Correcting an optical black level by feeding forward a calculated correction amount

An image processing apparatus constructed in a manner such that an image signal is inputted, the inputted image signal is clamped to a predetermined reference voltage in a clamp circuit, an optical black level of the clamped image signal is detected by a detection circuit, the detected optical black level and a predetermined black level are compared and a correction amount is calculated in an operation circuit, and the optical black level is corrected by operating the correction amount obtained by the operation circuit for the clamped image signal in a correction circuit.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an image processing apparatus of a VTR of 
a camera integrated type or the like. 
2. Related Background Art 
FIG. 1 is a block diagram showing a construction of a conventional image 
pickup apparatus. 
In the circuit construction shown in FIG. 1, reference numeral 1 denotes an 
image pickup element such as a CCD or the like for photoelectrically 
converting an image and outputting an image signal; 2 a circuit 
constructed by a double correlation sampling circuit for reducing noises 
included in the image signal outputted from the image pickup element 1 and 
an AGC circuit for adjusting a gain of the image signal (hereinbelow, the 
circuit 2 is referred to as an AGC circuit); 4 an A/D converter for 
converting the image signal outputted from a clamp circuit 3, which will 
be explained hereinlater, to a digital signal of eight or ten bits; 5 a 
signal processing circuit for performing a digital signal process to a 
digital image signal outputted from the A/D converter 4; 7 a clamp voltage 
generation circuit for integrating the image signal corresponding to an 
optical black portion from the image pickup element 1, namely, an optical 
black level (hereinbelow, referred to as an OB level) of a Y signal every 
horizontal line and for generating a clamp control voltage on the basis of 
the integration value; 6 a clamp pulse generation circuit for generating a 
timing pulse when clamping by a voltage generated by the clamp voltage 
generation circuit 7; and 3 the clamp circuit for setting a level of the 
image signal outputted from the AGC circuit 2 to the voltage generated by 
the clamp voltage generation circuit 7 by the timing pulse generated from 
the clamp pulse generation circuit 6 (hereinbelow, such a level setting is 
referred to as clamping). 
The specific operation of the conventional image pickup apparatus with such 
a construction will now be described by also referring to a timing chart 
of the clamp pulse in FIG. 2. 
In the circuit construction of FIG. 1, the image signal outputted from the 
image pickup element 1 is inputted to the AGC circuit 2 and is amplified 
to a predetermined signal level. The image signal outputted from the AGC 
circuit 2 is coupled by a capacitor 13 in an AC manner and is inputted to 
the clamp circuit 3. The image signal inputted to the clamp circuit 3 is 
clamped to the clamp voltage generated and outputted by the clamp voltage 
generation circuit 7 by the clamp pulse (refer to FIG. 2) outputted from 
the clamp pulse generation circuit 6 in the clamp circuit 3. 
The clamped image signal is inputted to the A/D converter 4 and is 
converted to the digital signal. After that, signal is subjected to a 
digital signal process by the signal processing circuit 5. The processed 
signal is recorded by a D-VTR 14 and is also displayed on a monitor 15. 
The digital signal converted by the A/D converter 4 is also inputted to 
the clamp voltage generation circuit 7. In the clamp voltage generation 
circuit 7, the optical black level (OB level) of the Y signal 
corresponding to an optical black (hereinbelow, referred to as an OB) of 
the image pickup element 1 is integrated every horizontal line or at 
preset intervals, thereby generating a clamp voltage such that the 
integration value is equal to a predetermined value. 
The clamp voltage which is generated from the clamp voltage generation 
circuit 7 constructs a closed loop among the clamp circuit 3, A/D 
converter 4, and clamp voltage generation circuit 7, so that a feedback is 
performed so as to hold the OB level of the signal which is inputted to 
the signal processing circuit 5 to be constant. 
The conventional image pickup apparatus performs the clamping of the image 
signal by the above-mentioned construction. 
In the conventional image pickup apparatus, however, as shown in 
explanatory diagrams with respect to a DC offset amount between the DC 
voltage and the OB level in a black period shown in FIGS. 3A and 3B, a DC 
offset exists between the DC voltage (1) in the black period and the OB 
level (2) of the image signal. Since the DC offset amount (3) 
[(3)=(2)-(1)] changes depending on the gain of the AGC, a tracing 
operation of the feedback is disturbed by a time constant when obtaining 
the OB integration value, so that the clamping operation is not correctly 
executed. 
