Television – Image signal processing circuitry specific to television – Noise or undesired signal reduction
Reexamination Certificate
1997-08-11
2001-08-28
Eisenzopf, Reinhard J. (Department: 2614)
Television
Image signal processing circuitry specific to television
Noise or undesired signal reduction
C348S606000, C348S624000, C348S620000
Reexamination Certificate
active
06281942
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to digital video signal filtering and, in particular, to a particularly efficient signal filtering mechanism for pre-processing digital video signals prior to compression and encoding of the digital video signals for subsequent decoding and display.
BACKGROUND OF THE INVENTION
With the advent of digital video products and services, such as Digital Satellite Service (DSS) and storage and retrieval of video streams on the Internet and, in particular, the World Wide Web, digital video signals are becoming ever present and drawing more attention in the marketplace. Because of limitations in digital signal storage capacity and in network and broadcast bandwidth limitations, compression of digital video signals has become paramount to digital video storage and transmission. As a result, many standards for compression and encoding of digital video signals have been promulgated. For example, the International Telecommunication Union (ITU) has promulgated the H.261 and H.263 standards for digital video encoding. Additionally, the International Standards Organization (ISO) has promulgated the Motion Picture Experts Group (MPEG), MPEG-1, and MPEG-2 standards for digital video encoding.
These standards specify with particularity the form of encoded digital video signals and how such signals are to be decoded for presentation to a viewer. However, significant discretion is left as to how the digital video signals are to be transformed from a native, uncompressed format to the specified encoded format. As a result, many different digital video signal encoders currently exist and many approaches are used to encode digital video signals with varying degrees of compression achieved.
The primary objective of any digital video signal encoder is to achieve a high degree of compression without a significant loss of video signal. Video signal compression is generally achieved by representing identical or similar portions of an image as infrequently as possible to avoid redundancy. As a result, an image which has only very coarse detail and very few distinct colors can be compressed to a much smaller representation in comparison to a compressed representation of an image with significant amounts of very fine detail and many distinct colors. Unfortunately, video cameras and other video signal acquisition equipment introduce noise into the video signal and, from the perspective of video signal processing, the noise is generally indistinguishable from fine detail in the subject of the video signal. For example, ordinary noise in a monochromatic image may be indistinguishable from the fine detail and texture of a terrycloth towel photographed up close.
Digital video signal compression typically involves a transformation, e.g., a discrete cosine transformation (DCT), in which pixels which are relatively close in value to one another are represented in a particularly compact form. Noise in a digital video signal has a particularly adverse effect on such compression since the noise is frequently unrelated to the subject matter of the video image and frequently renders portions of the digital video signal inappropriate for representation in such a compact form.
To achieve both enhanced image quality and greater compression, video signal encoders frequently filter a video signal prior to encoding the video signal. However, the use of a particularly strong filter achieves greater compression at the expense of greater signal loss, and a particularly light filter preserves more of the original signal at the expense of a smaller degree of compression. Thus, digital video signals which include significant noise force a choice between image clarity and compression rate, i.e., the rate between the amount of data required to represent the digital video signal in uncompressed and compressed forms.
In addition, noise introduced by inexpensive, low-cost video capture and processing equipment is not adequately addressed by currently available digital video signal filters. Currently available digital video signal filters typically focus on Gaussian noise. However, ever growing popularity of inexpensive video recording and processing equipment, such as video cameras, video tape, and personal computer video capture cards, of moderate or questionable quality introduces noise which is not Gaussian.
What is needed is a digital video signal filter which can better eliminate the types of noise found in digital video signals without compromising the clarity and quality of the image of the digital video signal to thereby simultaneously improve the quality and compression rate of the digital video signal.
SUMMARY OF THE INVENTION
In accordance with the present invention, a digital video signal is spatially and temporally filtered to remove unwanted noise of the digital video signal such that better image quality of the digital video signal and reduced size when compressed and encoded are simultaneously achieved. The temporal filter determines whether a current block of pixels represents the same subject matter represented by a block of pixels of a previous frame and temporally filters the current block if the blocks represent the same subject matter. By spatially filtering the current block prior to making such a determination, impulse noise is significantly reduced and the accuracy with which such a determination is made is enhanced substantially. Accordingly, the temporal filter can more aggressively filter the digital video signal without risking temporal blurring of the digital video signal.
The spatial filter is adaptive in that a frame of the digital video signal is divided into blocks and each block is filtered according to the amount of subject matter detail represented in the block. Blocks are chosen small enough to allow smoothing up to but not including edges in the represented subject matter and large enough to provide effective smoothing. The amount of detail of the subject matter represented in each block is determined by measuring the variance of pixels values of the block. A large variance indicates significant detail in the subject matter of the block and causes the block to forego spatial filtering. A moderate variance indicates moderate detail and/or significant noise in the subject matter of the block and causes the block to be lightly spatially filtered. A low variance indicates little or no detail in the subject matter of the block and causes heavy spatial filtering of the block. Since small blocks are evaluated separately, background sections of an image can be filtered heavily to remove nearly all noise up to and very near edges at which detail in the subject matter of the image is not so heavily filtered and therefore preserved.
For blocks which are spatially filtered, a median filter effectively removes impulse noise from the digital video signal, and a smoothing filter further reduces any remaining noise. The median filter effectively removes the impulse noise without blurring or smudging the impulse noise thereby removing the impulse noise without any affects which are annoying or perceptible to a human viewer of a display of a decoded digital video signal. In addition, such effective removal of impulse noise substantially improves the performance of subsequent filtering steps such as smoothing and temporal filtering. After application of the median filter, lightly spatially filtered blocks are filtered with a light smoothing filter and heavily spatially filtered blocks are filtered with a heavy smoothing filter.
After spatial filtering, the spatially filtered frame is adaptively temporally filtered to further remove noise from the digital video signal. However, temporal filtering of the spatially filtered frame is bypassed altogether if the current frame and the previous frame are so different that common subject matter between the frames is unlikely to be found. When the spatially filtered frame is temporally filtered, the spatially filtered frame is divided into blocks. The blocks are sufficiently small to have a relatively good likelihood of matc
Eisenzopf Reinhard J.
Lee & Hayes PLLC
Lo Linus H.
Microsoft Corporation
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