Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates generally to video signal gain
control, and more particularly, to the control of the gain
of video signals in accordance with a plurality of indepen-
dently related video signal gain determining key signals.
In television production, it is often desirable to
process a portion of a television signal less than the
standard full field raster. For example, a common function
of a digital special effects system is to compress a full
field of video to a size smaller tha~n the raster area
occupied by a full size field of the television signal. In
such a field of video, only the area corresponding to the
compressed video image is of intere~t. The surrounding
display area contains no information of interest. When this
video signal is outputted to a utilization device, it is
desirable for that device to know what area to process.
This is the function of a matte or key signal that accom-
panies the compressed or otherwise processed video signal
and defines the gain throughout the video signal interval.
A matte or video signal gain defining key signal
effectively specifies the area of interest or the area to be
processed. Key signals are commonly created during the
processing of a video signals, but can also be independently
created. For example, a key signal can be independently
created that represents selected shapes. Television signals
are commonly created by television cameras, graphic art
generators and other devices. Key signals can be created by
dedicated key signal generators, digital special effects
units, and other video signal processing devices. A
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c!~roma-key generator creates a key signal corresponding to a
specific color found in an input video signal and can be
used as a video signal gain defining key. For example, if
the video signal from a camera recelving an image of an
object against a blue background and chroma-keyed on the
blue color background, an object key can be produced from
the chroma key signal. In the digital special effects
system example discussed above, a key signal can be created
that corresponds to the area of the compressed field of
video.
A video gain defining key signal is used to control the
gain of video signals over a range and with a resolution
determined by the range and number of values the key signal
can take. Normally, such keys are linear so that there is a
one to one correspondence between each part of the key
signal and the video signal. Typically, the gain of a video
signal is adjusted by the key signal by multiplying the
level of the video signal by the key signal. When operating
in the analog domain, the instantaneous level or amplitude
of the video signal is multiplied with the instantaneous
value of the key signal on a continuous basis. Ordinarily,
an analog key signal has a value between zero and one.
A digital video signal is composed of discrete digitally
encoded samples defining pixels o~ the video image. Each
pixel may be defined by a plurality of different components,
such as a single luminance and a pair of chrominance compo-
nents. In systems for such video signals, a digital key
signal is used having a value between zero and one for each
pixel of the video image. When processedl each pixel of the
video signal is processed with its corresponding key signal.
The value of a key signal often varies with time. In an
analog system, the key value has an instantaneous value that
changes with time, whereas in a digital system each pixel
has a discrete associated key value.
One type of key signal is a bi-state kev signal. This
type of key signal has either a value of zero or one. When
processed with a video signal, the portion of the key signal
that has a value of zero is multiplied with the
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corresponding portlon of the video signal to yield a pro-
cessed or keyed video signal having a gain of zero, which is
indicative of the absence of a video image. This portion of
the ke~ed video signal appears as black when viewed on a
monitor. The portion o~ the video signal that is processed
with a key signal that has a value of one remains unchanged
and is viewed on a monitor unchanged. In the aforediscussed
object example, the portion of the key signal outside the
representation of the object has a value of zero. The
portion of the key signal corresponding to the inside of the
object has a value of one.
The value of a key signal often is correspondingly
processed proportionately with the gain of the video signal.
If the values of two key signals are processed with their
respective video signals representing two objects, the
result would be a display of one of the objects shaped in
the form of the other.
Another type of key signal is a linear key signal.
Unlike the bi state key signal which is restricted to either
a value of zero or one, a linear key signal may have several
values between zero and one. ~hereas, with the bi-state key
signal, one typically indicates the presence of the video
signal and zero the absence of the the video signal, the
value of the linear key signal represents a continuum of
change in gain of the video signal.
When a video signal is processed with a linear key
signal, the corresponding signals are multiplied, as in the
case of the bi-state key signal~ but because the linear key
can have any value between zero and one, the gain of the
video signal can be adjusted to any value between zero and
one. For example, if a point in a video signal is process
with a linear key that has a value of 0.5, the gain of the
video signal would be reduced in gain by one half. When
viewed on a monitor, the video signal would look
semi-transparent.
