GB2052209A

GB2052209A - White balance control for colour video cameras

Info

Publication number: GB2052209A
Application number: GB8021243A
Authority: GB
Original assignee: Sony Corp
Current assignee: Sony Corp
Priority date: 1979-06-28
Filing date: 1980-06-27
Publication date: 1981-01-21
Also published as: DE3024317C2; FR2461417B1; AT378648B; DE3024317A1; GB2052209B; NL8003785A; FR2461417A1; CA1139427A; JPS6237873B2; ATA335380A; AU5955580A; AU535331B2; JPS566585A

Abstract

A method of adjusting the white balance of a colour video camera comprises the steps of sampling a reference video signal VRF in a reference interval TB of an input video signal Vin, analog to digital converting the sampled reference signal VRF into a digital reference signal, memorizing the digital reference signal MRF in a microprocessor 30, sampling a signal in a video scanning interval Tp of the input video signal Vin, analog to digital converting the sampled signal into a digital signal, memorizing the digital signal MER in the microprocessor 30, computing the difference between the memorized signals MRF, MER, forming a white balance adjusting signal on the basis of the result of the computation thereby to perform coarse white balance adjustment, digital to analog converting the memorized digital signal in the reference interval into an analog reference signal, level-comparing the converted analog reference signal with the video signal Vin in the video scanning interval Tp of the input video signal Vin, and supplying the compared output to the microprocessor 30 to correct the white balance adjusting signal thereby to achieve the final white balance adjustment of the input video signal Vin. <IMAGE>

Description

SPECIFICATION White balance control for colour video cameras This invention relates to methods of adjusting white balance in a colour video camera, and to colour video cameras including a white balance control circuit.

A known white balance adjusting system is shown in Figure 1 of the accompanying drawings.

The image of an object 1 is projected through a lens 2 onto a camera tube 3, and horizontal and vertical synchronizing signals are supplied from a synchronizing signal generator 4 to the camera tube 3, which then produces a video signal. This video signal is supplied to a decoder 5, which forms colour difference signals R-Y and B-Y and a luminance signal Y.

The colour difference signals R-Y and B--Y are supplied to mixers 6R and 6B, respectively, the outputs of which are supplied to window comparators 7R and 78, respectively. The comparators 7R and TB have threshold values VTL and VTH which are respectively a predetermined width lower and higher than a reference level of the video signal in the blanking period. Thus, the comparators 7R and 7B produce signals showing that the input signal level is out of the range between the-threshold values and also showing whether the input signal level is above or below the range. The signals from the comparators 7R and 78 are applied to count control and up/down control terminals of aach of up/down counters 8R and 8B.When the colour difference signals R Y and 8-Y are higher in level than the threshold value VTH, the counters 8R and 88 count down, whereas when the signals are lower in level than the threshold value VTL, the counters 8R and 88 count up. To these counters 8R and 8B is supplied a clock signal from a clock generator 9. The contents of the counters 8R and 8B are applied to digital-to-analog (D/A) converters 1 OR and 1 or, respectively where dc voltages are produced in proportion to the magnitudes of the input contents. These dc voltages are supplied to gain control circuits 11 R and 1 13, which permit the luminance signal Y to be passed therethrough to the mixers 6R and 6B, respectively.

Thus, in this arrangement, if the colour difference signal R-Y or B--Y is out of the range between the threshold values Vm and VTL, the gain control circuits 11 R and 11 B are controlled in gain to adjust the level of a luminance signal by +AY, or +AYb, which is supplied to the mixers 6R and 6B. As a result, the colour difference signals R-Y and B-Y can be adjusted to be within the range between the threshold values VTH and Tm.

In this system, however, since the threshold values VTH and VTL are established by a circuit other than the circuit for the colour difference signal, there is a risk that the drift error difference between these circuits may increase due to change of circuit constants with lapse of time. In addition, since the up/down counters 8R and 8B count bit by bit, a substantial time will be taken to correct for a large input error.

According to the present invention there is provided a method of adjusting white balance in a colour video camera by using a microprocessor having a read-only-memory (ROM), a randomaccess-memory (RAM), a central processing unit (CPU) and data buses, which comprises the steps of: sampling a reference video signal in a reference interval of an input video signal while a monochromatic object is viewed by said camera; A/D converting said sampled reference signal into a digital reference signal; memorizing said digital reference signal in a first memory region of said RAM; sampling a colour-difference signal in a video scanning interval of said input video signal; A/D converting said sampled colour-difference signal into a digital colour difference signal; memorizing said digital colour-difference signal in a second memory region of said RAM; modifying said memorized digital colourdifference signal so as to coincide with said digital colour-difference signal memorized in said second memory region with said digital reference signal memorized in said first memory region; and adjusting a signal level of a luminance signal to be mixed with said colour-difference signal, in accordance with said modified memorized digital colour-difference memory signal in said second memory region.

