An apparatus for scrambling a television signal in which horizontal lines of the transmitted television picture are selectively inverted in accordance with the state of a pseudorandom noise generator. In order to permit correction of the throughput gain as well as the throughput offset at the receiving end of the communication link so as to obtain a desired level of picture quality, particularly in a satellite communication system, an analog calibration signal representing both black and white levels is inserted into a horizontal line of the vertical retrace interval at the transmission end of the link. This calibration signal is then detected at the receiving end of the link and the signal levels are adjusted as needed. In addition, a randomly-varying key code signal for resetting the pseudorandom noise generator is randomly inserted into another horizontal line of the vertical retrace period to control scrambling. The audio portion of the television signal is also scrambled by randomly shifting the audio carrier between first and second offset frequencies.

BACKGROUND OF THE INVENTION 
The present invention relates in general to video scrambling techniques for 
the protection of television transmissions, and more particularly, to a 
scheme for digitally-controlled analog encryption of video signals to be 
transmitted via satellite communication. 
A number of schemes for scrambling television signals have been proposed in 
recent years, particularly with the advent of subscription television 
wherein subscribers pay to receive noncommercial television programs 
generally transmitted to their receivers via coaxial cables. In such 
systems, the secure transmission of television programs to authorized 
subscribers only is essential to the commercial success of the system, and 
therefore, encoding systems of varying complexity have been developed to 
prevent unauthorized receipt of such programs by those who have not paid 
for this service. 
One general example of a system for scrambling and unscrambling 
communication signals is disclosed in the Hartung et al. U.S. Pat. No. 
3,919,642, issued on Nov. 11, 1975, in which video scrambling and 
unscrambling are effected by inversion of selected horizontal lines of a 
transmitted television picture in accordance with various possible modes 
including, inversion of alternate groups of equal numbers of lines, 
inversion or noninversion of lines selected on a line-by-line basis in 
accordance with a control signal transmitted with each line, and inversion 
or noninversion of lines in a preselected sequence in accordance with the 
output of a suitable random noise generator. Along with the scrambling and 
unscrambling of the video signal, this prior art system also provides for 
scrambling of the audio portion of the transmitted signal by offsetting 
the audio carrier and thereby increasing the frequency difference between 
the video and audio carriers. 
Another such system is disclosed in the Harna et al. U.S. Pat. No. 
3,732,355, issued May 8, 1973, in which a video signal is encoded by 
switching it alternately between delayed and undelayed modes several times 
during each field in response to a locally-generated phase-varying 
rectangular switching signal which is varied randomly at random intervals 
in response to a series of random-state control pulses from an inhibitable 
random pulse generator, giving a jittered effect to the picture as the 
alternately-displaced bands vertically shift position in a random manner. 
The inhibitable random pulse generator generates an output pulse 
indicative of a random one of a series of possible counting states, one or 
more of which may be inhibited to narrow selection of the remaining 
states. A counter included in the generator first has random noise applied 
to it for a predetermined period of time, leaving the counter in an 
unpredictable random counting state, and then, stepping pulses are applied 
to step the counter to the first uninhibited state, at which time counting 
action ceases and a representative output pulse is developed to control 
the switching between modes. 
Another system which is based upon the inverting of randomly-selected 
fields to effect scrambling of a television signal is disclosed in the 
Horowitz U.S. Pat. No. 4,075,660, issued Feb. 21, 1978. In this system, 
the transmitted signal is encoded by reversing the polarity of the video 
signal during randomly-selected fields and encoding bursts are injected 
into the composite signal prior to transmission to indicate whether or not 
the subsequent field has a video portion to be inverted. For purposes of 
encoding the audio portion of the transmitted signal, the program audio 
signals are modulated onto a suppressed carrier which is derived from the 
horizontal synchronizing signals and has a frequency equal to twice the 
horizontal line frequency. 
In addition to systems which utilize synchronized noise generators at both 
the sending and receiving locations to effect encoding of the transmissed 
television signals, various coding systems such as disclosed in the 
Sherman U.S. Pat. No. 4,081,832, issued Mar. 28, 1978, and the Loshin U.S. 
Pat. No. 4,025,948, issued May 24, 1977, have been provided wherein the 
program to be broadcast is encoded to effect electrical inversion of 
selected groups of lines of the program thereby to scramble and disguise 
both the video and audio signals, and simultaneously with the coding of 
the program, a punchcard is prepared which is forwarded to the subscriber 
for use in enabling a decoder associated with the subscriber television 
set to permit receipt of the transmitted television signals. 