In combining typical video signals, the total gain of
the two signals is never made more than one. Two video
- ~ signals of nominal gain would not be directly added, as
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their total gain would be two times the normal or nominal
gain of a video signal past through the system. Adjusting
the gain of a video signal with a key signal is one method
of reducing the gain of vldeo signals so that they can be
comblned with other video signals. Adding two gain adjusted
video signals, each of which has been multiplied with a key
signal of 0.5, would be permissible, as the gain of the
combined video signals would only be one. The resulting
video signal when viewed on a monitor would be a combination
of both video signals.
One method of combining two video signals whose gains
are nominal, i.e., not reduced, is to use an independent or
external key signal to process the video signals. In this
method, the independent key signal determines the amount of
each of the two signals present in the combined or mixed
video signals. The gain of one of the video signals is
reduced directly by the key signal in a video processor,
which multiplies the video signal by the key signal. The
gain of the other video signal is reduced by one minus the
value of the key signal, which is determined by a 1 K key
processor.
Generally, the gain of a video signal is processed by a
key signal as a last step prior to forming the desired
output video signal. There are situations, though, where it
is necessary to process a video signal whose gain has been
previously adjusted by another key signal. The way this has
been done previously is to adjust the gain of the video
signal without regard to any previous gain adjustment.
If, in the above example, the gain of the video signal
had been previously reduced at a point, by a key signal of
0.5, its gain would be 0.5 of its original value. When its
gain is reduced further by an independent key of, for
example, 0.3, the result gain of the signal is reduced to
0.15. When combined with the second video signal, the
combined video signal gain is only 0.85. This is unde-
sirable for two reasons. First, the independent key value
of 0.3 usually specifies the desired value for the gain of
the related video signal. Because this video signal had
~lready been reduced in gain, the processing by the indepen-
dent key reduces its gain to a level below the desired
level. Secondly, it is very desirable to fully use all the
gain available for the combined video signal. The combined
video signal, in this example, only has a gain of 0.85, not
the desired 1Ø
A desirable method of combining one or more gain
adjusted video signals, would take into account any previous
gain adjustments of the video signals. There is a need for
a method and apparatus which is able to combine previously
gain adjusted video signals so as to obtain video signals
with the desired gain specified by a key signal.
The present invention overcomes the limitations of the
previous method by taking into account the previous gain
reduction of video signals.
When gain adjusting a video signal with an independent
key signal, the independent key signal is considered the
desired level of gain reduction. If the video signal has
already been reduced in gain below the leve]. of the indepen-
dent key signal, it is preferred not to reduce its gain
further. This is because when the gain on a video signal is
reduced, information is lost. That lost informatlon cannot
be recovered by raising the gain of video signals. Raising
the gain adds false information or noise to the video
signal.
If the video signal has been reduced in gain by an
amount less than the desired level called for by the inde-
pendent key signal, then the gain of the video signal is
adjusted in accordance with the values of both the indepen-
dent key and the reciprocal of the value of the key signal
used to previously adjust the gain of the video signal. As
will be described further hereinafter with reference to the
preferred embodiments illustrated in the drawings, this can
be conveniently accomplished in one step by multiplying the
video signal with a key signal having a value corresponding
to the ratio of the value of the independent key signal to
the value of the key signal previously used to adjust the
gain of the video signal. The result of so adjusting the -
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gain of the video signal is the resulting video signal isreduced in gain by the precise amount to bring the to-tal
gain reduction as specified b~ the independent key
To adjust the gain of a previously gain adjusted video
signal with an independent key signal, it is preferred to
determine the minimum be-tween the values of the key signal
previousl~ used -~o adjust the gain of the video signal and
the independent key signal that identifies the desired gain
for the video signal. This is employed to determine which of
the two key signals speciEies the smaller video signal gain.