According to the present invention there is also provided a colour video camera of the type which provides a composite colour video signal formed of a luminance component and at least one colour-difference signal and having scanning intervals separated by blanking intervals, the camera including a white balance control circuit comprising: mixer means to combine, with said colourdifference signal, a colour-difference correction signal; adjustable gain control means providing as said colour-difference correction signal, a signal that varies with said luminance component and is proportional to a gain control signal applied thereto; means for sampling said colour-difference signal during one of said blanking intervals to provide a reference signal and during one of said scanning intervals to provide a sampled colour-difference signal; means for converting said reference signal and said sampled colour-difference signal to digital form; storage means having a plurality of storage locations for storing said converted reference signal and said converted sampled colourdifference signal in respective storage locations therein; means for modifying the stored converted sampled colour-difference signal so that the latter has substantially the same digital value as said converted reference signal; and means for providing as said gain control signal, a level proportional to the stored value of the modified converted stored sampled colour difference signal.

The invention will now be described by way of example with reference to the accompanying drawings, throughout which like references designate like elements, and in which: Figure 1 is a block diagram of a conventional white balance adjusting system; Figure 2 is a block diagram used for explaining a method according to the invention; and Figures 3 and 4 are explanatory diagrams to which reference is made in explaining the method.

Figure 2 shows an example of a system for performing a method according to the invention. A microprocessor 30 which includes a central processing unit (CPU) 31, a read-only memory (ROM) 32 having written therein a program which is to be executed by the CPU 31, a random access memory (RAM) 33 for storing or memorizing data, an input/output circuit 34 and so on. In the microprocessor 30, the CPU 31, ROM 32 and RAM 33 are interconnected by an address bus line 35, a data bus line 36 and a control bus line 37, and the CPU 31 and the input/output circuit 34 are interconnected by the data bus line 36 and the control bus line 37. The microprocessor 30 including such components is formed by a one chip large scale integrated circuit (LSI).

The signals from the mixers 6R and 68 are applied to the two stationary contacts of a change-over switch 12, which is operated in response to a signal from the microprocessor 30 to select either one of the signals and supply it to a sample-and-hold circuit 1 3. The signal from the sychronizing signal generator 4 is supplied to a sampling pulse generator 14, which then produces sampling pulses Sp, and Sp2, as shown in Figures 3B and 3C, corresponding to the blanking period TB and video scanning period Tp of an input video signal Vl" as shown in Figure 3A, respectively.The sampling pulses Sp, and 5p2 are respectively supplied to the two stationary contacts of a change-over switch 15, which is operated in response to a signal from the microprocessor 30 to select either of the sampling pulses Sp, and 5p2 and supply it to the sample .and-hold circuit 13. Thus, the sample-and-hold circuit 13 produces a signal and supplies it to the (+) terminal of a comparator 1 6. In addition, the contents at a given address in the RAM 33 of the microprocessor 30 is supplied to a D/A converter 17, and converted thereby to an analog signal, which is then applied to the (-) terminal of the comparator 16.Thus, the comparator 1 6 produces an output and supplies it to the microprocessor 30. The contents at the given address in the RAM 33 of the microprocessor 30 is also supplied to sample-and-hold circuits 1 8R and 18B through the D/A converter 17, and sampling pulses from the microprocessor 30 are supplied to the sample and-hold circuits 1 8R and 18B, which respectively produce voltages Vci and Vc2 corresponding to the contents at the given addresses. These voltages Vci and Vc2 are supplied to the gain control circuits 11 R and 11 B, respectively. The sychronizing signal from the synchronizing signal generator 4 is also supplied to the microprocessor 30.The microprocessor 30 is further supplied with a signal from a manual switch 1 9 for initiation of white balance adjustment, and supplies an error indicating signal and so on to an indicator device or element 20.

The ROM 32 has stored therein a program that will be described with reference to the flow chart shown in Figure 4. In this program, the colour difference signal R-Y is first adjusted and then the adjustment of the colour difference signal B-Y is performed.

As shown in the flow chart of Figure 4, the program execution is started, and at step 1 of the program, a decision is made as to whether the switch 19 is closed or not. So long as the switch 1 9 is open, step 1 is repeated. When the switch 1 9 is closed, the program is progressed to step 2 where the program for auto-white-balance is executed, that is, the switch 12, at step 2, is operated to the position where the mixer 6R for the colcur difference signal R-Y is connected.

Then, at step 3, the microprocessor 30 produces a digital value, for example, "80" (a number enclosed by quotation marks represents a hexadecimal value) used for setting the gain control circuit 11 R at a reference gain, and supplies it through the D/A converter 1 7 to the sample-and-hold circuit 18R.

