Note: Descriptions are shown in the official language in which they were submitted.
RCA ~,173~
7ZS7
~rllis is a divi.siona]. apl)lication of Canadian
applicAtioll Seria:L No. 2~,303, filed 19 March 1976.
I Tl~e ~)resent invention relates -to apparatus for
automatic gamma control of color signals generated by a
television film camera wherein the relative amplitude of the
color signals are adjusted.
S Automatic color balancing has been used to adjust
the black level balance and gains of the red and blue signals
relative to those of the green signals to provide white level
b.llance in the processing amplifier following a television
camera. This is particularly impcrtant to do when the pro-
gram material picked up by the television camera has been
recorded on motion picture film. Automatic color balancing
as shown in U.S. Patent No. 3,786,177, entitled "Apparatus
for Automatic Color Balancing of Television Camera Signals"
has been successfully used to ascertain when substantially
black or substantially white signals were to be provided to
the colorplexer of a television camera chain when the source
of program material comprise~ a sequenee of film sources.
Such a sequence of film sources may consist of motion picture
film, film clips of news interviews, on the spot news record-
ings, filmed commercials, all of which may be filmed under
widely varying lighting conditions and subsequent processing
resulting in a wide variation of density and color from
scene to scene, as well as from sequenee to sequence in the
final edited version to be teleeast. Motion pieture eolor
film of eurrent manufaeture typically consists of three
primary color dye layers identified as yellow, magenta and
cyan; improper processing, aging or a combination of these
affects can unbalance the final color represented by these
primary dye layers. Automatic color balancing of the type
(lescribed in U.S. Patent No. 3,786,177 serves to provide a
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101~7257
RCA 69,173
mor~ u~ orlllly ba]anced signal ~or transmission by ascertain-
ing ~or each scene a substantially black and substantially
whitc level; however, such a system does not necessarily
provide the dcsired color signal balance intermediate the
levels established for black, which represents a lack of
color information, and white which represents equal amounts
of color information. A lack of compensation in the area of
color information intermediate the estahlished black and
white levcls is particularly troublesome in the instances
where the motion picture film suffers a color defect due to
improper processing or as more commonly observed, the normal
aging of a previously recorded film.
Since a typical scene contains a significant amount
of detail which is reproduced only as shades of brightness,
it is necessary to have equal amounts of all three primary
colors to represent white and various shades of grey scale
throughout the picture. The loss of diminishing of one of
the primary co1ors results in a shift of the white or grey
areas of the scene content toward the remaining predominant
colors and the reproduced picture appears with an overall
color cast.
A typical example of this occurs in motion picture
film due to fading with time of one or more of the primary
dyes used in the manufacture of the color film. At present,
the cyan dye is the most susceptible to fading with time.
The loss of cyan on reproduction of the film in a telecine
camera results in a loss of blue-green color signal informa-
tion and the resultant picture is reproduced with the grey
areas of the picture content shifted toward a red-orange
color representing the predominance of yellow and magenta
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~087257 RCA 69,173
I dye layers. ~utomatic color balancinq based on establishing
black level and whitc lcvel does not compensate the grey
scale intermcdiate these levels.
Present practice to overcome the lack of correction
in the grey scale area requires either continuous manual moni-
toring of the gamma controls during the telecast or a preview-
ing of each segment of film in order to predetermine the
necessary gamma corrections in advance. A form of automatic
system has been proposed which requires a previewing of the
film by an operator who estimates the required gamma adjust-
ments and enters them on a perforated tape or record medium
which is then run in sychronism with the film during the
telecast to relieve the operator of continuous manual con-
trol of gamma adjustments. This present practice of preview-
ing or advance preparation is costly and time~consuming
requiring literally an hour of previewing in advance for
each hour of telecast. This is especially uneconomical when
feature films are telecast.