Whichever key signal specifies the smaller gain, it is
employed to determine the further gain adjustment of the
video signal. If the independent key signal specifies the
smaller gain, it is coupled to effect the further gain
adjusiment of the video signal. On the other hand, if the
independent key signal specifies a larger gain, the key
signal related to the video signal that previously caused
adjustment of the gain of the video signal determine the
further video gain adjustment of the video signal. In such
event, the result is the video signal's gain remains the
same as it was when previously adjusted by the related key
signal. This minimum key signal value determination has the
advantage of assuring that the gain of the resulting gain
adjusted video signal is not increased above the gain it had
as a result of the previous gain adjustment. In an embodi-
ment wherein a ratio of key signals is employed to effect
gain adjustment of the video signal, the key signal obtained
as a result o~ the minimum determination is used to form the
ratio relative to the key signal that determined the previous
gain adjustment of the video signal. As can be appreciated
from the foregoing, the ratio will be unity whenever the
value of the independent key signal is larger than that of
the key signal which determined the previous gain adjustment
of the video signal. Consequently, no further adjustment of
the gain of the video signal occurs under such circumstance.
To combine a gain adjusted video signal according to an
independent lcey signal with a video signal whose gain has
not been adjusted, the value of the input key signal
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si~nifying the input video signal gain and the value of the
indepe~dent key are detected. The gain of the input video
signal is ad,usted in accordance with the value of the
independent key signal and the reciprocal of the value of
the input key signal to produce a first gain adjusted video
signal. In addition, the difference between the gain of the
first gain adjusted video signal and a reference gain value,
usually, corresponding to unity video signal gain, is
detected. In response to the detected di~ference, the gain
of the video signal whose gain has not been previously
adjusted is adjusted to a val.ue determined by the detected
difference to produce a second gain adjusted video signal.
These two gain adjusted video signals are combined to
produce a combined output video signal.
Apparatus to effect such combining of a previously gain
adjusted video signal with a video signal whose gain has not
been previously adjusted advantageously employs a minimum
key signal value determining circuit to determine which of
the two independent and input key signals has the smaller
value. In the manner previously described, this minimum
circuit operates to insure that the resulting gain of the
combined video signal is not greater than that of the
reference value, or unity. Moreover, when employed with a
circuit for generating a ratio of the value of the indepen-
dent key signal taken against the value of the input key
signal, each of the gain adjustments of the two video
signals is able to be performed in a single step with a
single multiplier.
The present invention can be arranged to also combine
two previously gain adjusted video signals according to an
independent key signal. This is accomplished by first
detecting the values of the independent key signal and each
of the input key signals signifying the respective gains to
which the two input video signals have been previously
adjusted relative to a reference gain, typically unity. The
gain of one of the two input video signals is adjusted in
accordance with the value of the independent key signal and
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the reciprocal of the value of one of the input key signals
to produce a first gain adjusted video signal. The differ-
ence between the gain of the first gain adjusted video
signal and the reference gain is detected. In response to
-this detected difference, the gain of the other of the two
video signals is adjusted according to the detected differ-
ence and the other of the two input key signals to produce a
second gain adjusted video signal~ These two gain adjusted
video signals are combined to produce a combined output
video signal.
Apparatus to effect such combining of two previously
gain adjusted video signals advantageously employs two
minimum key signal value determining circuits to insure that
the resulting gain of the two gain adjusted video signals is
not greater than that of the reference value, or unity.
