusted, 
scrambled video signal for use by a descrambling unit to decode the 
scrambled video signal. The scrambled video signal is then delivered to a 
transmitter 80 for transmission through a channel. 
Preferably, the scrambling controller 62 provides scrambling information to 
the summer 78 when, for example, horizontal lines within one or both of 
the VBI's 24 and/or 26 are present at the summer 78 thereby to encode the 
VBI's of scrambled video signal with the scrambling information. The 
scrambling information may, however, be encoded onto any desired portion 
of the scrambled video signal and, furthermore, may be modulated onto the 
scrambled video signal in any other desired manner. Thus, for example, the 
summer 78 may be replaced with a modulator which modulates the audio 
signal associated with each horizontal line of the scrambled video signal 
with the scrambling information. 
Although the scrambling unit 60 can scramble, i.e. add or subtract, entire 
horizontal video lines, (including the horizontal synchronizing signals, 
the color burst signals and/or the active video signals) or any 
combination thereof, preferably, the scrambling unit 60 only adds and/or 
subtracts the active video portions of a horizontal line of video 
information from the active video portion of the previous horizontal line 
of video information to develop the scrambled video signal. When less than 
the entire horizontal line is added or subtracted to develop the scrambled 
video signal, the scrambling controller 62 passes the unscrambled portions 
unaltered by connecting the switch 70 to the null signal present at the 
switch input 76 when the unscrambled portions of the video signals are 
present at the input of the scrambling controller 62. Thus, for example, 
when the scrambled video signal is developed by combining only the active 
video portions of consecutive horizontal lines, the switch 70 is connected 
to the null signal at the switch input 76 when any of the horizontal lines 
in the VBI's 24 and 26 are present at the input of the scrambling unit 60 
and when the horizontal synchronizing signals and the color burst signal 
of each horizontal line within the video portions 28 and 30 are present at 
the input of the scrambling unit 60. 
Although the scrambling unit 60 has been described as inverting the delayed 
horizontal video line to subtract the delayed horizontal video line from 
the horizontal video line present at the input of the scrambling 
controller 62, the scrambling unit 60 could, instead, subtract the 
horizontal line of video information present at the input of the 
scrambling controller 62 from the delayed horizontal video line by, for 
example, inverting the horizontal video line present at the input of the 
scrambling controller 62 and adding this inverted signal to the delayed 
horizontal video line developed by the delay line 64. 
Still further, although the scrambling controller 62 has been described as 
adding and subtracting horizontal lines to or from one another, any other 
arithmetic combinations could be performed instead or in addition thereto, 
including, for example, multiplication and division. Alternatively, a less 
complex scrambling controller could provide only one of these arithmetic 
combining functions to scramble the video signal. Moreover, the delay line 
64 of the scrambling unit 60 could delay a horizontal video signal by more 
than one horizontal line scan time so that non-consecutive horizontal 
lines of video information are combined with one another by the scrambling 
unit 60. 
Referring now to FIG. 4, a scrambling unit 82 is illustrated. The 
scrambling unit 82 is similar to the scrambling unit 60, with like 
components numbered the same as the scrambling unit 60, except that the 
scrambling unit 82 includes a summer 84 connected in front of the delay 
line 64 and the summer 66. The summer 84 adds a scrambling signal, which 
is developed by the scrambling controller 62, onto each horizontal line of 
the original video signal before that horizontal line of the original 
video signal is delivered to the delay line 64 and to the summer 66. As a 
result, the summer 84 adds an additional level of masking to the scrambled 
video signal developed by the summer 66. The scrambling controller 62 can 
deliver any desired scrambling signal to the summer 84 including, for 
example, a predetermined scrambling signal or a randomly or pseudorandomly 
developed scrambling signal. In the later cases, the scrambling controller 
62 can further encode the scrambled video signal, via the summer 78, with 
an indication of the scrambling signal added onto each horizontal line of 
the original video signal by the summer 84. 
Referring now to FIG. 5, a descrambling unit 100 is shown connected to a 
receiver 102. The receiver 102 receives the scrambled video signal 
produced by one of the scrambling units 60 or 82 and delivers the 
scrambled video signal to a descrambling controller 104 and to an 
amplitude adjusting circuit 106. The descrambling controller 104 may 
comprise a horizontal and/or vertical sync detector and a microprocessor 
or other circuitry which recognizes the portions of the incoming scrambled 
video signal and which controls the operation of the descrambling unit 100 
in response thereto. The descrambling controller 104 may also include a 
timer which is reset by the vertical and/or horizontal sync detector and 
which indicates the portion of the incoming scrambled video signal present 
at the input of the descrambling unit 100. 