Thc concept of color correction on which the present
invention is based is that colors close to grey are assumed
to be grey and can be made grey without seriously affecting
a good color picture. The invention utilizes the availabil-
ity of the red, green and blue signals in a color television
camera to provide the necessary color signal samples to
adjust the colored grey scale. In a properly operating
color television camera, a grey scale represents equal
amounts of video signals in each of the red, green and
blue channels intermediate the black and white levels. A
detector samples the red, blue and green signal levels a-
vailable in the respective channels and when the three
signals are within a predetermined level of each other, an
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1~87Z57 RCA 69,]7-~
error correcting siqnal is applied to the appropriate gamma
circuit to ~-)alance the three signal levels so as to produce
a c~rey signal.
In accordance with the present inven-tion automatic
gamma correction apparatus comprises means providing a pair
of signal translating channels for a pair of related signals.
Gamma correction circuitry is included in one of the signal
translating channels. The gamma correction circuitry is
responsive to a control signal to alter the gamma correction
characteristic of the signal translating channel. The sys-
tem includes means for comparing the amplitude levels of the
pair of related signals in a relatively narrow range of
amplitude levels between the maximum and minimum levels of
the signals. Means coupled to the comparing means and to
the signal translating channels develop a control signal of
a value related to the differential lmplitude between the pair
of signals when both of the signals have an amplitude within
the narrow range of amplitude levels. The control signal
is applied to the gamma correction circuitry for altering
the gamma characteristic of its signal translating channel
in a direction so as to make the amplitude of the pair of
related signals equal.
A further feature of the present invention pro-
vides a logic circuit for determining when all of a plurality
of signals are within a predetermined amplitude range com-
prising first comparing means for developing a first output
signal when all of the signals are above an upper predeter-
mined amplitude. Second comparing means for developing a
second output signal when all of the signals are below a
a lower predetermined amplitude, and third comparing means
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I , coupled to the first and second compariny means for develop-
~ing a third output signal in response to coincidence of
.l the first and second output signals.
' The present invention will be better understood by
reference to the accompanying drawings and specification
in which:
FIGURE 1 is a block diagram of an automatic color
1, balance-automatic gamma system embodying the detector of the
;l invention;
i, , _ !.. ." ~
FIGURE 2 is a block diagram of a detector embody-
ing the present invention;
FIGURES 3 and 4 are waveforms depicting the opera-
~,l tion of the apparatus of FIGURE 2; and
Il FIGURE 5 is a block diagram of an adjustable gamma
15 ll correction circui L useful in implementing the present inven-
tion.
FIGURE 1 shows an automatic color balancing system
l; of the type shown in U.S. Patent No. 3,786,177.
... 1, . ............................... , . . !
. ~riefly, FIGURE 1 shows
~j an automatic color balancing system incorporated in a video
processing amplifier 10. Recl, green and blue color signals
: respectively, originate at camera tubes 101, 103, 105. The
;l original red, green and blue color signals are respectively
,¦ amplified in amplifiers 107, 109, 111 associated with the
25 1¦ camera head and are thereafter respectively applied to input¦
. terminals 113, 115, 117 of the processing amplifier 10. The
~ terminals 113, 115 and 117 are coupled to the input circuits
¦iof gamma correctors 127, 129, 131, respectively, and thence I .,.
Ilto output terminals 119, 121, 123. The gamma-corrected
I color signals appearing at th~ terminals 119, 121, 123 are
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10872S7
applied to a colorplexer 125 which combines them to provide
a composite video output signal.
The gamma correctors 127 and 131 are each preceded
by the cascade combination of a controlled gain amplifier
(133, 135) and a black-level control circuit (137, 139).
The black-level control circuits 137, 139 add varying amounts
of black-level pedestal to the red and blue color signals,
respectively, in response to first and second control signals,
respectively, as provided from analog memories 141, 143,
respectively. The information retained in the memories 141,
143 is obtained from the red, blue and green color signals
by a detector 145 which provides black-level correction infor-
mation during substantially black portions of the scene
presented to the camera tubes 101, 103, 105.
