Image sensing device arranged to perform a white compression process

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.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an image sensing device having a white 
compression circuit. 
2. 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. 
SUMMARY OF THE INVENTION 
This invention is directed to the solution of the above-stated problem of 
the prior art. It is therefore, an object of the invention to provide an 
image sensing device which is capable of accurately performing white 
balance control irrespectively of the color temperature of the image 
sensing object by setting the operating point of the white compression 
process at an optimum value according to the color temperature. 
To attain this object, an embodiment of this invention is provided with a 
white compression circuit which is arranged to have a variable 
characteristic and to have the variable characteristic controlled in 
association with automatic gain adjustment which is performed on a video 
signal after the white compression circuit. 
The above-stated arrangement of the embodiment optimizes the level of an 
input signal for the automatic gain control to ensure an improved S/N 
ratio. Further, in a case where white balance adjustment is to be 
accomplished by the automatic gain control, the white balance adjustment 
can be accurately accomplished. 
The above and other objects and features of the invention will become 
apparent from the following detailed description of an embodiment thereof 
taken in connection with the accompanying drawings.

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 Vrefl 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 by external inputs. 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 Vrefl 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&lt;G&lt;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: 
##EQU1## 
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 "Vrefl /2=RWB&lt;Vrefl&lt;BWB =2 Vrefl", 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.