Still further, the descrambling controller 104 may include a demodulator 
capable of extracting the scrambling information indicative of the way in 
which the scrambled video signal was generated which has been encoded onto 
the scrambled video signal. The demodulator delivers this information to 
the microprocessor for use in reproducing the original video signal. The 
information indicative of the way in which the scrambled video signal was 
produced may, instead, be stored in a memory associated with the 
descrambling controller 104, downlinked to the scrambling controller 104 
via a different communications channel or delivered to the descrambling 
controller 104 in any other desired manner. 
The descrambling controller 104 controls the operation of the amplitude 
adjusting circuit 106 to reverse the amplitude adjusting performed by the 
amplitude adjusting circuit 76 of the scrambling unit 60 or 82. The output 
of the amplitude adjusting circuit 106 comprises an amplitude adjusted 
scrambled video signal, marked in FIG. 5 as a signal A, which is delivered 
to a delay line 108 and to a summer 110. 
The delay line 108, which may comprise any desired digital or analog delay 
line, delays the scrambled video signal developed by the amplitude 
adjusting circuit 106 by same amount as the delay line 64 of the 
scrambling units 60 and 82. Thus, preferably, the delay line 108 delays 
the video signal for one horizontal line scan time, i.e., 63.6 
microseconds. The delay line 108 produces a delayed scrambled video 
signal, marked in FIG. 5 as a signal B+, which is delivered to a switch 
input 112 of a switch 114 and is also delivered to an inverter 116. The 
inverter 116 inverts the delayed scrambled video signal B+ to develop an 
inverted delayed scrambled video signal B- which is delivered to a switch 
input 118 of the switch 114. A switch input 120 of the switch 114 is 
connected to ground such that a null signal G (i.e., a ground or zero 
voltage reference signal) is developed at the switch input 120. 
The output of the switch 114, i.e., one of the signals B+, B- or G, is 
delivered to the summer 110 where it is combined with the signal A and a 
feedback signal to develop a descrambled video signal C. The descrambled 
video signal C substantially comprises the video signal delivered to the 
summer 66 of the scrambling units 60 and 82 and is provided to a feedback 
loop comprising a delay line 122, a switch 124 and an inverter 126 to 
produce the feedback signal. 
The delay line 122 delays the descrambled video signal C for twice the 
amount of time as the delay time of the delay line 108, i.e., twice the 
horizontal line scan time of the original video signal. The output of the 
delay line 122, marked in FIG. 5 as a signal D+, is delivered to an input 
128 of the switch 124 and is also delivered to the inverter 126. The 
inverter 126 develops an inverted version of the delayed descrambled video 
signal C, marked in FIG. 5 as a signal D-, and delivers this signal to an 
input 130 of the switch 124. An input 132 of the switch 124 is connected 
to ground to produce a null signal G (i.e., a ground or zero voltage 
reference signal) at the input 132. 
The operation of both of the switches 114 and 124 is controlled by the 
descrambling controller 104, as described hereinafter, so that, as noted 
above, signal C comprises a substantial replica of the signal developed by 
the summer 84 of the scrambling unit 82, i.e., the original video signal 
having a scrambling signal added thereto, or a substantial replica of the 
original video signal delivered to the scrambling unit 60. The signal C, 
however, will still include any scrambling information which may have been 
encoded onto the scrambled video signal. 
The descrambled video signal C is then delivered to a summer 140 which adds 
an inverted replica of the scrambling signal (which was added to the 
original video signal by the summer 84 in the scrambling unit 82) to 
subtract the scrambling signal from the video signal C and to produce a 
descrambled video signal comprising a substantial reproduction of the 
original video signal. Of course, if the scrambling system 60 of FIG. 3 
has been used to develop the scrambled video signal, the summer 140 is 
unnecessary. 
The summer 140 also allows the descrambling controller 104 to remove any 
other scrambling information or other extraneous signals which may have 
been added to the scrambled video signal by supplying an inverse of the 
signal to be eliminated to the summer 140 at the appropriate time. Of 
course, if the signal C comprises a portion of the video signal which has 
not been scrambled, for example, one of the VBIs 24 or 26 or the 
horizontal sync signals of a scrambled horizontal line of video, the 
descrambling controller 104 provides a ground signal to the summer 140 
when these portions of the signal C are present at the input of the summer 
140 to pass these portions of the signal C unaltered. 
The output of the summer 140, which comprises a substantial replica of the 
original video signal, is provided to an amplitude adjusting circuit 142 
which adjusts the amplitude of the descrambled video signal to the proper 
level for use in a television. The amplitude adjusting circuit 142 then 
delivers the descrambled video signal to a television (not shown) for 
display. 