The signal applied to the memory 141 by the detector
145 during black portions of the scene is responsive to the
difference between the amplitude of the red and the green
gamma-corrected color signals at the output circuits of the
gamma correctors 127, 129, respectively. The signal applied
.. . . . . .............. . .
to the memory 143 by the detector 145 during black portions
of the scene is responsive to the difference between the
amplitudes of the blue and the green gamma-corrected signals
at the output circuits of the gamma correctors 131, 129,
respectively. A black-level correction feedback loop 147
25 containing elements 137, 127, 145, 141 and the black-level
correction feedback loop 149 containing elements 139, 131,
145, 143 are arranged to degenerate the discrepancies in
the black-levels of the gamma-corrected blue color signal
(at 123) respectively, as compared to the black level of the
gamma-corrected green color signal (at 121).
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I The controlled gain amplifiers 133, 135 are
responsive to third and fourth control signals from analog
memories 151, 153, respectively. The information retained
in the memories 151, 153 is obtained from the red, blue and
green color signals by a deteetor 155 which provides white-
level correction information during substantially white
portions of the scene presented to the camera tubes 101,
103, 105. The signal applied to the memory 151 by the
detector 155 during white portions of the scene is responsive
to the difference between the amplitudes of the red and the
green color signals at the input circuits of the gamma
correctors 127, 129, respectively. The signal applied to
the memory 153 by the detector 155 during white portions of
the scene is responsive to the differenee between the ampli-
tudes of the blue and the green color signals at the inputcircuits of the gamma correctors 131, 129, respectively.
The white-level eorrection feedbaek loop 157 containing
elements 133, 137, 155, 151 and the white-level correetion
feedbaek loop 159 eontaining elements 135, 139, 155, 153
are arranged to degenerate the diserepancies in the amplitudes
of the red and the blue eolor signals, respectively, as
compared to the green color signal for neutral-color signals
(whites and greys), which is often referred to as "white-
level eorreetion."
Variations in the order of eascade conneetion of a
- eontrolled gain amplifier for white-level eorrection,
black-level corre~tion and gamma correction and variations
as to the points in that cascade combination from which error
signals are obtained are met in various processing amplifier
3 designs. The gamma correctors 127, 129, 131 increase the
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I gain of the black-going portions of the signals applied to
their input circuits compared to the white-going portions
of such signals. Taking error signals for black-level
correction from the output circuits of the gamma correctors
127, 129, 131 as shown in FIGURE 1 can increase the resolu-
tion against error of the black-level correction system.
Taking error signals for white-level correction before
gamma correctors 127, 129, 131 better preserves the resolu-
tion against error of the white-level correction system.
Taking error signals for white-level correction after black-
level circuits 137, 139 avoids having to provide separate
means for black-level restoration of the controlled gain
amplifier 133, 135 output signals, such black-level restora-
tion being necessary to provide proper biasing for the input
circuits of the detector 155.
Video processing amplifier 10 also includes detec-
tor 160 which, as will be described, samples the red, green
and blue color signals at the output of the gamma correctors
127, 129, 131 and provides gamma correcting feedback signals
to the ga~ma correctors so as to maintain a corrected grey
scale in accordance with the present invention.
FIGURE 2 shows the gamma correction detector 160
of FIGURE 1 in greater detail.
Gamma corrected red (R), green (G~ and blue (B)
video signals from gamma correctors 127, 129, 131 of FIGURE 1
are applied respectively to comparators 501, 502, 503 ~hich
provide increased output in response to signals above a
predetermined threshold and are therefore termed upper com-
parators. ~imultaneously, the same R, G and B color signals
are applied respectively to comparators 503, 504 and 506
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1087257 RCA 69,173
I which provide increased output in response to signals below
a second different threshold, and are therefore called
lower comparators. A reference source 520 comprising a
voltage divider with respective upper and lower threshold
voltage taps is coupled to the respective upper and lower
comparators to provide a predetermined level against which
the applied R, G, B signals are compared.
