Image sensing device

Abstract: In an image sensing device which performs a white compressing process before performing an automatic gain adjusting action on an image signal, a white com­ pression circuit is arranged to have a white compressing characteristic which is variable in association with the automatic gain adjustment; and the gain of an A/D converter which is provided for A/D converting the output of the white compression circuit is arranged to be adjustable.

Description: Background of the Invention: Field of the Invention: This invention relates to an image sensing device having a white compression circuit. Description of the Related Art: The dynamic range of a video signal obtained by an image sensing device is generally larger by a plurality of times than that of an ordinary standard TV signal. Therefore, for the purpose of securing an adequate dynamic range or S/N ratio, it has been practiced to carry out a white compression (knee) process within an analog or digital signal processing system. Various methods for attaining this purpose have been known. The conventional white compression process has been generally carried out at a fixed point after a white balance adjusting process. However, for example, it is conceivable to carry out the white balance adjustment by varying the reference voltages of the two ends of the ladder resistance of an A/D converter. In such a case, the white compression process must be carried out before the white balance adjustment process (in the case of R, G and B signals, for example) for the purpose of matching the input signal with the inside of the dynamic range of the A/D converter. Then, if the operating point of the white compression process is set beforehand at a level within a range desired for adequate white balance adjustment, the white compression characteristic becomes non-linear at a high luminance part within this range. The non-linear characteristic then results in a white compression process effected only for the R or B signal among other input signals according to the levels of input color signals. In such a case, therefore, the white balance adjustment cannot be accurately accomplished. Document JP-A-54 095 119 discloses an image sensing device comprising white compression means for performing a white compression process on color component signals of a color input video signal, gain control means for performing gain control over output signals of said white compression means, and control means for setting a white compression characteristic of said white compression means and a gain of said gain control means in association with each other. However, in case the color temperature of a pickup scene changes, the gain of said gain control means has to be manually adjusted by a variable resistor in order to equalize the color component output levels for a certain level of incident light. Furthermore, this adjustment has to be done separately for the color component signals R and B, since the difference between the characteristics of the R and the B channel varies in dependence on the level of the incident light. Thus, the adjustment is a time-consuming process. Summary of the invention: It is therefore an object of the present invention to provide an improved image sensing device which is capable of automatically and accurately performing white balance control for all color components irrespectively of the color temperature of the image sensing object. This object is achieved by an image sensing device as claimed in claim 1 Accordingly, the white compression operating points and the respective gains of the gain control means can be set for every color component signal in proportion to the input levels of the color component signals. As a result of the adjustment, the levels of the color component signals become equal to each other and the white balance is perfectly obtained also for a high luminance part of the signal above the compression operating point. Brief Description of the Drawings: Fig. 1 is a circuit diagram showing the arrangement of an embodiment of this invention. Fig. 2 is a graph showing the characteristics of essential circuits arranged according to the invention. Detailed Description of the Preferred Embodiment: Fig. 1 shows in a circuit diagram an embodiment of the invention. Referring to Fig. 1, an area sensor 1 is arranged to photo-electric convert an incident image. Sample-and-hold circuits 2, 3 and 4 are arranged to sample and hold video signals of colors R (red), G (green) and B (blue) read out from the area sensor 1. Reference numerals 5, 6 and 7 denote amplifying circuits. White compression circuits 8, 9 and 10 are arranged to have their characteristics variable by external voltages. A reference voltage source 11 is arranged to form a reference voltage Vref1 which is used for controlling the white compression characteristic of a G channel (for the G signal) and the gain of an analog-to-digital (hereinafter referred to as A/D) converter 13 for the G channel. A/D converters 12, 13 and 14 are arranged to convert the video signal outputs of the white compression circuits 8, 9 and 10 into digital video signals respectively. A digital signal processing circuit 24 is arranged to perform a digital processing action on the digital video signals converted by the A/D converters. Each of the A/D converters 12, 13 and 14 consists of a comparison circuit 15, 18 or 21, an encoder circuit 16, 19 or 22 and a ladder resistance circuit 17, 20 or 23. The gain of each A/D converter 12, 13 or 14 is variable by changing the reference voltage VRT of the ladder resistance circuit 17, 20 or 23 from the outside. A white balance control circuit 25 is arranged to detect a ratio between the outputs of an R sensor and a B sensor which are arranged to sense light obtained, for example, through a diffusing plate and to produce white balance control voltages RWB and BWB for controlling the white balance according to the output ratio. Further, Fig. 2 shows the characteristics of the essential circuits arranged according to this invention. In other words, Fig. 2 shows in a diagram the level of each channel obtained in a case where the color temperature of the object the image of which is to be sensed is high. The embodiment operates as follows: The video signals of colors R, G and B which are read out from the area sensor 1 are subjected to a pretreatment which is carried out by the circuits 2 to 7 respectively. After the pretreatment, these signals are supplied to the white compression circuits 8, 9 and 10 to undergo a white compression process which is performed in a manner as shown in Fig. 2. The white compressing operation point of each of these circuits 8, 9 and 10 is arranged to be variable by means of an external voltage. The white compression characteristics for the R and B channels are controlled by voltages RWB and BWB output from the white balance control circuit 25. Meanwhile the G channel is controlled in a fixed manner by the reference voltage Vref1 of the fixed reference voltage source 11. For example, in the event of an object of a high color temperature (when a level relation among the R, G and B signals is "G/2 = R < G < B = 2G", for example), the white balance control circuit 25 performs control in such a way as to obtain a relation which can be expressed as follows: Vref1 2 = RWB < Vref1 < BWB = 2 Vref1 As a result, white compressing operation points are set in proportion to the input levels of the R, G and B signals. The outputs of the white compression circuits 8, 9 and 10 are A/D converted by the A/D converters 12, 13 and 14. The gains of these A/D converters are arranged to be variable by changing a reference voltage VRT applied to the ladder resistance circuits 17, 20 and 23 which are included respectively in the A/D converters. More specifically, with a quantizing step assumed to be 255 CNT for an input signal level of 1 V when the reference voltage VRT is 1 (V), for example, the quantizing step becomes 255 CNT for an input signal level of 0.5 V if the reference voltage VRT is lowered to 0.5 (V). Then, this increases the gain of the A/D converter by two times. This gain control action is performed on the R and B channels by using the voltages RWB and BWB. Therefore, considering the above-stated color temperature with reference to the characteristic shown in Fig. 2, since there obtains the relation of "Vref1 / 2 = RWB < Vref1 < BWB = 2 Vref1", the gain of the R channel is increased to a value which is twice as high as that of the G channel while that of the B channel is reduced to one half. As a result of this adjustment, the levels of the R, G and B digital signals obtained through the A/D conversion become equal to each other and the white balance adjustment process is completed. Further, in this instance, the white balance is perfectly obtained also for a high luminance part of the signal above the white compressing operation point. (While the A/D converter is shown as having a linear characteristic in Fig. 2, the A/D converter may be arranged to have a non-linear characteristic). Each digital video signal encoded by the encoder circuit 16, 19 or 22 after completion of the white compression and white balance adjustment processes is processed by the digital signal processing circuit 24 and is digital-to-analog (D/A) converted (not shown) before it is output, for example, to a monitor or the like. As described in the foregoing, the operating characteristic of the white compression processing circuit which is provided for the purpose of obtaining an adequate dynamic range of the A/D converter and an adequate S/N ratio is arranged to be adjustable in association with the automatic gain adjustment such as white balance adjustment. This arrangement not only gives an improved S/N ratio but also ensures an adequate white balance adjustment for the high luminance part of the signal. The embodiment thus gives a good picture quality. In an image sensing device which performs a white compressing process before performing an automatic gain adjusting action on an image signal, a white compression circuit is arranged to have a white compressing characteristic which is variable in association with the automatic gain adjustment; and the gain of an A/D converter which is provided for A/D converting the output of the white compression circuit is arranged to be adjustable.

