Television – Image signal processing circuitry specific to television – Combined noise reduction and transition sharpening
Reexamination Certificate
1999-02-19
2002-06-11
Harvey, David E. (Department: 2614)
Television
Image signal processing circuitry specific to television
Combined noise reduction and transition sharpening
C348S627000
Reexamination Certificate
active
06404460
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to signal processing for imaging arrays and, more particularly, to image edge enhancement with background noise reduction in a CMOS image sensor.
BACKGROUND OF THE INVENTION
Integrated circuit technology has revolutionized various fields including computers, control systems, telecommunications, and imaging. In the field of imaging, the charge couple device (CCD) sensor has made possible the manufacture of relatively low-cost and small hand-held video cameras. An alternative low-cost technology to CCD integrated circuits is the metal oxide semiconductor (MOS) integrated circuit. Using MOS technology, the signal processing circuitry can be integrated alongside the imaging circuitry, thus allowing for a single integrated chip to form a complete stand-alone imaging device. To reduce the costs of imaging systems, there has been increasing pressure to reduce the number of chips and other components while still maintaining high image quality.
An example of a single chip used for CTV image processing is detailed in “A New Single Chip LSI for an NTSC CTV Signal Processing,” by Yoshimochi et al.,
IEEE, Transactions on Consumer Electronics,
Volume 35, August 1989, p. 297. As described for the Yoshimochi et al. device, the video signal processing stage shown in
FIG. 1
of that reference consists of an input clamp circuit (CLAMP), a black level stretcher (BLACK DET), a DC restoration circuit (DC RESTORATION), a switchable 3.58 MHz chroma trap (3.58 TRAP), a video delay line circuit to compensate the chroma signal delay time (DELAY LINE), a delay line type picture sharpness control (DL CONTOUR), a contrast control circuit (CONTRAST), and a brightness control circuit (CLAMP BRIGHT).
The delay line type picture sharpness control circuit (DL CONTOUR) of the Yoshimochi et al. device is shown in
FIG. 4
of that reference, which has been reproduced as
FIG. 1
herein. As illustrated in
FIG. 1
, the video input is received at a line A, which is coupled to the input of a delay line
10
, and is also coupled to the inverting input of an amplifier
14
. The output of delay line
10
is coupled to a line B, which is coupled to the input of a delay line
12
, is coupled to the non-inverting inputs of amplifiers
14
and
16
, and is coupled to a summer
26
. The output of delay line
12
is coupled to a line C, that is coupled to the inverting input of the amplifier
16
. The output of the amplifier
14
is coupled to a line D that is coupled through a limiter
18
to a summer
22
. The output of the amplifier
16
is coupled to a line E that is coupled through a limiter
20
to a summer
22
. The output of summer
22
is coupled to the input of a sharpness control circuit
24
. The output of sharpness control circuit
24
is coupled to a line F that is coupled to summer
26
. The output of summer
26
is coupled to a line G, which provides the video output of the system. The Yoshimochi et al. reference states that this delay line type picture sharpness control circuit achieves a ringingless horizontal contour enhancement when compared with a conventional second order differential type.
The operation of the circuit of
FIG. 1
is illustrated in the timing diagrams of
FIGS. 2A-2G
.
FIGS. 2A
,
2
B, and
2
C show the signals on the lines A, B, and C, respectively. As illustrated above, the signal on line B is produced by delaying the signal on line A for approximately one time period (t=approximately 160 ns) through the delay line
10
. Then the signal on line C is produced by delaying the signal on line B for approximately one more time period through the delay line
12
.
FIG. 2D
illustrates the output D of amplifier
14
that subtracts the signal shown in
FIG. 2A
from the signal shown in FIG.
2
B.
FIG. 2E
illustrates the output E of amplifier
16
, which subtracts the signal shown in
FIG. 2C
from the signal shown in FIG.
2
B.
FIG. 2F
shows the sum of the outputs D and E from the limiters
18
and
20
, once they have been added through the summer
22
, and then adjusted by the sharpness control
24
to become the signal F.
FIG. 2G
shows the output G of summer
26
, which adds the signals shown in
FIGS. 2B and 2F
.
The overall purpose of the circuit of FIG.
1
and its operations illustrated in
FIG. 2
can be described with reference to the timing diagrams of FIG.
3
. In general, the purpose of the circuit of
FIG. 1
is to enhance the transitions in the video signal that occur at the edge of an image. At the edge of an image, the signals produced by the pixels of the sensor array may have a sharp contrast from one pixel to the next, as the image is scanned out. In other words, at the edge of an image, one pixel may be receiving a signal at a low dark level, while the next pixel is receiving a signal at a high light level.
FIG. 3A
illustrates a transition at time t
1
, representing a transition from a pixel receiving a low dark signal to a pixel receiving a high light signal. The signal shown in
FIG. 3A
is for an ideal case where the image processing circuitry reacts instantaneously. However, in actual signal processing circuitry, the various components may contribute to a loss in resolution at higher spatial frequency, such that the transition that is begun at time t
1
, may not be completed until time t
2
. A timing diagram illustrating this effect is shown in FIG.
3
B. To compensate for this phenomena, one method is to enhance the edge of the image by adding an extra signal at the transitions to attempt to make the processed signal appear more like the signal shown in FIG.
3
A. The signal that results from enhancing the edge of the signal is illustrated in FIG.
3
C. It can be seen that the signal illustrated in
FIG. 3C
is comparable to the first half of the signal illustrated in FIG.
2
G. Thus, the type of transition enhancement shown in
FIG. 3C
is what the circuit of
FIG. 1
is attempting to produce.
One of the disadvantages of the circuit of
FIG. 1
is that, in addition to producing symmetrical overshoots around transitions in the picture, it also amplifies the noise simultaneously. In other words, the same process that enhances the edges for desired signal transitions also enhances undesirable noise transitions.
The present invention is directed to a method and apparatus that overcomes the foregoing and other problems in the prior art. More specifically, the present invention is directed to a method and apparatus for image edge enhancement with background noise reduction.
SUMMARY OF THE INVENTION
A method and apparatus for image edge enhancement with background noise reduction is disclosed. According to the method and apparatus, background noise is reduced through use of feed forward gain control and threshold control of a sharpness control amplifier. In a prior art circuit, the sharpness control amplifier was controlled only by a sharpness control signal. By controlling the sharpness control amplifier also with a feed forward gain control and a threshold control, the circuit can be made to have background noise reduction while maintaining a continuous input/output characteristic curve. According to the input/output characteristic curve, when the amplitude of the transitions of the video signal are below a particular threshold value, the amplification of the sharpness control amplifier is reduced by the gain control, such that low amplitude noise signals are reduced. When the amplitude of the transitions of the video signal, representing image edge transitions, are above the amplitude of the threshold level, then normal signal amplification is produced.
REFERENCES:
patent: 6148116 (2000-11-01), Park et al.
patent: 4-176266 (1992-06-01), None
Benson, K. Blair and Whitaker, Jerry C., “Broadcast Production Equipment, Systems, and Services—Color Cameras” and “Receivers—Video and Chroma Processing,”Televison Engineering HandbookFeaturing HDTV Systems, Revised Edition, McGraw-Hill, 1992, pp. 14.92-14.93 and 13.154-13.157.
Onga, Makota et al., “New Signal-Processing LSIs for the 8mm Camcorder,”IEEE Transac
Chen Datong
He Xinping
Harvey David E.
Omnivision Technologies, Inc.
Perkins Coie LLP
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