In addition to the fact that the foregoing systems are often quite 
complicated and expensive, sometimes involving the use of punchcards for 
decoding, the various schemes for scrambling the video and audio signals 
have often proven to be less than completely foolproof. However, even more 
importantly, while these prior art systems may operate in a satisfactory 
manner in the specific environment for which they have been particularly 
designed, that is, transmission directly to the subscriber via coaxial 
cable, various problems arise when such systems are called upon to operate 
under different conditions and circumstances. For example, where the video 
communication is to be by way of satellite communication, it is extremely 
important to maintain a very high signal quality since the video signal 
must be retransmitted for distribution. Thus, if the particular encoding 
scheme which is adopted results in an inherent deterioration of the signal 
quality, the system becomes impractical for such communication. 
One of the particular problems which results from the selective inversion 
of horizontal lines or combinations of horizontal lines in successive 
television fields, as suggested in the above-mentioned prior art systems, 
relates to the introduction of differential nonlinearities in the 
transmitted signals due to gain variations and level shifting in the 
system with the result that the dynamic range and quality of the 
transmitted television picture often suffers dramatically, resulting in a 
"washed out" picture. In particular, with the inversion of video 
information, a loss in the black level reference typically results which 
directly affects the picture quality of the transmitted video signals. 
Efforts to restore the black level of the video signal upon unscrambling 
by clamping the signal levels at the receiver to predetermined values have 
proven to be less than completely satisfactory, since such compensation 
fails to provide a dynamic correction of the problem which requires 
correction of both throughput gain and throughput offset. 
SUMMARY OF THE INVENTION 
Accordingly, one of the features of the present invention is to provide a 
digitally-controlled analog encryption system for scrambling video signals 
in which there is provided a restoration of the black level as part of the 
unscrambling operation, and this black level restoration is accomplished 
on the basis of level signals which are transmitted with the scrambled 
video from the sending end of the communication link. More particularly, 
in accordance with the present invention, one of the unused horizontal 
lines forming part of the vertical retrace interval is used to carry both 
the white and black calibration levels. In this way, a dynamic restoration 
of the black level of the video signal being unscrambled is made possible 
while at the same time permitting dynamic correction of the throughput 
gain, both types of correction being essential to ensure satisfactory 
picture quality. 
A further feature of the present invention relates to the provision of 
increased security in the scrambling of the video signal in a system 
wherein selective horizontal lines of the transmitted television picture 
are selectively inverted in accordance with the output of a pseudorandom 
noise generator (commonly known as a PN generator). This is accomplished 
by resetting the PN generator to a predetermined state in accordance with 
the output of a second PN generator. In this regard, a key is generated 
which is capable of setting the first-mentioned PN generator in both the 
scrambler control logic and descrambler control logic to a predetermined 
state. By inserting the key into a horizontal line in the vertical retrace 
interval of the clear video prior to application of the video to the 
scrambler, at a rate determined by the output of the second PN generator, 
the PN sequence which controls the scrambling and unscrambling operations 
will be reset and restarted at varying rates, greatly increasing the 
security of the encoding of the video signal to prevent unauthorized use 
thereof. 
To even further increase the security of the scrambling operation, the key 
which is used to reset and restart the PN generator which controls 
scrambling and unscrambling can be changed at random times, such as by a 
further PN generator or a microprocessor. 
A further feature of the present invention relates to the encoding of the 
audio portion of the transmitted television signal, which is accomplished 
by effecting two frequency offsets in the audio signal with the second 
frequency offset effecting a shifting of the baseband audio back and forth 
in frequency between first and second offset frequencies. This provides 
for greatly increased security as compared to the single fixed offset of 
the audio carrier, as disclosed in the above-mentioned Hartung et al. 
patent, which merely requires a knowledge of the extent of the shift in 
frequency of the carrier to effect a decoding of the signal. With the 
system provided in accordance with the present invention, the shifting of 
the baseband audio back and forth in frequency in accordance with one or 
the other of two frequency offsets controlled in accordance with the 
output of a PN sequence generator produces a highly-secure coding of the 
audio which is incapable of being deciphered simply on the basis of 
determination of a single frequency offset. 