Furthermore, a pair of ratio determining circuits are
utilized so that the gain adjustment of each of the two
video signals is able to be performed respectively in a
single step with a single multiplier. The manner in which
such minimum circuits and ratio determining circuits func-
tion to effect such results will become more apparent upon
consideration of the detailed descriptions of preferred
embodiments of the present invention illustrated by the
figures. Generally, however, in the operations performed
by these circuits, the minimum between the value of the
input key signal associated with a previously gain adjusted
video signal and the value of the independent key, which
signifies the desired gain for the video signal, is deter-
mined. The minimum value key signal is then further
processed to obtain a value inversely proportional to the
value of the associated input key. Consequently, when the
value of the independent key signal is greater than the
value of the associated input key signal, there will be no
further adjustment of gain of the previously adjusted video
signal. However, if the value of the independent key calls
for a reduction in gain that is less than that to which the
video signal has previously been adjusted, the minimum
circuit will function to provide a value corresponding to
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that of the independent key signal, whereby the gain oE the
video signal is reduced to the level called for by the
independent key.
Various of the above-mentioned and further features and
advantages of the present invention will be apparent Erom
the specific examples of an exemplary appara-tus and metho~
described hereinbelow with reference to the accompanying
drawinys, in which:
Figuxe 1 is a schematic block diagram of one embodiment
of a system for combining video signals of different pos-
sible gains in accordance with the presen-t invention;
Figure 2 is a schematic block diagram of another
embodiment oE a system for combining video signals of
different possible gains in accordance with the present
invention;
Figure 3 is a schematic block diagram of a further
embodiment of a system for combining video signals of
different possible gains in accordance with the present
invention;
Figure 4 is a schematic block diagram of one embodiment
of a circuit for adjusting the gain of video signals in
accordance with the present invention;
Figure 5 is a schematic block diagram of another
embodiment of a circuit for adjusting the gain of video
signals in accordance with the present invention; and
Figure 6 is a schematic block diagram of a further
embodiment of a circuit for adjusting the gain of video
signals in accordance with the present invention.
In accordance with the present invention, the video and
key signals may either be in digital or analog form,
although preferred form is digital. In an analog system,
video signals are processed continuously with their
corresponding key signals, although, at any one point in
time, only one point in the video signal is being processed
with the corresponding point in the key signal.
Further, the discussed embodiments are formed of
combinations of one or more of six basic circuit elements.
Each element can be implemented in a variety of ways in
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either an analog or digital system. Video signal gain
adjustment devices used to adjust the gain of a video signal
according to the value oE a key signal can be conveniently
implemented i~ a digital system as a digital multipIier.
These are commonly available in standard integrated circuit
pac~ages. In a non-real time signal processing system, the
video signal gain adjustment device can be implemented with
a microprocessor~ ~n analog video signal gain adjustment
device can be implemented as a simple analog multiply
circuit.
Another circuit element utilized in certain embodiments
of the present invention is a video signal combiner. The
video signal combiner is used to combine gain adjusted video
signals to form an output video signal composed o~ a
combination of input video signals after one or more of such
signals have been gain adjusted. In a digital system, this
element is easily implemented as a digital adder, which is
also commonly available in standard integrated circui-t
packages. In an analog system, the video combiner can
implemented as a simple mixing circuit.
A common element of preferred embodiments of the
invention is a minimum signal value determining circuit.
Such circuit is used to determine the minimum value between
two key signals. In a digital system, this is accomplished
by a digital comparator, which outputs as a key value signal
the minimum of the values of two input key signals. ~n a
digital system, these values are discrete numbers fed
serially to the minimum signal value determining circuit.
In an analog system, a minimum signal value circuit is
conveniently implemented as an analog ~ND gate, which has
the precise desired property.
Yet another circuit element employed in certain
preferred embodiments of the present invention is a signal
differencing circuit, which is used to determine a key value
remainder signal. In a digital system, a digital adder can
be used to add the negative of a received key signal to one.
In an analog system, the value of a key signal is subtracted
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Erom a reference level signal, typically representative of
unity signal gain, to produce the desired difference output.
A summing circuit is used to determine the sum of the
values of two key signals. In a digital system, this can be
implemented as a common digital adder. These as available
in standard integrated circuit packages. In an analog
system, the summing circuit can be implemented as analog add
circuit. A summing circuit is employed in embodiments of
the present invention where it is desired to provide a key
signal indicative of the adjusted yain of the output video
signal derived in accordance with the present invention and
provided to a signal utilization device.
