Pulse or digital communications – Bandwidth reduction or expansion – Television or motion video signal
Reexamination Certificate
1998-06-29
2003-12-16
Kelley, Chris (Department: 2613)
Pulse or digital communications
Bandwidth reduction or expansion
Television or motion video signal
Reexamination Certificate
active
06665344
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a downconverting decoder for downconverting and decoding high resolution encoded video for display by a lower resolution receiver.
BACKGROUND OF THE INVENTION
The international standard ISO/IEC 13818-2 (Generic Coding of Motion Pictures and Associated Audio Information: Video) and the “Guide to the use of the ATSC Digital Television Standard” describe a system, known as MPEG-2, for encoding and decoding digital video data. According to this system, digital video data is encoded as a series of code words in a complicated manner that causes the average length of the code words to be much smaller than would be the case if, for example, each pixel in every frame was coded as an eight bit value. This type of encoding is also known as data compression.
The standard allows for encoding of video over a wide range of resolutions, including higher resolutions commonly known as HDTV. In MPEG-2, encoded pictures are made up of pixels. Each 8×8 array of pixels is known as a block, and a 2×2 array of blocks is known as a macroblock. Compression is achieved by using well known techniques including (i) prediction (motion estimation in the encoder and motion compensation in the decoder), (ii) two dimensional discrete cosine transform (DCT) which is performed on 8×8 blocks of pixels, (iii) quantization of the resulting DCT coefficients, and (iv) Huffman and run/level coding. In MPEG-2 encoding, pictures which are encoded without prediction are referred to as I pictures, pictures which are encoded with prediction from previous pictures are referred to as P pictures, and pictures which are encoded with prediction from both previous and subsequent pictures are referred to as B pictures.
An MPEG-2 encoder
10
is shown in simplified form in FIG.
1
. Data representing macroblocks of pixel values are fed to both a subtractor
12
and a motion estimator
14
. In the case of P pictures and B pictures, the motion estimator
14
compares each new macroblock (i.e., a macroblock to be encoded) with the macroblocks in a reference picture previously stored in a reference picture memory
16
. The motion estimator
14
finds the macroblock in the stored reference picture that most closely matches the new macroblock.
The motion estimator
14
reads this matching macroblock (known as a predicted macroblock) out of the reference picture memory
16
and sends it to the subtractor
12
which subtracts it, on a pixel by pixel basis, from the new macroblock entering the MPEG-2 encoder
10
. The output of the subtractor
12
is an error, or residual, that represents the difference between the predicted macroblock and the new macroblock being encoded. This residual is often very small. The residual is transformed from the spatial domain by a two dimensional DCT
18
. The DCT residual coefficients resulting from the two dimensional DCT
18
are then quantized by a quantization block
20
in a process that reduces the number of bits needed to represent each coefficient. Usually, many coefficients are effectively quantized to zero. The quantized DCT coefficients are Huffman and run/level coded by a coder
22
which further reduces the average number of bits per coefficient.
The motion compensator
14
also calculates a motion vector (mv) which represents the horizontal and vertical displacement of the predicted macroblock in the reference picture from the position of the new macroblock in the current picture being encoded. It should be noted that motion vectors may have ½ pixel resolution which is achieved by linear interpolation between adjacent pixels. The data encoded by the coder
22
are combined with the motion vector data from the motion estimator
14
and with other information (such as an indication of whether the picture is an I, P or B picture), and the combined data are transmitted to a receiver that includes an MPEG-2 decoder
30
.
For the case of P pictures, the quantized DCT coefficients from the quantization block
20
are also supplied to an internal loop that represents the operation of the MPEG-2 decoder
30
. Within this internal loop, the residual from the quantization block
20
is inverse quantized by an inverse quantization block
24
and is inverse DCT transformed by an inverse discrete cosine transform (IDCT) block
26
. The predicted macroblock, that is read out of the reference picture memory
16
and that is supplied to the subtractor
12
, is also added back to the output of the IDCT block
26
on a pixel by pixel basis by an adder
28
, and the result is stored back into the reference picture memory
16
in order to serve as a macroblock of a reference picture for predicting subsequent pictures. The object of this internal loop is to have the data in the reference picture memory
16
of the MPEG-2 encoder
10
match the data in the reference picture memory of the MPEG-2 decoder
30
. B pictures are not stored as reference pictures.
In the case of I pictures, no motion estimation occurs and the negative input to the subtractor
12
is forced to zero. In this case, the quantized DCT coefficients provided by the two dimensional DCT
18
represent transformed pixel values rather than residual values, as is the case with P and B pictures. As in the case of P pictures, decoded I pictures are stored as reference pictures.
The MPEG-2 decoder
30
illustrated in
FIG. 2
is a simplified showing of an MPEG-2 decoder. The decoding process implemented by the MPEG-2 decoder
30
can be thought of as the reverse of the encoding process implemented by the MPEG-2 encoder
10
. The received encoded data is Huffman and run/level decoded by a Huffman and run/level decoder
32
. Motion vectors and other information are parsed from the data stream flowing through the Huffman and run/level decoder
32
. The motion vectors are fed to a motion compensator
34
. Quantized DCT coefficients at the output of the Huffman and run/level decoder
32
are fed to an inverse quantization block
36
and then to an IDCT block
38
which transforms the inverse quantized DCT coefficients back into the spatial domain.
For P and B pictures, each motion vector is translated by the motion compensator
34
to a memory address in order to read a particular macroblock (predicted macroblock) out of a reference picture memory
42
which contains previously stored reference pictures. An adder
44
adds this predicted macroblock to the residual provided by the IDCT block
38
in order to form reconstructed pixel data. For I pictures, there is no reference picture so that the prediction provided to the adder
44
is forced to zero. For I and P pictures, the output of the adder
42
is fed back to the reference picture memory
42
to be stored as a reference picture for future predictions.
The MPEG encoder
10
can encode sequences of progressive or interlaced pictures. For sequences of interlaced pictures, pictures may be encoded as field pictures or as frame pictures. For field pictures, one picture contains the odd lines of the raster, and the next picture contains the even lines of the raster. All encoder and decoder processing is done on fields. Thus, the DCT transform is performed on 8×8 blocks that contain all odd or all even numbered lines. These blocks are referred to as field DCT coded blocks.
On the other hand, for frame pictures, each picture contains both odd and even numbered lines of the raster. Macroblocks of frame pictures are encoded as frames in the sense that an encoded macroblock contains both odd and even lines. However, the DCT performed on the four blocks within each macroblock of a frame picture may be done in two different ways. Each of the four DCT transform blocks in a macroblock may contain both odd and even lines (frame DCT coded blocks), or alternatively two of the four DCT blocks in a macroblock may contain only the odd lines of the macroblock and the other two blocks may contain only the even lines of the macroblock (field DCT coded blocks). The coding decision as to which way to encode a picture may be made adap
Bugg George A.
Kelley Chris
Zenith Electronics Corporation
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