It is therefore an object of the present invention to provide a 
digitally-controlled analog encryption circuit which is capable of use 
with satellite communication systems. 
It is another object of the present invention to provide a system of the 
type described which is capable of providing secure encoding of video 
signals without discernible deterioration in signal quality. 
It is a further object of the present invention to provide a system of the 
type described in which enhanced security in the coding of both video and 
audio portions of the transmitted television signal is achieved in a 
system capable of maintaining high signal quality. 
The objects of the present invention are achieved by a system which 
includes, at the transmitting end, an analog multiplexer connected in 
series with a scrambler in the form of a video inverter. The clear video 
is applied to the analog multiplexer which serves to insert into an unused 
horizontal line of vertical retrace interval of the television signal both 
black and white reference levels and a key for controlling the scrambling 
of the video signal. A scrambler control logic circuit includes a PN 
generator which is driven from the horizontal sync signals detected from 
the clear video supplied at the output of the analog multiplexer, the 
output of which controls the video inverter to effect inversion of 
selected horizontal lines of the television signal and this PN generator 
in the scrambler control logic is selectively reset in accordance with a 
key which has been inserted into the video signal in a selective manner by 
the analog multiplexer. A key logic control circuit which is operated from 
the horizontal sync signals includes a code selector whose output is 
applied to a shift register for storage of the selected key therein. A 
second PN generator in the key logic control circuit is driven at a lower 
rate keyed to the horizontal sync signals and the output thereof operates 
a key control circuit to selectively shift the key out of the shift 
register to the analog multiplexer in accordance with the state of the key 
control PN generator. 
The descrambler which is located at the receiving end of the link is 
provided in the same form as the scrambler. The unscrambler control logic 
includes a PN generator which is selectively reset by the key received 
with the scrambled video signal and serves to control the operation of a 
video inverter to effect reinversion of the selected horizontal lines 
previously inserted during the scrambling operation. At the output of the 
video carrier, the unscrambled video is applied to a black level 
restoration circuit which readjusts the black level of the video signals 
in accordance with the black level calibration signal detected in the 
received video. The white level calibration signal is used at the same 
time to correct the throughput gain of the amplifiers, so that gain 
variations and level shifting are both eliminated from the received video 
signal. 
In scrambling the audio portion of the transmitted television signal, the 
audio is applied to a balanced mixer where it is mixed with a fixed 
frequency provided at the output of a first phase locked loop. The signal 
provided at the output of the balanced mixer is applied to a filter where 
one sideband is filtered out and the result is applied to a second 
balanced mixer to which there is also randomly applied one or the other of 
two modulating frequencies selected by the output of a PN sequence 
generator. The output of the second balanced mixer is then applied through 
a bandpass filter and the resulting scrambled audio is then ready for 
transmission. 
These and other objects, features and advantages of the present invention 
will become more apparent from the following more detailed description of 
the present invention when taken in conjunction with the accompanying 
drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the embodiment of the present invention illustrated in FIG. 1 clear 
video received on line 1 is applied to a scrambler 10, which may be 
provided in the form of a switchable gain amplifier, via an analog 
multiplexer 12, which serves to multiplex onto the video signal various 
reference and control signals under control of the scrambler control logic 
14 and key logic control circuit 16. The audio portion of the television 
signal to be transmitted is received on line 2 and is applied to a 
spectrum filter 22 which is controlled by audio scrambler control 24 in 
response to timing signals received from the scrambler control logic 14 
and key logic control circuit 16. The scrambled audio is provided at the 
output of the spectrum shifter 22 via an amplifier 26. 
The analog multiplexer 12 which receives clear video on line 1 has the 
purpose of multiplexing reference control signals onto the video signal so 
as to ensure increased security and to enhance the picture quality of the 
video signal at the receiving end of the communication link. As is well 
known, between successive fields of a television picture, there is a 
vertical blanking interval during which the beam is blanked out and 
positioned for the start of the next field. The vertical blanking interval 
conventionally includes a group of equalizing pulses, some wider vertical 
synchronization pulses, a further group of equalizing pulses, followed 
finally by a number of yet unused horizontal lines which appear before the 
first line of video information in a new field. Using well-known 
multiplexing techniques, the analog multiplexer 12 under control from the 
scrambler control logic 14 utilizes two of the unused horizontal lines 
provided within the vertical blanking interval for transmission of 
information concerning white and black reference levels as well as a key 
reference for controlling the scrambling and descrambling operations to be 
performed at the transmitter and receiver ends, respectively, of the 
communication link. 