Another circuit element employed in preferred
embodiments of the present invention is a signal value ratio
determining circuit. The ratio determining circuit enables
convenient generation of a composite key signal that is
indicative of the reciprocal of the value of an input key
signal that signifies the gain of an associated input video
signal relative to a reference level, typically,
representative of unity video signal gain, and of the value
of an independent key signal indicating the desired gain for
the video signal. The ratio determining circuit
conveniently generates the composite key signal by dividing
the independent key siqnal value by the value of the input
key signal to obtain the ratio. In a digital system, this
can be implement by an arithmetic processor circuit, which
is commercially available. In The preferred embodiment, the
independent and input key signals are employed as address
signals for a look-up table in a ROM-type memory that stores
values corresponding to the ratio of the two key signals.
While such em~odiments limits the resolution available, it
is fast and low-cost. Analog division circuits are known in
the art and can be constructed to perform the desired ratio
determination.
In one embodiment of the present invention, a system
is arranged to process an input video signal, whose gain has
been previously adjusted in accordance to a value of an
associated input key signal, is further processed according
~12- AV 3235 C2
1 3 1 462()
to the value of an independent key. Referring to Figure 1,
a previously gain adjusted video signal A', 50, is coupled
to a video signal gain adjustment circult 52 to produce an
output video si~nal A", 54, for further adjustmen-t of its
gain in accordance with a processing or request key signal
KP, 56 having a value indicating the desired gain for the
output video signal A", 54.
Key signal KP, 56, is generated in response to the
input key signal KA, 58, associated with video signal A',
50, and an independent key signal KI, 60, and has a value
determined by the values of the input and independent key
signals. The associated key signal KA, 58, is an indicator
of previous gain adjustment of the input video signal Vl.
The independent key, most often an ~xternally generated key
signal indicates the level of gain reduction wished for the
output video signal V". If the video signal A', 50, has
been processed according to a key signal which has reduced
the video signal's gain below the level of the independent
key signal KI, it is desired no further gain reduction
occur.
It is the task of minimum signal value determining
circuit 62 to determine whether the input key siynal KA, 58,
indicates the video signal A', 50, has already been reduced
in gain by the same amount or more than that requested by
the independent key signal KI. If value of the key signal
KA, 58, is less than that of the independent key signal KI,
60, the minimum circuit 62 outputs as a request key signal
KD, 64, the input key signal KA, 58. If the value of the
input key signal KA, 58, is more than that of the
independent key signal KI, 60, the minimum circuit 62
outputs the independent key signal KI, 60, as the request
l~ey signal KD, 64. If the values of the two signals are
equal, the minimum circuite 62 may be arranged to provide
either signal as the request key signal KD, 64.
The output of minimum circuit 62 is coupled to a ratio
signal value determining circuit 66. The input key siynal
KA, 5~, and the request key signal KD, 64, are coupled to
the input of the ratio circuit 66, which determines the
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ratio of the value of the request key signal KD to the value
of the input key signal KA. That ra-tio is output as the
selected process key signal KP, 56, and is used -to further
adjust the gain of the input video signal Al, 50 in
accordance with the value of the selec-ted process key signal
to generate the output video signal A", 54 with a selected
gain.
If the value of the request key signal KD, 64, is
selected by the minimum circuit 62 to have the value of the
input key signal KA because the gain of the input video
signal V' has already been reduced by more than the value of
the independent key signal, then the ratio provided by the
ratio circuit 66 will be equal to one, since it will receive
at both of its inputs a value corresponding to that of the
input key signal KA. As a result, the input video signal
A', 50, will not have its gain further adjust it in value,
since the multiplier circuit 59 receives a process key
signal KP over line 56 which has a value of unity. As a
result, the gain of the input video signal A', 50 is not
further reduced, which is the desired result because the
gain of the input video signal was previously a~justed to a
value less than that represented by the independent key
signal KI, 60.