Claims: An image sensing device comprising: a) white compression means (8, 9, 10) for performing a white compression process on color component signals (R, G, B) of a color input video signal; b) gain control means (12, 13, 14) for performing gain control over output signals of said white compression means; and c) control means (25) for setting a white compression characteristic of said white compression means and a gain of said gain control means in association with each other, characterized in that d) said control means (25) is arranged to detect a ratio between said color component signals (R, G, B) and to generate a control signal (RWB, BWB) for setting said white compression characteristic of said white compression means and said gain of said gain control means according to said detected ratio. An image sensing device according to claim 1, characterized in that said gain control means is an A/D-converter (12, 13, 14) . An image sensing device according to claim 2, characterized in that said gain control is accomplished by varying a reference voltage (VRT) of a ladder resistance circuit (17, 20, 23) of said A/D-converter (12, 13, 14) in accordance with said control signal (RWB, BWB) . An image sensing device according to claim 1, characterized in that said control means is a white balance control circuit (25) and said control signal is a white balance control signal (RWB, BWB) according to a color temperature of an object. An image sensing device according to claim 1, characterized in that said input video signal is an output of an image sensing means (1) being arranged to convert image light of an object into an electrical signal.

Inventors: OHARA EIJI C O TAMAGAWA JIGYOS, OHARA, EIJI, C/O TAMAGAWA JIGYOSHO
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1990
Main classification: H04N9
IPC codes: H04N5, H04N9