The scrambler 10 receives the video signals on which there is encoded the 
white and black reference and key reference signals from the output of the 
analog multiplexer 12 and operates to invert selected horizontal lines of 
the transmitted television signal under control of the scrambler control 
logic 14. As a result of such encoding of the video signals, it is 
inevitable that nonlinearities due to gains and level shifting will be 
introduced into the video signal resulting in a commensurate degradation 
in the quality of the picture reproduced from the transmitted signals 
after decoding. Basically, it is extremely difficult to get all of the 
gains and all of the offsets to repeat throughout the system; accordingly, 
some means must be provided to correct the throughput-gain as well as the 
throughput offset. This is accomplished in accordance with the present 
invention by adding a calibration signal at the input to the video decoder 
which represents zero and maximum signal levels (black and white) to 
permit adjustment of the gain and linearity of the amplifier provided at 
the receiving end of the link. These calibration signals are then measured 
at the receiving end of the link to determine both black and white levels, 
and both the inverted and noninverted lines are detected and the offsets 
and gain variations which have been introduced during the scrambling at 
the receiving end of the link are corrected. 
FIG. 2 illustrates as a schematic diagram one horizontal line in the 
vertical retrace interval showing the insertion of the black and white 
reference levels between consecutive horizontal sync pulses. It should be 
noted that this diagram is not to scale and does not show all of the 
details of the standard television signal except for the provision of the 
black and white reference levels which are utilized in accordance with the 
present invention. As seen in FIG. 1, a circuit 20 comprising resistors 17 
and 18 connected in series between a fixed voltage source V+ and ground 
provides on the line 19 the white reference voltage level. Similarly, the 
black reference voltage level is supplied from ground on line 21 to the 
analog multiplexer 12, where these signals will be inserted into the video 
received on line 1 in the manner seen in FIG. 2. 
The details of the scrambler control logic 14 are illustrated in FIG. 3. 
This circuit provides for control of the selected inversion of horizontal 
lines in the video signal to effect encoding thereof, the selected 
inversion being controlled in accordance with the state of a pseudorandom 
noise generator 30 which operates in response to the horizontal sync 
pulses derived from the video signal via sync detector 34. The state of 
the generator 30 is controlled by the output of key detector 32. The 
output of generator 30 is supplied through AND gate 37 to the scrambler 
10, the control of the AND gate 37 being provided from the horizontal sync 
pulses via a pair of one-shot multivibrator circuits 36 and 38. In 
addition, the control signals for the analog multiplexer 12 are provided 
by a multiplex control circuit 39 in response to the vertical and 
horizontal sync pulses supplied by the sync detector 34. 
The video signals which have been encoded in the analog multiplexer 12 to 
include the black and white reference and key reference signals are 
supplied on line 3 to the key detector circuit 32 and sync detector 
circuit 34 simultaneously with application thereof to the scrambler 10. 
The key detector circuit 32 detects the key which has been encoded on the 
video signal by the analog multiplexer 12 and sets the PN generator 30 to 
the state indicated thereby. In this regard, the key may take the form of 
a simple reset pulse which serves to reset the PN generator 30 to an 
initial state or, preferably, takes the form of a coded sequence which 
serves to set the PN generator 30 to the state indicated by the key code. 
The sync detector 34 detects the leading edge of the horizontal sync pulse 
and uses this signal to clock the PN generator 30. This signal also is 
delayed by the one-shot 36 to produce a signal coincident with the 
beginning of video in the signal on line 3 and a further delay by one-shot 
38 produces a gate signal which has the same length as the horizontal 
video scan line. This gate signal provided at the output of one-shot 38 
enables the AND gate 37 to apply the output of the PN generator 30 to the 
scrambler 10, which may take the form of a switchable gain amplifier. In 
this way, depending on the output from PN generator 30, successive 
horizontal lines will be either inverted or not inverted producing a 
scrambling of the video on the output line 4. 
