Digital scheme for noise filtering of optical black and...

Details Digital scheme for noise filtering of optical black and... Digital scheme for noise filtering of optical black and...

2000-08-30

2004-12-07

Ho, Tuan (Department: 2615)

Television

Camera, system and detail

Combined image signal generator and general image signal...

C348S241000

Reexamination Certificate

active

06829007

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, which provides digital optical black and offset correction and noise filtering.
BACKGROUND OF THE INVENTION
Great strides 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
, 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
FIGS. 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
. 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
FIGS. 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. 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 is not desired, since it is better to cancel the offset in analog domain for an optimum dynamic range.
Our copending application Ser. No. 09/353,919, as shown in
FIG. 4
, provides a CCD signal processing method that provides optical black offset correction using a moving average filter scheme such that the optical black pixels are averaged at the beginning of each line and offset DAC,
418
, are updated in order to cancel the offset. The analog front end (AFE) converts the CCD output signal to digital data to allow subsequent digital signal processing. At the input of the AFE, the DC level of the CCD output signal is clamped to the input dynamic range. For better noise performance and dynamic range, correlated double sampling is applied to the clamped input signal. The output of correlated double sampler (CDS) is amplified by a programmable gain that varies exponentially with linear control. Then the amplified analog signal is converted to digital data. The optical black value and channel offset are corrected in order to maximize the dynamic range.
Using a feedback loop having a switch
410
that closes during optical black level sampling of the signal, a digital averager
412
averages the optical black pixels. A comparator
414
compares the desired optical black level with the averaged optical black level. It provides an up and down control signal to the up/down counter
416
. Counter/register
416
counts up or down until the output of the ADC
408
converges to the desired optical black level. Digital-to-analog converter
418
converts the output of the counter into an analog voltage to be applied to the image signal output from CDS
402
. This circuit arrangement, however, will take an unknown repetition of feedback lines to cancel the optical black level offset. Also, if the PGA gain is too high, the accuracy of the cancellation may be poor.
The second embodiment in our copending application Ser. No. 09/353,919, provides a CCD signal processing method that provides optical black offset correction using a moving average filter scheme such that the optical black pixels are averaged at the beginning of each line and offset DACs, DAC-C
612
and DAC-F
614
, are updated in order to cancel the offset. Specifically, as shown in
FIG. 6
, circuit
600
includes a mixed signal technique that corrects the offset and optical black value in the analog domain using a coarse and fine adjustment mode. Digital optical black correction circuit
616
determines the necessary amount that the analog offset of the image signal should be adjusted. DAC-C
612
and DAC-F
614
provide offsets in the coarse and fine adjustment modes, respectively. This highly programmable technique can be used both in discrete and continuous time systems and does not require any off-chip components.
In operation, CCD image lines are shifted vertically to a line register, then the pixels on this line are shifted horizontally to an output pin. This process causes a gradual increase in the optical black value within the frame, which needs to be corrected. As shown in
FIG. 7
, there may be an initial jump in the optical black value for the first line of the image frame or field. This jump is caused by different exposure times. Afterwards there is a gradual increase in the average value. In addition to the slow ramp due to the shift in the optical black value during the image read mode, line noise exists as shown; thus, if correction DACs are updated every line, there will be line noise. If DAC updates are conducted over a fixed number of user programmable lines, then there may be visible bands on the image. Moreover, the average differs from line to line since some of the optical black pixels may be defective, i.e. hot and cold optical black pixels. A hot pixel is a defective pixel that generates too much charge, and a

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