In operation, the descrambling controller 104 recognizes when the portions 
of the incoming scrambled video signal which have been scrambled are 
present and, if desired, reads information from the scrambled video signal 
indicative of the way in which the scrambled video signal has been 
scrambled. More specifically, the descrambling controller 104 decodes the 
way in which the last two horizontal lines of the scrambled video signal 
were generated, i.e., the scrambled video signals marked in FIG. 5 as 
signals A and B+, and uses this information to control the operation of 
the switches 114 and 124 as indicated by Table 1 to descramble the 
scrambled video signal. 
When the scrambled video signal has only been scrambled during particular 
portions thereof, for example, the active video portions of each of the 
horizontal lines within the video portions 28 and 30, the descrambling 
controller 104 controls the switches 114 and 124 such that the other 
portions of the incoming scrambled video signal, i.e., the horizontal 
lines within the VBIs 24 and 26 and/or the horizontal synchronizing 
signals and the color burst signal of each horizontal line, are passed 
through the summer 110 while the switches 114 and 124 are connected to the 
ground signals G through switch inputs 120 and 132, respectively. 
On the other hand, when a scrambled portion, such as an active video signal 
within a horizontal line of the video portions 28 or 30, is present at the 
input of the descrambling unit 100, the descrambling controller controls 
the switch 114 to be connected to one of the signals B+, B- or G at the 
switch inputs 112, 118 or 120, respectively, and controls the switch 124 
to be connected to one of the signals D+, D- or G at the switch inputs 
128, 130 or 132, respectively, according to the truth table given in Table 
1. 
TABLE 1 
______________________________________ 
Operation Of 
Operation Of 
Scrambling Unit 
Scrambling Unit 
Switch 70 With 
Switch 70 With 
Operation Operation 
Respect To The 
Respect To The 
Of Of 
Signal B+ Signal A Switch 114 
Switch 124 
______________________________________ 
+ + B- D+ 
+ - B+ D- 
+ G G G 
- + B- D- 
- - B+ D+ 
- G G G 
G + B- G 
G - B+ G 
G G G G 
______________________________________ 
The first column of Table 1 expresses the operation of the switch 70 of the 
scrambling unit 60 or 82 when producing the horizontal line of the 
scrambled video present at the output of the delay line 108, i.e., the 
signal B+. The second column of Table 1 expresses the operation of the 
switch 70 of the scrambling unit 60 or 82 when producing the horizontal 
line of the scrambled video signal present at the input of the scrambling 
unit 100, i.e., the signal A. In these columns, the symbol "+" indicates 
that the switch 70 was connected to the switch input 68 (i.e., the 
positive input), the symbol "-" indicates that the switch 70 was connected 
to the switch input 74 (i.e., the negative or inverting input) and the 
symbol "G" indicates that the switch 70 was connected to the switch input 
76 (i.e., the ground or null signal input). The third and fourth columns 
of Table 1 express the operation of the switches 114 and 124, 
respectively, by identifying the signals passed by the switches 114 and 
124 under the conditions of the first two columns. In the third and fourth 
column, and as indicated above, the signal B+ represents the output of the 
delay line 108, the signal B- represents the output of the inverter 116, 
the signal D+ represents the output of the delay line 122, the signal D- 
represents the output of the inverter 126 and the signal G represents the 
null or ground signals present at the switch inputs 120 and 132. 
Thus, as indicated in Table 1, when the switch 70 of the scrambling unit 60 
or 82 is connected to the output of the delay line 64 through the switch 
input 68 (i.e., the positive input) to produce both of the most recent 
horizontal lines of the scrambled video signal, i.e., the scrambled 
signals A and B+, then the switch 114 of the descrambling unit 100 should 
be connected to the negative input 118 (i.e., to the signal B-), while the 
switch 124 should be connected to the positive input 128 (i.e., to the 
signal D+). Also, for example, if the switch 70 of the scrambling unit 60 
or 82 is connected to the positive input 68 to produce the signal B+, and 
the switch 70 is connected to the negative input 74 to produce the signal 
A, then the switch 114 should be connected to the positive input 112, 
i.e., to the signal B+, and the switch 124 should be connected to the 
negative input 130, i.e., to the signal D-. As a further example, when the 
signal A is developed by adding a null signal G to the original video 
signal, both of the switches 114 and 124 of the descrambling unit 100 
should be connected to the ground or null signal G. 