In accordance with the present invention, increased security in the 
scrambling of the video signal is accomplished by varying the key which 
resets and restarts the PN generator 30 in the scrambler control logic 14 
at random times. This control is provided by the key logic control circuit 
16, the details of which are illustrated in FIG. 4. A shift register 40 
receives a coded key from a code selector 42 and this key will be 
selectively shifted out onto line 5 to the analog multiplexer 12 by a key 
control 48 under control of a PN generator 44 driven from the output of a 
divider 46 to which are applied the horizontal sync pulses obtained from 
the scrambler control logic 14. The code selector 42 may simply take the 
form of a manually-settable switch circuit, or it may be provided as a 
microprocessor or even another PN generator circuit, both of which are 
capable of providing randomly-variable key codes to the shift register 40. 
In operation of the circuit of FIG. 4, the PN generator 44 is driven at a 
reduced rate keyed to the horizontal sync pulses applied to the divider 
circuit 46. The randomly-variable output of PN generator 44 is then 
applied to the key control circuit 48 which selectively applies clock 
signals to the shift register 40 to shift out the contents thereof onto 
line 5 to the analog multiplexer 12. For example, each time the output of 
generator 44 is a "1" the key control circuit 48 will detect this 
condition and apply clock signals to the shift register 40 to read out the 
contents thereof. Thus, a key code will be applied to the line 5 extending 
to the analog multiplexer 12 at a random rate and this key code will be 
encoded in an available horizontal line forming a part of the vertical 
retrace interval. At the same time the video signal is applied to the 
scrambler 10, as seen in FIG. 3, the key detector 32 will detect the key 
which has been encoded on the video signal and reset the PN generator 30 
to that particular key code. Since the key code is multiplexed onto the 
video signal at random times, the PN generator 30 in the scrambler control 
logic 14 will be restarted at that particular code at random times, 
further complicating the coding of the video signals and rendering them 
more secure against unauthorized use. This is even further enhanced where 
the code selector 42 takes the form of a microprocessor or further PN 
generator circuit from which new key codes are generated at random times. 
It is also possible, as apparent from the foregoing description, to 
eliminate the key detector 32 in the scrambler control logic 14 and to 
apply the key output on line 5 from the key logic control circuit 16 
directly to the PN generator 30 as well as to the analog multiplexer 12. 
However, from the point of view of standardizing the scrambler and 
descrambler systems, the use of the key detector 32 in the scrambler 
system may be desirable. 
As seen in FIG. 5, the spectrum shifter 22 which serves to scramble the 
audio portion of the transmitted television signal, includes, in series, 
an audio interface circuit 51, a first balanced mixer 50, a first filter 
54, a second balanced mixer 55 and a bandpass filter 58. At the 
transmitter end of the communication link, the filter 54 in the spectrum 
shifter comprises a highpass filter; whereas, at the receiving end of the 
link, the filter 54 in the corresponding spectrum shifter will comprise a 
lowpass filter. The audio scrambler control circuit 24, on the other hand, 
comprises a first modulating signal generating circuit 49 comprising a 
phase-locked loop 52 and divider 53 providing a modulating frequency to 
the balanced mixer 50, and a second modulating frequency generating 
circuit 59 comprising a phase-locked loop 56 connected to a divider 57 for 
providing a modulating frequency to the balanced mixer 55. 
The operation of the audio scrambler portion of the system in accordance 
with the present invention, which is identical to the descrambler provided 
at the receiving end of the communication link, will be described in 
conjunction with the spectral components illustrated in FIGS. 6A-6G. The 
audio is supplied on line 2 via the audio interface 51 to one input of the 
balanced mixer 50 having a spectral range generally indicated in FIG. 6A. 
To the other input of the balanced mixer 50 there is provided the output 
of the modulating signal generator 49, which is locked to a multiple of 
the horizontal sync frequency of the video system. Thus, the input 
frequency of 15,750 Hz is converted at the output of generator 49 to a 
stable frequency of 47,250 Hz, which becomes the carrier frequency for the 
modulated audio frequency signal obtained at the output of the interface 
circuit 51, as seen in FIG. 6B. The filter 54 connected to the output of 
the mixer 50 serves to filter out one sideband of the modulated signal and 
applies this filtered signal to one input of the balanced mixer 55. The 
other input of the mixer 55 is derived from the modulating signal 
generator 59, which is capable of selectively generating one of two 
modulating signals each of which represent a multiple of the horizontal 
sync frequency of the video system. In this regard, the divider 57 which 
is connected to the phase-locked loop 56 is responsive to the output of a 
PN sequence generator, such as the generator 44 which forms part of the 
key logic control circuit 16, to vary the output of the generator 59 
selectively between 46,725 Hz and 46,620 Hz. As a result, the signal 
applied to the balanced mixer 55 is mixed back down to baseband with a 
second frequency offset which shifts the baseband audio back and forth in 
frequency across the baseband in accordance with the modulating frequency 
selected by the output of the PN sequence generator applied to line 6, as 
seen in FIG. 6D or 6F, respectively. The bandpass filter 58 then filters 
out the carrier and upper sidebands to leave only the lower sideband, as 
seen in FIGS. 6E and 6G, respectively. 
