Optical black and offset correction in CCD signal processing

Details Optical black and offset correction in CCD signal processing Optical black and offset correction in CCD signal processing

2000-11-01

2004-09-14

Ho, Tuan (Department: 2612)

Television

Camera, system and detail

Combined image signal generator and general image signal...

C348S250000

Reexamination Certificate

active

06791607

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to image processing, and, more particularly, to an analog front end for a charge coupled device and CMOS imager, which provides digital optical black and offset correction.
BACKGROUND OF THE INVENTION
Advances in integrated circuit design and manufacturing have enabled low cost, highly integrated, high performance image processing products, including the digital electronic cameras. A conventional camera comprises an image sensor, typically an array charge coupled device (CCD), an analog front end (AFE) and a digital image processor. Most analog front ends having optical black and offset calibration include schemes that integrate the error signal on a capacitor during an optical black period and feed back the voltage generated to the input to cancel the offset or the optical black value during the video interval.
As shown in circuit
100
of
FIG. 1
, the CCD
102
which is an integrated array of photocells used in digital imaging is connected to a capacitor
104
and a clamp circuit
106
for AC coupling. The AFE connected to the capacitor
104
generally includes three main elements: a correlated double sampler
108
(CDS), a programmable gain amplifier
110
(PGA), and an analog to digital converter
112
(ADC). The fundamental goal in any camera design is to extract as much dynamic range from the image sensor without adding any noise with the subsequent circuitry.
The specific operation of the conventional image process apparatus
100
with such a construction is described referring to the timing charts of CCD
102
output in
FIG. 2
a
and
2
b
. Particularly, the output of the CCD
102
contains a reset pulse, the reference level and the video level. Output from the CCD
102
is sampled twice by CDS
108
such that the first sample is taken during the reference level and the second sample is taken during the video signal. The difference is the corresponding CDS
108
output. This difference between the optical black level and the video level represents the actual image value for any given pixel.
As shown in
FIG. 2
b
, a dark cell does not produce a zero differential output, due to the dark currents of the photocells, which may vary from pixel to pixel and line to line in a frame. Due to the dark current or “optical black level” and the internal offsets of all amplifiers used in the CDS
108
, PGA
110
, and ADC
112
, the resulting ADC
112
output for a dark cell will not be zero. Further complicating the matter, the CDS
108
offset and the optical black level are multiplied by the gain from the PGA
110
. Thus, in order to achieve the ideal dynamic range for the signal, the black level and the offsets must be removed.
The function of the CDS
108
, as illustrated in
FIG. 2
a
and
2
b
, is to sense and produce a voltage difference between the reference level and the video level of each pixel. The most important benefit of using CDS
108
is to reduce noise. In addition to the capturing of the video data by subtracting the reference levels from the video levels, any noise common to these two signals are removed by the CDS
108
.
One approach for canceling an offset in switched capacitor amplifiers is to put the amplifier in unity gain feedback during the sampling phase. This way the input offset is also sampled and canceled during the amplification phase. For applications, however, where high speed and high closed loop gain are required, stable amplifiers at unity gain feedback can not be maintained. In addition, this approach will not correct the optical black level.
Another approach corrects the optical black level using the feedback circuit
300
displayed in FIG.
3
. It integrates the optical black error on an integrator and applies a negative feedback to the input of the PGA
306
. The feedback circuit operates to control the level of the analog optical black signal to a predetermined level.
This technique, however, lacks the flexibility of digital programmability and requires analog circuit complexity, sometimes even off-chip capacitors. This technique also relies on device matchings which may cause a yield issue. It is also not suitable for discrete time (switched capacitor) systems because of the latency at the amplifier outputs. In the alternative, however, post digital optical black correction techniques are not desired, since it is better to cancel the offset in analog domain for an optimum dynamic range.
An approach disclosed in U.S. Pat. No. 6,049,355 which is incorporated by reference herein includes a clamping circuit
400
for use in a video camera that varies the level of the image signal utilizing level varying circuitry
401
which receives an analog feedback control signal from control signal generating unit
408
and converts the varied image signal to a digital image signal using ADC
402
. Level detection circuit
404
generates a digital zone value signal that corresponds to a level of a reference zone of the digital image signal. Further, the error detection circuit
406
digitally detects an amount of error between the digital zone value signal and a predetermined zone value having a non-zero factional portion. From the detected amount of error, it generates a digital error signal that has a predetermined number of data bits. The circuit
400
then switches between a coarse adjustment mode and a fine adjustment mode in accordance with a level of the digital error signal. In the coarse adjustment mode, a pulse width modulated signal from the most significant bits of the digital error signal is used to apply an coarse adjustment to the incoming signal. In the fine adjustment mode the pulse width modulated signal from the least significant bits of the digital error signal, and generates from the generated pulse width modulated signal the analog feedback control signal.
The difficulty existing with this scheme is that the loop gain must be switched. Such precision is not easily achieved. This approach relies heavily upon pulse width modulation and an integrator feedback to generate a coarse and fine adjustment using most and least significant bits respectively.
Thus, there exists a need for an all digital programmable optical black and offset correction circuit for CCD signal processing suitable for discrete time switched capacitor systems .
SUMMARY OF THE INVENTION
To address the above-discussed deficiencies of the analog front end circuitry having optical black and offset correction, the present invention teaches an offset and optical black correction circuit having a digitally programmable bandwidth. A first embodiment of the image processing apparatus in accordance with the present invention includes a sampling circuit to sample the incoming optical black signal output from a CCD. An analog-to-digital converter converts the sampled signal for processing by a digital detector circuit which detects the average optical black level of the sampled signal. The sum of the channel offset and optical black level present at the output of the digital detector circuit as a digital error signal is averaged for a given number of lines and optical black cells per line by a digital averager included within the digital detector circuit. A digital comparator receives a digital reference signal and the averaged optical black and channel offset data output from the digital averager to compare the optical black signal with the reference signal. This difference is transmitted to a correction circuit to correct the optical black level which feeds back an adjustment to be applied to the analog image signal. More particularly, the correction circuit includes a digital-to-analog converter (DAC) that converts the difference into an analog signal to be applied as an adjustment to the analog image signal at the input of the image processing apparatus. Thus, the error signal is fed back to the analog channel through the DAC in order to modify the existing signal such that a desired optical black level at the output of the ADC exists.
In a second embodiment, the correction circuit includes a first and a second DAC, use

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