At step 4, the switch 1 5 is operated to the position where the sampling pulse corresponding to the blanking period is given.

Then, at step 5, the sampled and held level of the signal is stored in the microprocessor 30 at a first storage address. In this case, the contents, MRF at the first storage address is read out and supplied through the D/A converter 1 7 to the (-) terminal of the comparator 1 6 where it is sequentially compared with the signal level applied to the (+) terminal, so that the contents MRF at the first storage address is sequentially modified until these inputs to the comparator 1 6 become equal to each other. Therefore, the contents MRF stored at the first storage address finally coincides with the value held in the sampleand-hold circuit 1 8.

At step 6, the switch 1 5 is operated to the position where the sampling pulse corresponding to the video scanning period is given.

Then, at step 7 the signal level sampled and held by the circuit 13 is stored in the microprocessor 30 at a second storage address.

Consequently, the level of the video signal in the blanking period is stored at the first address of the RAM 33 in the microprocessor 30, and a video signal level in the video scanning period is stored at the second address.

At step 8, the computation of (the contents MER at the second address) - (the contents MRF at the first address) is performed.

At step 9, the contents MER at the second address is replaced by the "80" minus the result of computation, in the case where the remainder or result of the computation is positive or the "80" is decreased by the difference value, whereas if the remainder is negative, the "80" is increased by the same value. Therefore, the contents at the second address is substantially equal to that at the first address.

Then, at step 10, the microprocessor 30 supplies the contents MER from the second address to the sample-and-hold circuit 1 8R through the D/A converter 1 7 to be held.

Thus, in the sample-and-hold circuit 18R is established a potential which is corrected by the result of the computation mentioned above, and thereby the level of the colour difference signal R-Y is made close to the level of the blanking period with AYr adjusted. That is, the level of the video signal VER, as shown in Figure 3A, is approximately equal to the reference signal VFR. In this way, the level in the blanking period is made substantially equal to the level of the colour difference signal, R-Y, but since a residual error is introduced by the gain control circuit 11 R and the irregularity of the luminance signal level Y, the final, convergent accuracy is achieved by the next process.Since the level of the luminance signal is slightly changed by the rocking or vibration of a portable video camera, the previously described correction alone does not provide a satisfactory accuracy.

At step 11, a decision or discrimination is made as to whether the contents MER at the second address lies in a predetermined, controllable range or not.

If the contents MER at the second address is out of the range, or overflowed into "00" or "FF" (F=1 5), an error indication is made by the indicator device 20 at step 12. Then, the program is progressed to step 20 for adjustment of the colour difference signal B-Y, by-passing the steps for adjustment of the colour difference signal R-Y. This indicates that it is impossible to adjust the automatic white balance, and in this case, after switching to the manual mode, the optical system of the video camera is adjusted or corrected, for example, a filter is changed, and then the above-mentioned process is repeated.

If, on the contrary, the contents MFR is within the range, or when "yes" is decided in the flow chart, the automatic adjustment is possible and thus the program is progressed to the next step 13.

At step 13, the microprocessor 30 supplies the contents MRF from the first address through the D/A converter 1 7 to the comparator 16, where it is compared with the level of the colour difference signal R-Y.

Then, at step 14, a decision is made on the compared output. If the colour difference signal R-Y level is larger, the program is progressed to the step 15, where "1" is substracted from the contents MER at the second address. If the colour difference signal R-Y level is smaller, the program is progressed to the step 16, where "1" 'is added to the contents MER at the second address.

Then, at step 17, a decision is made as to what number N of times the above operations have been repeated so far. If the repetition of the operations is, for example, N=1 5 times or below, the program is returned to the step 10.

If the repetition number N is decided to be 1 6 at the step 17, the program is progressed to the step 20. Thus, if the initial value is in the controliable range, the white balance adjustment is completed within sixteen repetitions.

At step 20, the switch 12 is operated in the position at which the mixer 6B for the colour difference signal B-Y is connected, and the adjustment of the colour difference signal B-Y is performed by use of the first and third addresses in the same way as in the previous description.

When the adjustment of the colour difference signal B-Y has been completed, the program stops.

Thus, in this circuit arrangement, when the switch 1 9 is closed while a white object 1 is being viewed, the white balance adjustment is automatically performed. Then, when the colour difference signals R-Y and BY have been completely adjusted, the correction signals used for the white balance adjustment of the colour difference signals R-Y and B-Y are stored in the form of digital values at the second and third addresses in the microprocessor 30.These digital values, when the original object 1 is viewed, are alternately read out, for example, at each vertical synchronizing signal and supplied to the sample and-hold circuits 1 8R and 1 8B, whereby a correctly white-balanced picture can be obtained from the video camera.