In the event the value of the input kev signal KA is
greater than that of the independent key signal KI, the
minimum circuit 62 and the ratio circuit ratio 66 cooperate
to provide a selected processing key signal KP whose value
corresponds to that of the independent key signal KI. As a
result the gain of the input video signal A', 50, is reduced
by an amount determined by the value of the independent key
signal KI to form an output video signal A", 5~, with the
selected gain as if it had been processed by the independent
key signal XI, 60, alone, instead of by both the input key
signal KA, 58, and the independent key signal.
Referring now to Figure 2 t which illustrates a system
to combine an input video signal KA, 70, whose gain has been
previously adjusted to a level determined by the value of
the input key signal KA, 7~, with another input video signal
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~, 94, w~ose gain has not been reduced, but is at the
nominal reference level, typically, unity gain. sOth o~
these input video signals A' and B have their respective
gains adjusted in accordance with the present invention
according to the value of an independent key sign~l KI, 80,
and the reciprocal of the value of an input key signal RA,
78, with the value of the input key signal signifying the
gain to which one of the input signals, namely, KA, has been
prevlously adjusted. The system illustrated in Figure 2 is
simi].ar to that illustra-ted in Figure 1, in that the
processing of the input video s;.gnal A' to form the output
video signal A" is identical to that described with
reference to the embodiment of Figure 1. As previously
described, the input video signal A', 70, has its gain
adjusted by a video signal multiplier circuit 72 to produce
the output video signal A", 74, having the selected gain.
The selected process key signal KPA, 76, is generated
from the input key signal KA, 78, associated with the input
video signal A', 70, and the independent key signal KI, 80.
This selected process key signal KPA is generated through
the cooperation of a minimum signal value-determining
circuit 82 and a ratio determining circuit 86. As described
with reference to the embodiment illustrated in Figure 1,
the minimum circuit 82 operates to determine whether the
input key signal KA, 78, signifies that the gain of the
input video signal A', 70, has been reduced by the same
amount or more than that signified by the value of the
independent key signal KI. If the value o~ the input key
signal KA, 7~, is less than that of the independent key
signal KI, 80, the minimum circuit 82 outputs the value of
the input key signal KA, 78, as the request key signal KD,
8~. On the other hand, if the value of the input key signal
KA, 78, is greater than that of the independent key signal
KI, 80, the minimum circuit 82 outputs the value of the
independent key signal KI, 80, as the request key signal KD,
84. As in the embodiment of Figure 1, the output of minimum
circuit 82 i.s coupled to the inputs of the ratio circuit 86.
The input key signals KA, 78, and the request key signal KD,
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1 ~ 1 4 62~
84, are operated upon by the ratio circuit 86 to ~letermine
the ratio of the value of the request key signal KD to the
value of the input key signal KA. This ratio signal is
coupled as the selected process key signal KPA, 76, and is
used by the multiplier circuit 72 to adjust the gain of the
input video signal A', 70 and form a first output gain
adjusted video signal A", 74 with the selected gain.
This first output gain adjusted video signal A", 74, is
coupled to a first input of a video combiner 96, where it is
combined with the other input video signal s, 94, after it
has been also gain adjusted to form an output video signal
C, 97, in the form of a combination of two separate video
signals. The input video signal s, 94, has its gain
adjusted in accordance with the difference between a
selected reference level, typically unity, and the value of
the request ~ey signal KD, 84, which difference value forms
a second selected process key signal Kps~ 98 This gain
adjustment is performed by a video signal multiplier circuit
90 to generate a second output gain adjusted video signal
s', 88. The two output gain adjusted video signals A", 74,
and B', 88, have respective gain which when added, or
combined, together equal one. sy design, the input video
signal A' is reduced in gain by at least the value of KD,
84, and therefore, the other input video signal B, 94, is
reduced in gain by an amount corresponding to l-KD. The
reduction in gain of the input video signal B, 94, is
accomplished by the signal differencing circuit 92, which
takes the output of the minimum circuit 82, namely, KD, 84,
and subtracts it from unity to form the second selected
process signal KPB, 98, which is coupled to the video
mul-tiplier circuit 90. Using the selected process key
signal KPB, 98, the video multiplier circuit 90 operates on
the input video signal B, 9~, to form the second gain
adjusted output video signal B', 88.
