Pulse or digital communications – Bandwidth reduction or expansion – Television or motion video signal
Reexamination Certificate
2000-02-23
2003-02-18
Rao, Andy (Department: 2713)
Pulse or digital communications
Bandwidth reduction or expansion
Television or motion video signal
C375S240250, C375S240050
Reexamination Certificate
active
06522693
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This present invention relates generally to the field of data compression, and, more specifically, to a system and method for reencoding segments of buffer constrained video streams, such as MPEG video streams.
2. Discussion of the Prior Art
The digital video compression techniques are essential to many applications because the storage and transmission of uncompressed video signal requires very large amounts of memory and channel bandwidth. The dominant digital video compression techniques are specified by the international standards MPEG-1 (ISO/IEC 11718-2) and MPEG-2 (ISO/IEC 13818-2) developed by the Moving Picture Experts Group (MPEG), part of a joint technical committee of the International Standards Organization (ISO) and the International Electrotechnical Commission (IEC). These standards were developed for coding of motion pictures and associated audio signals for the variety applications involving the transmission and storage of compressed digital video, including high-quality digital television transmission via coaxial networks, fiber-optic networks, terrestrial broadcast or direct satellite broadcast; and in interactive multimedia contents stored on CD-ROM, Digital Tape, Digital Video Disk, and disk drives. The standards specify the syntax of the compressed bit stream and the method of decoding, but leave considerable latitude for novelty and variety in the algorithm employed in the encoder.
Some pertinent aspects of the MPEG-2 video compression standard will now be reviewed with further reference to commonly-owned U.S. Pat. No. 5,231,484, the contents and disclosure of which is incorporated by reference as if fully set forth herein.
To begin with, it will be understood that the compression of any data object, such as a page of text, an image, a segment of speech, or a video sequence, can be thought of as a series of steps, including: Step 1) a decomposition of that object into a collection of tokens; Step 2) the representation of those tokens by binary strings that have minimal length in some sense; and Step 3) the concatenation of the strings in a well-defined order. Steps 2 and 3 are lossless; i.e., the original data is faithfully recoverable upon reversal. Step 2 is known as entropy coding.
Step 1 can be either lossless or lossy. Most video compression algorithms are lossy because of stringent bit-rate requirements. A successful lossy compression algorithm eliminates redundant and irrelevant information, allowing relatively large errors where they are not likely to be visually significant and carefully representing aspects of a sequence to which the human observer is very sensitive. The techniques employed in the MPEG-2 standard for Step 1 can be described as predictive/interpolative motion-compensated hybrid DCT/DPCM coding. Huffman coding, also known as variable length coding, is used in Step 2. Although, as mentioned, the MPEG-2 standard is really a specification of the decoder and the compressed bit stream syntax, the following description of the MPEG-2 specification is, for ease of presentation, primarily from an encoder point of view.
The MPEG video standards specify a coded representation of video for transmission. The standards are designed to operate on interlaced or noninterlaced component video. Each picture has three components: luminance (Y), red color difference (CR), and blue color difference (CB). For 4:2:0 data, the CR and CB components each have half as many samples as the Y component in both horizontal and vertical directions. For 4:2:2 data, the CR and CB components each have half as many samples as the Y component in the horizontal direction but the same number of samples in the vertical direction. For 4:4:4 data, the CR and CB components each have as many samples as the Y component in both horizontal and vertical directions.
An MPEG data stream consists of a video stream and an audio stream that are packed, with systems information and possibly other bit streams, into a systems data stream that can be regarded as layered. Within the video layer of the MPEG data stream, the compressed data is further layered. A description of the organization of the layers will aid in understanding the invention.
The layers pertain to the operation of the compression scheme as well as the composition of a compressed bit stream. The highest layer is the Video Sequence Layer, containing control information and parameters for the entire sequence. At the next layer, a sequence is subdivided into sets of consecutive pictures, each known as a Group of Pictures (GOP). A general illustration of this layer is shown in FIG.
3
. Decoding may begin at the start of any GOP, essentially independent of the preceding GOP's. There is no limit to the number of pictures that may be in a GOP, nor do there have to be equal numbers of pictures in all GOP's.
The third or “Picture” layer is a single picture. A general illustration of this layer is shown in FIG.
4
. The luminance component of each picture is subdivided into 16×16 regions; the color difference components are subdivided into appropriately sized blocks spatially co-situated with the 16×16 luminance regions; for 4:4:4 video, the color difference components are 16×16, for 4:2:2 video, the color difference components are 8×16, and for 4:2:0 video, the color difference components are 8×8. Taken together, these co-situated luminance region and color difference regions make up the fifth layer, known as “macroblock” (MB). Macroblocks in a picture are numbered consecutively in raster scan order.
Between the Picture and MB layers is the fourth or “Slice” layer. Each slice consists of some number of consecutive MB's. Slices need not be uniform in size within a picture or from picture to picture.
Finally, as shown in
FIG. 5
, each MB consists of four 8×8 luminance blocks and
8
,
4
, or
2
(for 4:4:4, 4:2:2 and 4:2:0 video) chrominance blocks. If the width of the luminance component in picture elements or pixels of each picture is denoted as C and the height as R (C is for columns, R is for rows), a picture is C/16 MB's wide and R/16 MB's high.
The Sequence, GOP, Picture, and Slice layers all have headers associated with them. The headers begin with byte-aligned “Start Codes” and contain information pertinent to the data contained in the corresponding layer.
A picture can be either field-structured or frame-structured. A frame-structured picture contains information to reconstruct an entire frame, i.e., two fields, of data. A field-structured picture contains information to reconstruct one field. If the width of each luminance frame (in picture elements or pixels) is denoted as C and the height as R (C is for columns, R is for rows), a frame-structured picture contains information for C×R pixels and a frame-structured picture contains information for C×R/2 pixels.
A macroblock in a field-structured picture contains a 16×16 pixel segment from a single field. A macroblock in a frame-structured picture contains a 16×16 pixel segment from the frame that both fields compose; each macroblock contains a 16×8 region from each of two fields.
Each frame in an MPEG-2 sequence must consist of two coded field pictures or one coded frame picture. It is illegal, for example, to code two frames as one field-structured picture followed by one frame-structured picture followed by one field-structured picture; the legal combinations are: two frame-structured pictures, four field-structured pictures, two field-structured pictures followed by one frame-structured picture, or one frame-structured picture followed by two field-structured pictures. Therefore, while there is no frame header in the MPEG-2 syntax, conceptually one can think of a frame layer in MPEG-2.
Within a GOP, three “types” of pictures can appear. An example of the three types of pictures within a GOP is shown in FIG.
6
. The distinguishing feature among the picture types is the compression method used. The first type, Intramode pictur
Gonzales Cesar A.
Kouloheris Jack L.
Lu Ligang
Morris, Esq. Daniel P.
Rao Andy
Scully Scott Murphy & Presser
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