There is, consequently, a problem such that the OB level of the image 
signal fluctuates when the signal process is performed, so that the stable 
image signal cannot be obtained. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide an image processing apparatus 
which can obtain an image signal whose signal level is stable. 
Another object of the invention is to provide an image pickup apparatus 
which can obtain an image signal having a stable optical black level. 
To accomplish the above objects, according to an embodiment of the 
invention, there is provided an image processing apparatus comprising: 
input means for inputting an image signal; clamp means for clamping the 
image signal to a predetermined reference voltage; detecting means for 
detecting an optical black level of the clamped image signal; operating 
means for comparing the optical black level detected by the detecting 
means and a predetermined black level, thereby calculating a correction 
amount; and correcting means for correcting the optical black level by 
operating the correction amount obtained by the operating means for the 
clamped image signal. 
According to another embodiment of the invention, there is provided an 
image processing apparatus comprising: input means for inputting an image 
signal; clamp means for clamping the image signal to a predetermined 
reference voltage; detecting means for detecting an optical black level of 
the clamped image signal; operating means for comparing the optical black 
level detected by the detecting means and a predetermined black level, 
thereby calculating a correction amount; and correcting means for 
correcting the optical black level by feeding forward the correction 
amount obtained by the operating means for the clamped image signal. 
With the construction as mentioned above, the image processing apparatus 
which can obtain the image signal whose signal level is stable can be 
provided. 
According to still another embodiment of the invention, there is provided 
an image pickup apparatus comprising: photoelectric converting means for 
photoelectrically converting an image pickup light of an object and 
outputting an image signal; clamp means for clamping the image signal to a 
predetermined reference voltage; reference voltage generating means for 
generating the reference voltage; detecting means for detecting an optical 
black level of the clamped image signal; operating means for comparing the 
optical black level detected by the detecting means and a predetermined 
black level, thereby calculating a correction amount; and correcting means 
for correcting the optical black level by operating the correction amount 
obtained by the operating means for the clamped image signal. 
According to further another embodiment of the invention, there is provided 
an image pickup apparatus comprising: an image pickup element for 
converting an image pickup light of an object to an image signal; an AGC 
circuit for adjusting a gain of the image signal converted by the image 
pickup element; a clamp circuit for clamping the image signal to a 
predetermined reference voltage; a reference voltage generation circuit 
for generating the reference voltage; an A/D conversion circuit for 
converting the image signal clamped by the clamp circuit to a digital 
image signal; a detection circuit for detecting an optical black level of 
the digital image signal; an operation circuit for comparing the optical 
black level detected by the detection circuit and a predetermined black 
level, thereby calculating a correction amount; and a correction circuit 
for correcting the optical black level by operating the correction amount 
obtained by the operation circuit for the clamped image signal. 
With the construction as mentioned above, according to further another 
object of the invention, the image pickup apparatus which can obtain the 
image signal having the stable optical black level can be provided. 
The above and other objects and features of the present invention will 
become apparent from the following detailed description and the appended 
claims with reference to the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
An image pickup apparatus of an embodiment of the invention will now be 
described with reference to block diagrams of FIGS. 4 and 5. 
Since blocks designated by reference numerals 1 to 6 shown in FIG. 4 have 
the same constructions as those of the blocks 1 to 6 in FIG. 1 described 
in the related background art, their overlapped descriptions are omitted. 
Reference numeral 9 denotes a DC voltage generation circuit for generating 
a DC voltage as a predetermined reference voltage to clamp a signal 
inputted to the clamp circuit 3. According to the invention, since a clamp 
control voltage by an integration value of the OB is not fed back but a 
clamp voltage is generated by the DC voltage generation circuit 9, an 
influence such as a level fluctuation or the like due to a change in gain 
as in the conventional apparatus is hardly exerted. Reference numeral 8 
denotes an OB level correction circuit for comparing the OB level of the 
image signal digital converted by the A/D converter 4 and a predetermined 
black level, operating an offset amount at which the both levels coincide, 
and feeding forward the operation result for the OB level of the image 
signal, thereby performing a correction of an addition or a subtraction. 