Referring now to Figure 3, a preferred embodiment is
illustrated of a system to combine two previously gain
adjusted input video signals according to an independent key
KI, 110. The system illustrated in Figure 3 is similar to
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1 3 1 ~1 6~
the embodiment of Figure 2, in that the gain adj~lstment of
the input video signal ~', 100, to -form a first gain
adjusted output video signal A", 104, in accordance with the
input key signal KA and the independent key signal KI is
identical. The previously gain adjusted input video signal
A', lO0, is coupled to a video signal multiplier circuit 102
to have its gain adjusted thereby to produce the first gain
adjusted output video signal A", 104.
The selected process key .signal KPA, 106, is generated
from thr input key signal KA, 108, associated with the input
video signal A', lO0, and the independent key signal KI,
110. As previously described with reference to the
embodiments of Figures 1 and 2, the selected process key
signal KPA, 106, is generated through the cooperation of a
minimum circuit 112 and a ratio circuit 116 to insure that
the gain o~ the input video signal B', 100, is never
increased above the value o~ its gain as received on line
100. The first gain adjusted output video signal A"
provided by the multiplier circuit 102 is coupled by line
104 to one input of a video signal combiner 126, where it is
combined, or added with a second gain adjusted output video
signal B", 118, to form an output video signal C', 127,
which is a composite of the two gain adjusted output video
signals.
The second gain adjusted output video signal B", 118,
is derived from the second input video signal B', 114, by
adjusting the gain of that signal in accordance with the
value of a second selected process key signal KPs, 128~ In
this embodiment, the second input video signal has a gain
which has been adjusted previously in accordance with the
value of a second input key signal KB, 130. When combined,
the two gain adjusted output video signals A", 104, and B",
118, form an output video signal C', 127, which has a
combined signal gain of unity. By design, the input video
signal A' is reduced in gain by an amount determined by the
values of the input key signal KA, 108, and the independent
key signal KI, 110. The amount is determined by the request
key signal KDA, 114. Therefore, the second input video
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I31~61~)
signal ~', 114, should have its gain adjusted in accordance
with the minimum of the value of its associated input key
signal KB, 130, and the value of the dif~erence be-tween the
reference value, normally, unity, and the first request key
signal KDA. This difference is determined by the
differencing circuit 122. A second minimum circuit 136
receives the difference signal provided hy the differencing
circuit 122 and ,the second input key signal KB, 130, and
responsibly provides as a second re~uest key signal KDP,
132, the minimum value of the two signals received at its
input. This second request key signal KDB, 132, is coupled
to one of the inputs of a second ratio circuit 134~ This
ratio circuit 134 also receives the second input key signal
KB, 130, and responsibly provides at its output a second
selected process key signal KPB, 128, whose value is set at
the ratio of the second request key signal to the value of
the second input key signal. This second selected process
key signal is provided over line 128 to the control input of
the video signal multiplier circuit 120 to adjust the gain
of the second input video signal R', 114, and thereby form
the second gain adjusted output video signal s", 118. As
previously described, this video signal is coupled to the
video combiner 126 for addition to the first gain adjuted
output video signals A", 104, to form the output video
signal C', 127.
In the system of Figure 2, the output video signal C,
97, is treated as an unprocessed video signal by any
downstream usage device. This is because, the gain of the
combined gain adjusted output video signals A", 74, and B',
88, is one. In the embodiment of Figure 3, the gains of
both input video signals A', 100, and B', 114, have been
previously reduced prior to being processed by the system.