When a video signal exceeds the predetermined
lower threshold signal level, the output of the upper com-
10 parators 501, 502, 503 responds with a logic 1 and when the
video signal does not exceed the predetermined signal level,
the comparators respond with a logic 0. Similarly, when a
color signal exceeds the predetermined upper threshold
signal level, the output of the lower comparators 504, 505,
15 506 respond with a logic 0 and when the color signal does
not exceed the predetermined signal level, the comparators
respond with a logic 1.
The outputs of the upper comparators 501, 502, 503
represented by a logic 1 or logic 0 are coupled to AND gate
20 507. The output of AND gate 507 is a logic 1 only when all
of the inputs representing upper comparators 501, 502, 503
are logic 1.
The outputs of lower comparators 504, 505, 506
represented by a logic 1 or logic 0 are coupled to AND gate
25 509. The output of AND gate 509 is a logic 1 only when all
of the inputs representing lower comparators 504, 505, 506are logic 1.
The outputs of AND gates 507, 509 and a gating
signal 514 are coupled to AND gate 508 so that AND gate
508 provides an output represented as logic 1 only when
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1~87257 RCA 69,173
I both inputs ~rom AND gates 507 and 509 are at logic 1 and
~lle gate signal is presellt,
The ~ and G signals are applied to a differential
amplifier 510 to develop a difference signal R-G which is
applied to memory circuit 5].2.
Similarly, the B and G signals are applied to a
differential amplifier 511 to develop a difference signal
G-Y which is applied to a memory circui.t 513.
Memor,ies,512 and 513 each comprise a field effect
transistor (FET) 514, 515 and capacitors 516, 517 respectively.
The capacitors 516 and 517 are each high quality polysty.rene
capacitors capable of retaining a charge on themselves for
hours at a time. Together with FET's 514 and 515,~capacitors
516 and 517 form sample and hold memory ci.rc~its:,
The output of AND gate 508 in the form of a logic :
1 serves to close the FET switches in memories 512 and 513
to sample the R-G and B-G signals respectively only if an
input corresponding to a logic 1 output from both the AND
gates 507 and 509 is present during the gating signal applied
to AND gate 508. The outputs of the memories 512 and 513
are coupled to the adjustable gamma circuits 127, 131 of
FIGURE 1 to provide the control signal for controlling the
grey scale gamma of the red and blue color signals respectively
relative to the reference green color signal over the entire
dynamic range of the applied signals. A gamma circuit
suitable for the adjustable gamma corrector circuits 127,
131 of FIGURE 1 is illustrated in FIGURE 5 and described
in detail in United States Patent 3,970,774 of Lucas John
Bazin and Gary Ray Peterson ~
entitled "Electronic Siqnal
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I Mixer". ~s shown in FIGURE 5, a video signal Vin is
coupled to linear amplifier lO0 and non-linear amplifier
200. The outputs of amplifiers lO0, 200, Vin and Vin
respectively,are coupled to a resistive mixer 300, 400
to provide a first summation of linear and non-linear signals,
and further coupled to a voltage multiplier 600 to provide
a second summation of linear and non-linear signals. A
control voltage 700, for example, the output signal of
memories 512 or 513 is coupled to the voltage multiplier
500 to vary the value of a coefficient "Y" which serves to
multiply the second summation of linear and non-linear
signals. The first and second combined signals are summed
in amplifier 500 to provide an output signal VO which may
be varied from Vin to Vin-4 in response to the applied con-
trol voltage 700.
