Pulse or digital communications – Bandwidth reduction or expansion – Television or motion video signal
Reexamination Certificate
2000-01-12
2003-04-01
Kelley, Chris (Department: 2613)
Pulse or digital communications
Bandwidth reduction or expansion
Television or motion video signal
Reexamination Certificate
active
06542541
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to video decoders, and more particularly, to a method and apparatus for decoding encoded MPEG video data stream into raw video data.
BACKGROUND OF THE INVENTION
MPEG Background
Moving Pictures Experts Group (“MPEG”) is a committee under the International Standards Organization (“ISO”) and the International Electronics Commission (“IEC”) that develops industry standards for compressing/decompressing video and audio data. Two such standards that have been ratified by MPEG are called MPEG-1 and MPEG-2. MPEG-1 is documented in ISO/IEC 11172 publication and is fully incorporated herein by reference. MPEG-2 is disclosed in ISO/IEC publication 11172 and 13818, and is also incorporated herein by reference.
MPEG-1 was developed with the intent to play back compressed video and audio data either from a CD-ROM, or transfer compressed data at a combined coded bit rate of approximately 1.5 Mbits/sec. MPEG-1 approximates the perceptual quality of a consumer videotape (VHS). However, MPEG-1 was not intended for broadcast quality. Hence, MPEG-1 syntax was enhanced to provide efficient representation of interlaced broadcast video signals. This became MPEG-2.
MPEG-1 and MPEG-2 can be applied at a wide range of bit rates and sample rates. Typically MPEG-1 processes data at a Source Input Resolution (SIF) of 352 pixels×240 pixels at 30 frames per second, at a bit rate less than 1.5 Mbits/s. MPEG-2, developed to serve the requirements of the broadcast industry, typically processes 352 pixels×240 lines at 30 frames/sec (“Low Level”), and 720 pixels/line×480 lines at 30 frames/sec (“Main Level”), at a rate of approximately 5 Mbits/sec.
MPEG standards efficiently represent video image sequences as compactly coded data. MPEG standards describe decoding (reconstruction) processes by which encoded bits of a transmitted bit stream are mapped from compressed data to the original raw video signal data suitable for video display.
MPEG ENCODING
MPEG encodes video sequences such that RGB color images are converted to YUV space with two chrominance channels, U and V. A MPEG bitstream is compressed by using three types of frames: I or intra frames, P or predicted frames, and B or bidirectional frames. I frames are typically the largest frames containing enough information to qualify as entry points. Predicted frames are based on a previous frame and are highly compressed. Bi-directional frames refer both to future and previous frames, and are most highly compressed.
MPEG pictures can be simply intra-coded, with no motion compensation prediction involved, forward coded with pel prediction projected forward in time, backward coded with pel prediction backward in time, or bi-directionally coded, with reference to both forward and backward pictures. Pictures can be designated as I (formed with no prediction involved as a still image from the image data originating at the source, e.g., a video camera), P (formed with prediction from forward pictures) or B (formed with prediction both from a forward picture and/or a backward picture). An example of display sequence for MPEG frames might be shown as follows:
IBBPBBPBBPBBIBBPBBPB
Each MPEG picture is broken down into a series of slices and each slice is comprised of a series of adjacent macroblocks.
MPEG pictures can be progressive sequence or interlaced. For the interlaced GOP comprises of field and/or frame pictures. For frame pictures, macroblock prediction scheme is based upon fields (partial frames) or complete frames.
MPEG encoder decides how many pictures will occur in a GOP, and how many B pictures will be interleaved between each pair of I and P pictures or pair of P pictures in the sequence. Because of picture dependencies, i.e., temporal compression, the order in which the frames are transmitted, stored or retrieved, is not necessarily the video display order, but rather an order required by the decoder to properly decode pictures in the bitstream.
MPEG compression employs two fundamental techniques: Motion compensation and Spatial Redundancy. Motion compensation determines how predicted or bidirectional frames relate to their reference frame. A frame is divided into 16×16 pixel units called macroblocks. The macroblocks in one frame are compared to macroblocks of another frame, similarities between the frames are not coded. If similar macroblocks shift position between frames, the movement is explained by motion vectors, which are stored in a compressed MPEG stream.
Spatial redundancy technique reduces data by describing differences within corresponding macroblocks. Spatial compression is achieved by considering the frequency characteristics of a picture frame. The process uses discrete cosine transform (“DCT”) coefficients that spatially tracks changes in color and brightness. The DCTs are done on 8×8 pixel blocks. The transformed blocks are converted to the “DCT domain”, where each entry in the transformed block is quantized with respect to a set of quantization tables. Huffman coding and zig-zag ordering is used to transmit the quantized values.
MPEG DECODING
MPEG Video decoders are known in the art. The video decoding process is generally the inverse of the video encoding process and is employed to reconstruct a motion picture sequence from a compressed and encoded bitstream. Generally MPEG video bitstream data is decoded according to syntax defined by MPEG standards. The decoder must first identify the beginning of a coded picture, identify the type of picture, and then decode each individual macroblock within a particular picture.
Generally, encoded video data is received in a rate or a video buffer verifier (“VBV”). The data is retrieved from the channel buffer by a MPEG decoder or reconstruction device for performing the decoding. MPEG decoder performs inverse scanning to remove any zig zag ordering and inverse quantization to de-quantize the data. Where frame or field DCTs are involved, MPEG decoding process utilizes frame and field Inverse Discrete Cosine Transforms (“IDCTS”) to decode the respective frame and field DCTs, and converts the encoded video signal from the frequency domain to the spatial domain to produce reconstructed raw video signal data.
MPEG decoder also performs motion compensation using transmitted motion vectors to reconstruct temporally compressed pictures. When reference pictures such as I or P pictures are decoded, they are stored in a memory buffer. When a reconstructed picture becomes a reference or anchor picture, it replaces the oldest reference picture. When a temporally compressed picture, also referred to as a target frame, is received, such as P or B picture, motion compensation is performed on the picture using neighboring decoded I or P reference pictures. MPEG decoder examines motion vector data, determines the respective reference block in the reference picture, and accesses the reference block from the frame buffer.
After the decoder has Huffman decoded all the macroblocks, the resultant coefficient data is then inverse quantized and operated on by an IDCT process to transform macroblock data from a frequency domain to data in space domain. Frames may need to be re-ordered before they are displayed in accordance with their display order instead of their coding order. After the frames are re-ordered, they may then be displayed on an appropriate device.
FIG. 1
shows a block diagram of a typical MPEG decoding system, as is known in the art. Shown in
FIG. 1
are a MPEG Demux
10
, a MPEG video decoder
11
and an audio decoder
12
. MPEG Demux
10
receives encoded MPEG bit stream data
13
that consists of video and audio data, and splits MPEG bit stream data
13
into MPEG video stream data
14
and MPEG audio stream data
16
. MPEG video stream data
14
is input into MPEG video decoder
11
, and MPEG audio stream data
16
is input into an MPEG audio decoder
12
. MPEG Demux
10
also extracts certain timing information
15
, which is provided to video decoder
11
and audio decoder
12
. Timing information
15
enable video dec
Luna Amelia Carino
Wang Jason N.
Williams Richard L.
Blakely , Sokoloff, Taylor & Zafman LLP
Sony Corporation
Vo Tung T.
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