CA1087257A - Signal limit detection circuit - Google Patents

Abstract

APPARATUS FOR AUTOMATIC GAMMA
CONTROL OF TELEVISION COLOR SIGNALS

Abstract of the Disclosure A plurality of comparators are used to compare the red, blue and green color signals of a color television camera against a first and second predetermined signal level threshold corresponding to selected values of grey scale. The outputs of the comparators are combined to ascertain the difference in signal amplitude between the red, blue and green signals with respect to the first and second predeter-mined signal thresholds which represent grey. If the ampli-tude deviation of the red, blue and green color signals is within the threshold limits, a control signal adjusts the gamma characteristics of the blue and red color channels to match the amplitude of the red and blue color signals to the amplitude of the green color signal to correct to grey. If the red, blue and green color signal deviation is without the threshold limits, the color signal is assumed correct and no adjustment is made.

Description

RCA ~,173~

~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 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 ~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 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 RCA 69,173 ~0~7Z57 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 11 , ~ . .

RCA 69,173 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).

1087257 RCA 69,173 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 1~87257 RCA 69,173 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 , .

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 lI

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 ~87257 RCA 69,173 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 -, . .. .. .

RCA 69,173 ~8725~

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 ~0~7257 RCA 69,173 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 1087257 RCA 69,173 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

Claims (4)

Claims

1. A logic circuit for determining when all of a plurality of input signals are within a predetermined amplitude range comprising:
first comparing means for developing first output signals when all of said signals are above a lower pre-determined amplitude corresponding to one limit of said amplitude range, said first comparing means including:
first and second comparators;
reference voltage means producing a threshold voltage corresponding to said one limit of said amplitude range coupled to each of said first and second comparators;
means coupling a different one of said plurality of input signals to each of said first and second comparators, each of said first and second comparators being operative to produce a first logic state when the signal applied thereto exceeds said one limit and a second logic state when the signal applied thereto is less than said one limit;
second comparing means for developing second output signals when all of said input signals are below an upper predetermined amplitude corresponding to the other limit of said amplitude range;
said second comparing means including:
third and fourth comparators;
said reference voltage means producing a threshold voltage corresponding to said other limit of said amplitude range coupled to each of said third and fourth comparators;
means coupling a different one of said input signals to each of said third and fourth comparators, each of said third and fourth comparators being operative to produce said second logic state when the input signal applied thereto exceeds said other limit and to produce said first logic state when the input signal applied thereto is less than said other limit;
gating means coupled to said first and second comparing means for developing a third output signal in response to coincidence of said first and second output signals;
said gating means including:
a first AND gate coupled to said first and second comparators;
a second AND gate coupled to said third and fourth comparators;
a third AND gate coupled to said first and second AND gates to produce a logic state indicating when all of said input signals are within said predetermined amplitude range.

2. A logic circuit according to Claim 1 wherein said first logic state equals a 1 and said second logic state equals a 0.

3. A logic circuit according to Claim 1 wherein said first logic state equals a 0 and said second logic state equals a 1.

4. A logic circuit according to Claim 1 wherein said third AND gate has coupled thereto a third control signal having said first and second logic state to provide a further control of the logic state output of said third AND gate.