The operation of the circuitry of FIGURE 2 can bebetter understood with reference to FIGUR~ 3 which illustrates
differences in the red, green and blue color signals within
the predetermined threshold level and in FIGURE 4 differences
in red, green and blue color signals which occur outside
the predetermined threshold level. In FIGURES 3 and 4 wave-
forms 3a and 3b illustrate a video waveform with a time
base representing a single horizontal line of a television
picture frame with a duration of 63.55 microseconds in
accordance with NTSC standards. The amplitude of waveforms
3a and 3b illustrate a video signal which varies uniformly
from black at the beginning of the active picture time
following the blanking interval to white at the end of the
active picture time preceding the next blanking interval.
0 The R, G, B video signal levels for black level and white
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1 level are presumed to be established either by means of the
apparatus lllustrated in FIGURE 1 or similar means. Typical
values for black level and white reference levels are 5g and
95%, respectively. By means of upper and lower threshold ad-
~~ 5 justment 520, voltages corresponding to 45% and 55~ of full
scale are selected as predetermined thresholds for grey. It
will be noted, as previously discussed, that a lack of video
signal information represents black level, an equal amount of
R,G,B, video signal information at full scale represents
white and equal amounts of R,G, and B video signal information
at an amplitude of 50~ represents grey.FIGURE 3 illustrates
the operation of the systems of FIGURE 2 when only the red
color signal shown in waveform 3a is different in value from
the blue and greensignals bu~ within the threshold limits esta-
blished by the upper and lower comparators in conjunction
with reference 520. Outputs from each of the upper R,G, and
B comparators shown as waveforms 3b,3c are summed in the
upper AND gate 507, while the outputs of each of the lower R,
G, B comparators shown as waveforms 3e and 3f are summed in
the lower AND gate 508.The outputs of upper AND gate 507
shown as waveform 3d and of lower AND gate 508 shown as wave-
form 3e are summed in AND gate 508, resulting in output wave-
form 3h from AND gate 508 which enables sample and hold memo-
ries 512 and 513 to establish control values responsive to the
difference signal R-G from difference amplifier 510 and B-G
from difference amplifier 511. The control values established
at memories 512 and 513 respectively are coupled to the red
and blue channel gamma control to change the transfer charac-
teristic of the gamma circuit 127,131 to correct the red and
3 blue color signals to a level equivalent to the reference
green color signal over the entire dynamic range of the applied
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1 signals thereby producing grey.
S:imilarly, in FIGURE 4 a red color signal deviating
from the blue and green color signals, is shown in waveform
4a;however, as illustrated in waveforms 4b, 4c, 4e, 4f and
- 5 their summation in AND gate input waveforms 4d and 4g, the
resultant output waveform from AND gate 508 indicates the
red color signal is outside the selected threshold limit.
AND gate 508 does not provide a signal in this instance to
enable sample and hold memories 512 and 513 to establish
new control values responsive to the R-G and B-G difference
signals, since by definition if a color signal exceeds the
selected upper and lower thresholds (i.e., not within the
"window"), it is assumed that the color belongs in the
overall picture and it is not desirable to make it equivalent
to grey.
As noted in conjunction with FIGURE 2, final AND
gate 508 has as its input a gating signal which is required
in addition to a logic 1 from AND gates 507,509 for an
output to be provided at gate 508. In the illustrated em-
bodimentsS a "wide gate slgnal" in the form of a logic 1derived from the horizontal and vertical line frequency is
used in addition to a logic 1 from AND gates 507,509 to
enable a logic 1 at the output of gate 508 during approxi-
mately 90% of the active picture area to prevent shading and
edge transient phenomena during edge portions of the picture
from initiating gamma correction. In summary, the red, greenand blue color signals are compared during the active picture
area at selected grey scale values and when their deviation
is small or close to grey, the red and blue grey scale gamma
transfer characteristic is matched to the green grey scale
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I c3amm.l characteristic to proclucc ~rey. If the red, blue and
rccll deviatioll is lar~3cr than the estab1ished threshold,
the color si~na1 is prcsumed to be a correct color and a
corrcction to c3rey is no-t made.
: 25
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