Therefore, it is possible that either or both of the input
video signals have been reduced in gain below the level
requested by the independent key signal KI. When this
occurs, the gains of one or both of the two input video
signals A", 104, and B", 118, could be reduced to a value
whereby the gain of the combined output video signal C',
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1 3 1 4 6,'~
127, is one or less. Consequently, in the embodiment of
Figure 3, A combined output key signal KC, 140, is formed by
summing the two request key signals KDA, 114, and KDs, ~32,
in the summiny circuit 138. This output key signal
represent the actual reductions to which the gains of the
two gain adjusted output video signals A", 104, and ~", 118,
respectively, have been set.
In summary,-the above three embodiments all implement a
form of the present invention. Common to each is the
initial deter~ination as to whether the input video signal
or signals have already been reduced in gain by more than
that requested by the independent key signal. If an input
video signal has been reduced in gain by more than tha-t
requested by the key signal, then the video signal is
further processed by a key signal of one, which results in
no further reduction of gainO If the video signal has been
reduced in gain by less than that requested by an
independent key signal, then a process key signal is
determined which, when used to further process the gain of
the video signal, reduces the gain of the video signal by an
amount such that the total reduction in gain on the output
video signal is the amount requested by the independent key
5 ignal.
In the embodiment of Figure 2, the input video signal
whose gain has not been previously adjusted is processed
with a difference key signal, and combined with another
video signal whose gain is adjusted by a related key signal.
The result of this process of the gain of the two input
video signals is a new output video signal which is formed
as a combination of the the two gain adjusted video signals,
and has a net gain of unity.
In the embodiment of Figure 3, both input video signals
have had their gains adjusted previously. These two signals
have their gains further adjusted to form a new output video
signal which is a combination of the the two input video
signals and has a net gain of one or less. Thus this system
also provides an output key signal associated with the new
output video signal indicating the gain.
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1 .') 1 ~ ~ 2S
Referring to Figure 4, the input video signal A' has
its gain adjusted in accordance with the value of a request
or independent key signal KI and the reciprocal of the value
of an input key signal KA in successive and separate steps
by a pair of multipliers 201 and 203 ln the communication
:Line 205 of the input video signal A'. The input key signal
KA has a value that indicates the gain o~ the input video
signal. This indicated gain signifies the gain of the input
video signal ~' relative to a reference gain, typically
unity. As a result of the two multiplications, an output
video signal ~''' is provided having a selected gain
determined by the value of -the key signal RI. In the
embodiment illustrated by Figure 4, the two multiplications
are shown as having the video signal A' gain first adjusted
by the reciprocal KP of the value of the input key signal KA
present on line 207 and, thereafter, by the key signal KI.
The sequence of multiplications however can be reversed,
such as in the embodiment of Figure 5.
Moreover, referring to Figure 6, the two key signals KI
and KA can first be combined in a inversely proportional
relationship relative to the input key signal KA by a ratio
circuit 225, whereby the input video signal A' on line 223
only has to be multiplied one time in a multiplier circuit
221 to form the desired output video signal A".
Referring to Figure 5, an embodiment is shown similar
to the embodiment of Figuxe 1, except the sequence of
multiplications of the gain of the input video signal
relative to the independent key KI and input key KA is
reversed. As illustrated therein, a minimum operation is
performed on the two key signals KA and KI by the minimum
circuit 213 to determine whose value indicates a smaller
video signal gain. Whichever key signal indicates the
smaller gain value, it is output by the minim~m circuit 213
to the first multiplier circuit 201 in the input video
signal path 205. The second multiplier circuit 203 in that
path receives the reciprocal KP2 of the input key value
provided by the reciprocal circuit 215 to multiply the
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S 2 S
results of the first multiplier circuit 201, and thereby
provide the outpu-t video signal A "' with the desired gain.
While the embodiments disclosed herein are preferred,
it will be appreciated that they are merely examples, and
that various alternatives, modifications, variations or
improvements thereon may be made by those skilled in the art
from -this teaching, which are intended to be encompassed by
the following cla,ims.