In this way, the white balance adjustment is carried out, and the reference level of the video signal in the blanking period and the video signal level in the video scanning period are produced from the same signal line, so that there is no risk that drift error or the like will occur. In addition, since coarse correction is made by taking the difference between the levels and then fine correction is carried out, the time taken for the adjustment can be decreased.

While for the white balance adjustment the coarse adjustment is made at steps 1 to 10, and more precise, fine adjustment is performed at steps 11 to 20 shown in the flow chart of Figure 4, only the steps 1 to 10 may be used with the other steps 11 to 20 omitted, if necessary.

Moreover, although the digital signal corresponding to the reference signal VRF is stored in the RAM 33 at the steps 4 to 5, the digital signal corresponding to the video signal VER is stored in the RAM 33 at steps 6 to 7, and these stored contents are compared at step 8, it is of course possible to reverse the order of the steps.

Moreover, the microprocessor 30 used permits any complex adjustment to be easily performed using the program, so that the circuit arrangement used can be simplified.

While the balance adjustment for the colour difference signals is made in the system described above, this adjustment may be applied to the colour difference signals G-R and G-B in a three-tube camera.

Moreover, auto-black balance can be effected.

with the same arrangement merely by changing the program, and it is also possible to make completely automatic adjustment in three stages, auto-white balance to auto-black balance to autowhite balance.

Claims

'1. A method of adjusting white balance in a colour video camera by using a microprocessor having a read-only-memory (ROM), a randomaccess-memory (RAM), a central processing unit (CPU) and data buses, which comprises the steps of: sampling a reference video signal in a reference interval of an input video signal while a monochromatic object is viewed by said camera; A/D converting said sampled reference signal into a digital reference signal; memorizing said digital reference signal in a first memory region of said RAM; sampling a colour-difference signal in a video scanning interval of said input video signal; A/D converting said sampled colour-difference signal into a digital colour difference signal; memorizing said digital colour-difference signal in a secondary memory region of said RAM; modifying said memorized digital colourdifference signal so as to coincide with said digital colour-difference signal memorized in said second memory region with said digital reference signal memorized in said first memory region; and adjusting a signal level of a luminance signal to be mixed with said colour-difference signal, in accordance with said modified memorized digital colour-difference memory signal in said second memory region.
2. A method according to claim 1 further comprising the steps of: D/A converting said memorized digital reference signal in said first memory region into an analog reference signal; level comparing said converted analog reference signal with said colour-difference signal; modifying said memorized digital colourdifference signal in said second memory region in bit steps in accordance with the difference level amount produced by said comparison; and adjusting said signal level of said luminance signal in accordance with said modified digital colourdifference signal in said second memory region.
3. A method according to claim 2 further comprising the steps of: checking whether or not said digital colourdifference signal memorized in said second memory region is within the adjustable range of a auto-white balance system; and indicating an error condition when said checking step shows said memorized digital colourdifference signal not to be within said range.
4. A method according to claim 2 comprising the steps of: adjusting said signal level of said luminance signal in two colour-difference channels sucessively.
5. A colour video camera of the type which provides a composite colour video signal formed of a luminance component and at least one colour-difference signal and having scanning intervals separated by blanking intervals, the camera including a white balance control circuit comprising: mixer means to combine, with said colourdifference signal, a colour-difference correction signal; adjustable gain control means providing as said colour-difference correction signal, a signal that varies with said luminance component and is proportional to a gain control signal applied thereto; means for sampling said colour-difference signal during one of said blanking intervals to provide a reference signal and during one of said scanning intervals to provide a sampled colour-difference signal; means of converting said reference signal and said sampled colour-difference signal to digital form; storage means having a plurality of storage locations for storing said converted reference signal and said converted sampled colourdifference signal in respective storage locations therein; means for modifying the stored converted sampled colour-difference signal so that the latter has substantially the same digital value as said converted reference signal; and means for providing as said gain control signal, a level proportional to the stored value of the modified converted stored sampled colourdifference signal.
6. A camera according to claim 5 further comprising manually actuated means for selectively initiating a white balance adjustment operation.
7. A camera according to claim 5 further comprising means for detecting whether the stored converted sampled colour-difference signal is outside said range.
8. A camera according to claim 5 wherein said means for providing said gain control signal includes digital-to-analog converting means for providing a level corresponding to said stored converted sampled colour-difference signal, and sample-and-hold means for sampling said level during predetermined intervals and providing said gain control signal to said adjustable gain control means.
9. A method of adjusting white balance in colour video camera, the method being substantially as hereinbefore described with reference to Figures 2 to 4 of the accompanying drawings.
10. A colour video camera of the type which provides a composite colour video signal formed of a luminance component and at least one colour-difference signal and having scanning intervals separated by blanking intervals, the camera including a white balance control circuit substantially as hereinbefore described with reference to Figures 2 to 4 of the accompanying drawings.