Pulse or digital communications – Bandwidth reduction or expansion – Television or motion video signal
Reexamination Certificate
1999-10-21
2003-10-28
Kelley, Chris (Department: 2713)
Pulse or digital communications
Bandwidth reduction or expansion
Television or motion video signal
Reexamination Certificate
active
06639942
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to signal processing, and in particular, to a method and apparatus for estimating and controlling the number of bits output from a video coder.
2. Description of the Related Art
Numerous international video coding standards have been established over the last decade. MPEG-1, for example, defines a bitstream for compressed video and audio optimized to fit into a bandwidth of 1.5 Mbits/sec. This rate is special because it is the data rate of uncompressed audio CDs and DATs.
MPEG-1 is defined to begin with a relatively low-resolution video sequence of about 352×240 frames×30 frames/sec., but use original high (CD) quality audio. The images are in color, but are converted into YUV space (a color space represented by luminance (Y) and two color differences (U and V)).
The basic scheme of MPEG-1 is to predict motion from frame-to-frame in the temporal direction, and then to use discrete cosine transforms (DCTs) to organize the redundancy in the spatial directions. The DCTs are performed on 8×8 blocks, and the motion prediction is done in the luminance channel (Y) on 16×16 blocks (each of the 16×16 Y and the corresponding 8×8 U and V block pairs is considered to be a macroblock).
In other words, given the 16×16 block in a current frame to be coded, a close match to that block in a previous or future frame (there are backward prediction modes where later frames are sent first to allow interpolating between frames) is desired.
The DCT coefficients of either the actual data, or the difference between the block and the close match, are “quantized,” in that they are coarsely represented by fewer number of bits by means of (shifting and) integer dividing by a quantization parameter to yield quantization levels. By quantization, it is desired that many of these DCT coefficients will become “0” and drop out.
The result of the coding, including the motion vectors and the quantization levels are variable length coded using fixed tables. The quantization levels are zigzag scanned and ordered into a one—dimensional array. Each nonzero level is represented by a codeword indicating a run—length of zeros preceding in the scan order, the nonzero value of the level that ended the run and whether more nonzero levels are to be coded in the block. Compression is achieved by assigning shorter codewords to frequent events and longer codewords to less frequent events.
In the MPEG standard, there are three types of coded frames. There are “I” frames, or intra-coded frames, that are simply a frame coded as a still image, without using any past history. Then there are “P” frames, or predicted frames. P-frames are predicted from the most recently reconstructed I- or P-frame (from the point of view of the decompressor). Further, each macroblock in a P-frame can either be characterized by a motion vector from a close match in the last I or P-frame and blocks of DCT coefficients of the motion compensated difference values associated with the motion vector (inter coded), or simply be characterized by the blocks of DCT coefficients of the macroblock itself (intra-coded), if no suitable match exists.
In “B” (bidirectional) frames matching blocks are searched for in the past and/or future I or Pframes. The macroblock can be motion compensated by only the forward vector and using DCT blocks from the past frames, or by only the backward vector and using DCT blocks from the future frames or by both forward and backward vectors and using the average of the DCT blocks from past and future frames. The macroblock can also be simply intra-coded. Thus, after coding, a typical frame sequence may resemble the following sequence: IBBPBBPBBPBBIBBPBBPB . . . , where there are 12 frames from I to I.
Unlike MPEG-1, that is strictly meant for progressive sequences, another standard, MPEG-2 was developed. MPEG-2 can represent interlaced or progressive video sequences. The MPEG-2 concept is similar to MPEG-1, but included extensions to cover a wider range of applications. The primary application targeted by MPEG-2 is the all-digital transmission of broadcast television quality video at coded bit rates between 4 and 9 Mbit/sec. The most significant enhancement in MPEG-2 is the addition of syntax for efficient coding of interlaced video (16×8 block size motion compensation).
Several other enhancements such as alternate scan, intra VLC, nonuniform quantization resulted in improved coding efficiency for MPEG-2. Other key features of MPEG-2 are the scalable extensions that permitted the division of a continuous video signal into two or more coded bit streams representing the video at different resolutions, picture quality or picture rates.
H.261 is a video coding standard designed for data rates that are multiples of 64 Kbit/sec. This standard is specifically designed to suit ISDN lines.
As in MPEG standards the coding algorithm utilized is a hybrid of inter-picture prediction, transform coding and motion compensation. The data rate of the coding algorithm can be set between 40 Kbit/sec. and 2 Mbit/sec. Inter-picture prediction aids in the removal of temporal redundancy, while transform coding removes spatial redundancy and motion vectors are used to help the codec compensate for motion. To remove any further redundancy in the bitstream, variable length coding is utilized.
As in the MPEG standards, H.261 allows the DCT coefficients to be either intra coded or inter coded from previous frames. In other words the 8×8 blocks of DCT coefficients of the actual data or the motion compensated difference values are quantized and variable length coded. They are multiplexed onto a hierarchical bitstream along with the variable length coded motion vectors.
A similar standard, H.263, is a compression standard originally designed for low bit rate communication, but can use a wide range of bit rates. The coding algorithm is similar to that of H.261, but improves H.261 in certain areas. Specifically, half-pixel precision is used for motion compensation, as opposed to full pixel precision and a loop filter used by H.261. Additionally, H.263 includes unrestricted motion vectors, syntax-based arithmetic coding, advance prediction and forward and backward frame prediction similar to MPEG, called P-B frames. This results in the ability to achieve the same video quality as in H.261 at a drastically lower bit rate.
Unrestricted motion vectors point outside the picture. That is, the edge pixels are used as predictions for the “not existing” pixels. There is a significant gain achieved if there is movement along the edge of the picture.
Through advance prediction, overlapped block motion compensation is used for the P-frames. That is, four 8×8 vectors, instead of one 16×16 vector are used for some of the macroblocks in the picture, and motion vectors are allowed to point outside the picture. Four vectors require more bits, but give better prediction.
A “P-B” frame consists of two pictures being coded as one unit. The name P-B actually was derived from the name of picture types in MPEG (P-frames and B-frames). Thus, a P-B-frame consists of one P-frame that is predicted from the last decoded P-frame and one B-frame that is predicted from both the last decoded P-frame and the P-frame currently being decoded. The last picture is called a B-picture because parts of it may be bi-directionally predicted from the past and future P-frames.
As a result of the above characteristics, for relatively simple sequences, the frame rate can be doubled with this mode without greatly increasing the bit rate. For sequences with a lot of motion, P-B-frames do not work as well as B-frames in MPEG, since there are no separate forward and backward vectors in H.263. A motion vector for the P-frame is scaled to yield the backward vector for the B frame and scaled and augmented by a delta vector to yield the forward vector for the B frame.
Another compression standard is MPEG-4. From a video compression perspective, MPEG-4 is closely related t
Gray Cary Ware & Freidenrich LLP
Kelley Chris
Toshiba America Electronic Components Inc.
Tu Tung Vo
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