Because the descrambled video signal C is fed back to the summer 110 to 
produce subsequent horizontal lines of the descrambled video signal C, it 
is important that initially, for example, when the descrambling unit 100 
is first turned on, the signal C is a substantial reproduction of the 
original video signal delivered to the summer 66 of the scrambling units 
60 and 82. Therefore, at least one horizontal line within each frame of a 
scrambled video signal should be left unaltered so that the descrambling 
unit 100 can properly sync up to the scrambled video signal, i.e., produce 
an initial signal C which is a reproduction of the signal scrambled by the 
summer 66 of the scrambling units 60 and 82, and which, when fed back as a 
feedback signal to the summer 110, operates to decode subsequent 
horizontal lines of the scrambled video signal correctly. This unscrambled 
horizontal video line can be located at any desired position within a 
frame of the scrambled video signal including being within one or both of 
the VBIs 24 and/or 26. 
In the situation where only active video signals within the video portions 
28 and 30 are combined to produce the scrambled video signal, the first 
horizontal line of each of the video portions 28 and 30 within a frame of 
the scrambled video signal is preferably passed through the scrambling 
units 60 and/or 82 by adding a null or ground signal thereto. In this 
manner, the descrambling unit 100 can sync up to the first scrambled 
horizontal line within each of the video portions 28 and 30 by simply 
connecting the switches 114 and 124 to the null signals G at the switch 
inputs 120 and 132, respectively. This operation ensures that the first 
horizontal line of the descrambled video signal C within each of the video 
portions 28 and 30 is a substantial reproduction of the video signal 
scrambled by the summer 66 of the scrambling units 60 and 82. Furthermore, 
in this case, the switch 124 should be connected to the null signal G at 
the switch input 132 when the second horizontal line of the scrambled 
video signal within each of the video portions 28 and 30 is present at the 
input of the summer 110 to decode the second horizontal line of scrambled 
video within the video portions 28 and 30 properly. 
Although Table 1 has been developed for the situation in which the delay 
line 64 of the scrambling units 60 and 82 and the delay line 108 of the 
descrambling unit 100 delay a horizontal line of video information by one 
horizontal line scan time and the delay line 122 of the descrambling unit 
100 delays a horizontal line of the descrambled video signal by twice the 
horizontal line scan time, similar truth tables can be developed according 
to the principals set forth herein for situations in which the delay lines 
64, 108, and 122 delay the associated signals by other amounts. 
Furthermore, similar scrambling units and descrambling units could be 
developed, using the principles described herein, which combine video 
signals from three or more horizontal lines of a video signal to scramble 
and descramble the video signal. 
Although the scrambling units 60 and 82 and the descrambling unit 100 have 
been described as including analog components, one skilled in the art 
could easily substitute digital components therefor. For example, in the 
scrambling units 60 and 82, an analog to digital (A/D) convertor could be 
used to convert the original video signal into digital form while a 
microprocessor, responsive to the digitized video signals, could scramble 
the video signal according to the principles described herein to produce a 
digitized scrambled video signal. The digitized scrambled video signal 
could then be converted to analog by a digital to analog (D/A) convertor 
and transmitted through a channel. In such a system, a microprocessor, 
responsive to a digital version of the original video signal, could store 
the original video signal in a random access memory (RAM) for one 
horizontal line scan time. The microprocessor could then (1) add the 
stored digital delayed horizontal video line to a digitized version of the 
horizontal video line present at the input of the scrambling unit, (2) 
subtract one of the current digitized version of the horizontal video line 
present at the input of the scrambling unit and the stored digital delayed 
horizontal video line from the other of the current digitized version of 
the horizontal video line present at the input of the scrambling unit and 
the stored digitized delayed horizontal video line, or (3) pass the 
digitized version of the horizontal video line present at the input of the 
scrambling unit unaltered, to produce the digitized scrambled video 
signal. The microprocessor could also digitally encode the scrambling 
information onto the digital scrambled video signal. 
Likewise, in the descrambling unit 100, the output of the receiver 102 
could be converted from analog to digital in an A/D convertor and supplied 
to a microprocessor which would recognize the scrambling information 
encoded onto the digitized scrambled video signal. The microprocessor 
could then add or subtract previous digitized horizontal lines of video 
information stored in a RAM and add or subtract previously descrambled 
video signals to or from the horizontal line of video information present 
at the input of the descrambling unit to produce a digitized version of 
the descrambled video signal. The digitized descrambled video signal could 
then be amplitude adjusted and converted from digital to analog in a D/A 
convertor. 
The foregoing detailed description has been given for clearness of 
understanding only, and no unnecessary limitations should be understood 
therefrom, as modifications will be obvious to those skilled in the art.