As can be seen from FIG. 6, the audio scrambling operation performed in 
accordance with the present invention represents a double modulation 
scheme involving two frequency offsets in which the audio carrier is 
shifted randomly in accordance with the output of a PN sequence generator, 
thereby materially increasing the security of the scrambled signal as 
compared to prior schemes. This feature in combination with the random 
variation in key code generation and transmission as well as the 
incorporation of black and white level reference signals into the video 
signal at the transmission end of the communication link provide for a 
combination of increased security and picture quality. 
As seen in FIG. 7, the descrambler system which operates in accordance with 
the features of the present invention is essentially a complement of the 
scrambler system of FIG. 1. The most apparent difference between these two 
systems is that the descrambler system has no requirement for the analog 
multiplexer 12 and the key logic control circuit 16. On the other hand, 
the descrambler system includes a gain and offset equalization circuit 70 
for control of the gain and offset in the descrambled video signal. 
The descrambling operation begins with receipt of the scrambled video 
signal at the input of the descrambler 10'. At the same time, the 
descrambler control logic 14', having the same configuration shown in FIG. 
3, extracts the horizontal and vertical sync signals as well as the key 
carried by the video signal and controls the descrambler 10' to 
selectively re-invert the previously-inverted horizontal lines of the 
video which occurred during the scrambling operation. 
The unscrambled video appearing at the output of the descrambler 10' is 
then applied to the gain and offset equalization circuit 70 where gain and 
offset are equalized in accordance with the black and white levels carried 
by the video signal itself. As seen in FIG. 8, the detected horizontal and 
vertical sync pulses derived from the received video signal are applied to 
the reference line detector 71 which detects the horizontal line in the 
vertical retrace interval carrying the black and white reference levels. 
Upon detecting the proper horizontal line, the detector 71 triggers 
one-shot 72, which provides an enabling output signal to the offset 
equalizer circuit 73 for the time period t.sub.1, during which the black 
reference level is being received, as seen in FIG. 2. The equalizer 
circuit 73 adjusts the offset at this time on the basis of the received 
black level, providing for sampling and holding of this reference value 
until receipt of the next frame, at which time a new reference will be 
received. 
When the one-shot 72 resets at the end of time t.sub.1, the one-shot 74 is 
triggered, providing an enabling signal to the gain equalizer 75 for the 
time period t.sub.2 during which the white reference level is received, as 
seen in FIG. 2. A sample and hold circuit in the gain equalizer 75 samples 
the reference level and holds it until receipt of the next frame. 
On the basis of the received black and white reference levels, both the 
offset and gain of the circuits in the descrambler system are dynamically 
adjusted to maintain the dynamic range and quality of the video signal. 
Such dynamic correction is clearly superior to fixed bias correction and 
other known schemes, especially in the area of satellite communications. 
The descrambling of the audio signal is accomplished in a manner 
complementary to the scrambling operation. The audio descrambler control 
24' is responsive to an output from one-shot 77, which is triggered by the 
key detector in the descrambler control logic 14' to reconstruct the 
frequency control signal, which controls the shifting of the audio back 
and forth in frequency across the baseband by the spectrum shifter 22'. 
Thus, as can be seen from the foregoing description, the present invention 
provides a digitally-controlled analog encryption circuit which is capable 
of providing secure encoding of video signals without discernible 
deterioration in signal quality, providing dynamic correction of both 
throughput gain and throughput offset on the basis of black and white 
reference levels transmitted with the video signal itself. 
While we have shown and described several embodiments in accordance with 
the present invention, it is understood that the invention is not limited 
thereto but is susceptible of numerous changes and modifications as are 
known to those of ordinary skill in the art, and we therefore do not wish 
to be limited to the details shown and described herein but intend to 
cover all such changes and modifications as are obvious to one skilled in 
the art.