FIG. 5 is a block diagram showing a construction of the OB level correction 
circuit 8. Reference numeral 10 indicates an OB integration value 
detection circuit for integrating the image signal corresponding to an 
optical black portion of the image pickup element 1 in the image signal, 
namely, an optical black level (OB level) of a Y signal as a luminance 
signal every horizontal line or at preset intervals, thereby detecting its 
integration value; 11 a correction data operation circuit for comparing 
the OB integration value detected by the OB integration value detection 
circuit 10 and a predetermined black level, thereby calculating correction 
data such that the OB integration value and the predetermined black level 
coincide; and 12 an addition-subtraction circuit for performing an 
addition or a subtraction of the correction data calculated by the 
correction data operation circuit 11 for the OB level of the image signal 
inputted to the OB level correction circuit 8. 
The specific operation of the embodiment with the above-mentioned 
construction will now be described. 
In the circuit construction shown in FIG. 4, the image signal outputted by 
the image pickup element 1 is inputted to the AGC circuit 2 and is 
amplified to a predetermined signal level. The image signal outputted from 
the AGC circuit 2 is coupled by the capacitor 13 in an AC manner and is 
inputted to the clamp circuit 3. The image signal inputted to the clamp 
circuit 3 is clamped by a predetermined voltage generated by the DC 
voltage generation circuit 9 at a timing of the clamp pulse (refer to FIG. 
2) outputted by the clamp pulse generation circuit 6 in the clamp circuit 
3. The clamped image signal is inputted to the A/D converter 4 and is 
converted to a digital signal. After that, the digital signal is inputted 
to the OB level correction circuit 8. 
The subsequent processes in the OB level correction circuit 8 will now be 
described with reference to the block diagram of FIG. 5. 
The digital image signal inputted to the OB level correction circuit 8 is 
supplied to the addition-subtraction circuit 12, and also is inputted to 
the OB integration value detection circuit 10. The OB integration value 
detection circuit 10 integrates the OB level of the Y signal corresponding 
to the optical black portion of the image pickup element 1 in the image 
signal every horizontal line or at preset intervals, thereby detecting an 
OB integration value. 
The OB integration value detected by the OB integration value detection 
circuit 10 is compared with the predetermined black level in the 
correction data operation circuit 11 and correction data to make the OB 
integration value coincide with the predetermined black level is 
calculated. In the addition-subtraction circuit 12, the correction data 
calculated by the correction data operation circuit 11 is added or 
subtracted to/from the OB level of the image signal inputted to the OB 
level correction circuit 8, thereby performing a correction so that the OB 
level of the image signal is equal to the predetermined black level. After 
that, the corrected image signal is outputted by the OB level correction 
circuit 8. In FIG. 4, at least the OB level correction circuit 8 and 
signal processing circuit 5 construct an IC. An output of the signal 
processing circuit 5 is transmitted to the D-VTR 14 and monitor 15. 
As mentioned above, since the integration value of the OB level is not fed 
back as a clamp voltage but the integration value of the OB level is made 
coincide with the predetermined black level in the OB level correction 
circuit 8, a level fluctuation or the like due to the influence by the AGC 
gain as in the conventional apparatus doesn't occur and the stable and 
correct image signal can be obtained. 
With such a construction, the OB level of the image signal which is 
inputted to the signal processing circuit 5 is held constant. 
As described above, according to the invention, even when the DC offset 
amount between the reference voltage in the black period, namely, a fixed 
DC voltage and the OB level of the image signal due to, for example, an 
increase in gain of the AGC, the OB level of the image signal doesn't 
fluctuate when the signal process is performed, so that a stable image 
signal can be outputted. Although the embodiment has been explained with 
respect to the example in which the image signal from the CCD is 
processed, it will be obviously understood that the invention can be also 
applied to an apparatus for processing an image signal inputted by another 
input means. 
Many widely different embodiments of the present invention may be 
constructed without departing from the spirit and scope of the present 
invention. It should be understood that the present invention is not 
limited to the specific embodiments described in the specification, except 
as defined in the appended claims.