Pre-DCT residue filter

Details Pre-DCT residue filter Pre-DCT residue filter

1998-10-16

2001-11-20

Lee, Thomas D. (Department: 2624)

Image analysis

Image enhancement or restoration

Image filter

C382S261000

Reexamination Certificate

active

06320987

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to video signal processing. More particularly, the present invention related to a method and apparatus for filtering a digital video signal prior to performing DCT and Quantization operations on the resultant signal.
2. The Background Art
In order to minimize the bandwidth required when transmitting video signals from one location to another, the video signals are often compressed to decrease the amount of data that will later be transmitted over a band width limited transmission path.
Many compression routines exist today which use Discrete Cosine Transform (DCT) and Quantization operations known to those of ordinary skill in the art. Some of these compression routines include various forms of MPEG compression such as MPEG-I, MPEG-II, and MPEG-4, and other compression routines such as H.261, and H.263.
Reference designations appearing in more than one figure are intended to refer to substantially similar structures. Effort is made to identify differences between figures, if any.
FIG. 1
is a schematic diagram of a generalized video processing system.
Referring to
FIG. 1
, video source
10
provides a video signal to source driver
14
which converts the RGB signal out of source
12
into YUV format. YUV is a digital representation well known to those of ordinary skill in the art.
The output of driver
14
is then processed by digital signal processor (DSP)
16
, and either processed by display driver
18
to be displayed on display
20
, or to be sent to a transmitting station over transmission lines
22
.
The processing performed by DSP
16
can be as simple as compressing the video for transmission over transmission lines
22
, or may also involve filtering to counter adverse effects inherent in the compression routine which cause the output signal to be inaccurately reproduced when decompression and display take place within a receiving station.
Briefly, between two images, motion compensation techniques are used to find image residue, which has smaller than expected values than the original image. The residue image is then transformed using DCT. DCT is a transform well known to those of ordinary skill in the art.
In order to digitize and achieve a satisfactory compression, DCT coefficients are quantized before Huffman Coding. However, the combination of quantization and DCT causes ringing and other artifacts in the reconstructed images, especially when the quantization step is large.
To reduce these artifacts, filters can be applied during the coding and decoding processes. Several in-loop filters have been introduced in video coding standards.
Two compression algorithms in common use in the video processing, and which utilize filtering schemes, are H.261 and H.263. These two filtering schemes will be briefly described, and their respective disadvantages noted.
FIG. 2
is a schematic diagram of a prior art DSP using the H.261 algorithm and in-loop filtering.
Referring to
FIG. 2
, an incoming video signal is provided into DSP
16
to a difference apparatus
30
which computes the pixel-by-pixel difference between the incoming data block and the previous data block, providing the output signal through a switch
32
to a DCT apparatus
34
.
DCT apparatus
34
performs DCT operations, operations which are required for all forms of MPEG, H.261, and H.263 compression. DCT operations and the specific requirements of MPEG compression are well-known to persons of ordinary skill in the art.
Following DCT operations, the output of DCT apparatus
34
is provided to quantizer
36
where quantizing steps well known to those of ordinary skill in the art are performed. The output of quantizer
36
is provided to transmission path
22
(FIG.
1
), and also to inverse quantizer
38
. As described earlier, the signal provided to transmission path
22
is provided over long distances to a receiver which decompresses the signal into a display frame, and then provides that display frame to a display device (receiver and display device not shown).
Inverse quantizer
38
and inverse DCT apparatus
40
together invert the compression previously accomplished, and provide the resulting signal to a summing apparatus
42
, the output of which is provided to picture buffer
44
, the output of which is then filtered by filter
46
.
Depending on the condition of switch
48
, the output of filter
46
is provided either to summing apparatus
42
, or to difference apparatus
30
.
Switches
32
and
48
are ganged switches controlled by coding controller
50
. If both of switches
32
and
48
are in position “A”, the system operates on previously stored data, filtering the data already present in picture buffer
44
, passing it through the DCT and Quantization apparatus, and back thru the inverse quantization apparatus and the inverse DCT apparatus
40
, and back into picture buffer
44
. In this mode, difference apparatus
30
and summing apparatus
42
are bypassed, since there is no comparison needing to be made with a previous block of video data.
Alternatively, if both of switches
32
and
48
are in position “B”, DSP
16
accepts new video data, compares, using difference apparatus
30
, new image data to old data which has been previously stored and then filtered, and then performs DCT and Quantization functions on the result.
When switches
32
and
48
are in position “B”, thus switching filter
46
into the processing path, reconstructed images are filtered, resulting in smoother images as well as less data required to be transmitted over transmission lines
22
. However, a significant drawback of this prior art filtering solution is that the filter coefficients are fixed, and therefore there is little or no flexibility to change the filtering based upon the type of images arriving from the external environment to difference apparatus
30
. Thus, filter
46
only has on-off control.
FIG. 3
is a schematic diagram of a prior art DSP using the H.263 algorithm and in-loop filtering.
Referring to
FIG. 3
, DSP
16
comprises the same features and structures as the apparatus of
FIG. 2
, except for the absence of filter
46
from
FIG. 2
, and the presence of new filter
60
in FIG.
3
.
The apparatus of
FIG. 3
functions substantially similarly to the apparatus of
FIG. 2
, except for the difference in the filtering. Filter
60
in
FIG. 3
is commonly used by those of ordinary skill in the art. Although filter
60
is similar to filter
46
of
FIG. 2
, filter
60
filters video data prior to that data entering picture buffer
44
.
A filter
60
as used with the H.263 algorithm, as is known to those of ordinary skill in the art, is a block-edge filter applied on reconstructed data to reduce blocking artifacts. The filtering strength is fixed, but the active region is controlled on the quantization steps.
In each of the FIG.
2
and
FIG. 3
cases, the filters function as intended. However, in both cases, the encoder and decoder must each have the same ones of the filters, in order for the decoded image to be properly viewed. For example, if a filter
46
is used, both encoder and decoder must have that filter
46
. Alternatively, if a filter
60
is used, both encoder and decoder must have that filter
60
.
A second drawback of the prior art filters is that one or more filter coefficients are fixed, rather than being variable which would add flexibility.
The present invention solves both of these problems by providing a filter that is utilized prior to DCT and quantization operations, and which does not have to be duplicated in the decoder apparatus.
OBJECTS AND ADVANTAGES OF THE INVENTION
It is an object and advantage of the present invention to provide a pre-DCT filtering apparatus for video streams.
It is a further object and advantage of the present invention to provide a video filter which is only required to be present in the encoding apparatus.
It is let a third object and advantage of the present invention to provide a filtering apparatus having variable filtering coefficients.
SUMMARY OF THE INVENTION

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