Pulse or digital communications – Bandwidth reduction or expansion – Television or motion video signal
Reexamination Certificate
1999-03-22
2003-06-03
Le, Vu (Department: 2613)
Pulse or digital communications
Bandwidth reduction or expansion
Television or motion video signal
Reexamination Certificate
active
06574278
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of image processing and, in particular, to a method and apparatus for performing real-time data encoding.
2. Background Information
Over the years, the Motion Picture Experts Group (MPEG) has developed a number of standards for digitally encoding (also commonly referred to as compressing) audio and video data (e.g., the well-known MPEG-1, MPEG-2 and MPEG-4 standards). Recently, particular attention has been drawn to the MPEG-2 standard [ISO/IEC 13818-2:1996(E), “Information technology—Generic coding of moving pictures and associated audio information: Video”, 1996], which generally describes a bit-stream syntax and decoding process for broadcast quality digitized video. The MPEG-2 standard is widely used in emerging state-of-the-art video delivery systems including digital versatile disk (DVD, sometimes referred to as digital video disk), direct broadcast satellite (DBS) (e.g., digital satellite television broadcasts) and high-definition television (HDTV).
The rising popularity of the MPEG-2 standard may well be attributed to its complex video compression technology that facilitates the broadcast quality video. Compression is basically a process by which the information content of an image or group of images (also referred to as a Group of Pictures, or GOP) is reduced by exploiting the spatial and temporal redundancy present in and among the image frames comprising the video signal. This exploitation is accomplished by analyzing the statistical predictability of the signal to identify and reduce the spatial and temporal redundancies, thereby reducing the amount of storage and bandwidth required for the compressed data. The MPEG-2 standard provides for efficient compression of both interlaced and progressive video content at bit rates ranging from 4 Mbps (for DVD applications) to 19 Mbps (for HDTV applications).
FIG. 1
illustrates a block diagram of the complex elements of an example prior art MPEG-2 encoder for compressing video data.
As shown in the block diagram of
FIG. 1
, encoder
100
is generally comprised of an intra-frame encoder
102
, an inter-frame encoder
04
a multiplexer
106
and a buffer
108
, which controls the rate of broadcast of the compressed video data. Each of the intra-frame encoder
102
and inter-frame encoder
104
will be described in turn, below.
Simplistically speaking, compression by intra-frame compressor
102
may be thought of as a three-step process wherein spatial redundancy within a received video frame is identified, the frame is quantized and subsequently entropy encoded to reduce or eliminate the spatial redundancy in the encoded representation of the received frame. The identification of spatial redundancy within a frame is performed by transforming spatial amplitude data of the frame into a spatial frequency representation of the frame using the discrete cosine transform (DCT) function
110
. The DCT function is performed on 8×8 pixel “blocks” of luminance (brightness) samples and the corresponding blocks of chrominance (color differential) samples of the two-dimensional image, generating a table of 64 DCT coefficients. The block of DCT coefficients is then compressed through Quantizer (Q)
112
. Quantization is merely the process of reducing the number of bits required to represent each of the DCT coefficients. The quantizing “scale” used can be varied on macroblock (16×16 pixel) basis. The quantized DCT coefficients are then translated into a one-dimensional array for encoding
114
via variable length encoding and run length encoding. The order in which the quantized DCT coefficients are scanned into encoder
114
affects the efficiency of the encoding process. In general, two patterns for scanning the block of quantized DCT coefficients are recognized, the zigzag pattern and the alternate scan pattern, each of which are depicted in
FIG. 2
as pattern
200
and
250
, respectively. Those skilled in the art will appreciate that with prior art intra-frame compression such as that employed by intra-frame encoder
102
, the zigzag scan pattern
200
is typically used as it produces long runs of zeroes, as the block of DCT coefficients are transformed run-length/value pairs for the variable length encoding process. The quantized, entropy encoded DCT coefficients along with the quantization tables are then sent to MUX
106
for broadcast and/or storage through rate control buffer
108
.
Inter-frame compressor
104
reduces the temporal redundancies existing between frames in a group of pictures and is typically a complex process of motion estimation between frames and fields of the frames using reconstructed past and predicted future frames as a reference. Accordingly, inter-frame compressor
104
is depicted comprising motion estimator
116
which statistically computes motion vectors to anticipate scene changes between frames, anchor frame storage
118
to store reconstructed prior frame data (from the quantized DCT coefficients) and predicted frame storage
120
to store a predicted future frame based on information received from the motion estimator
116
and current frame information. In addition, inter-frame compressor
104
is depicted comprising inverse quantizer
122
, inverse DCT
124
and a summing node
126
to reconstruct the present or past frames for storage in anchor frame storage
118
.
Those skilled in the art will appreciate that the MPEG-2 standard provides for three types of video frames and that the type of frame determines how the motion estimation for that frame is to be accomplished. The three frame types are Intra-frame coded (I-frame), Predictably encoded frames (P-frame) and bidirectionally interpolated frames (B-frame). I-frames are encoded based only on the content within the frame itself and are typically used as reference and synchronization frames. That is, the separation between I-frames is used to denote Groups of Pictures (GOPs). P-frames are encoded based on the immediate past I- or P-frames (also referred to as anchors), and B-frames are encoded based on past or future I- and P-frames (thus the need for anchor and predicted frame storage
118
and
120
, respectively). Predicting content based on frame data is graphically illustrated with reference to FIG.
3
.
Turning to
FIG. 3
, a graphical representation of a typical GOP sequence of frames is presented
300
denoting an IBBPBBI sequence (commonly referred to as a GOP (
6
,
3
) sequence by those skilled in the art). As shown in
FIG. 3
, encoding of I-frame
302
does not rely on any prior or future frame. Encoding of B-frame
304
utilizes information from past frames (e.g., I-frame
302
) as well as future I and/or P-frames (e.g., P-frame
306
).
If the frame sequence contains interlaced content, field prediction is also performed in calculating the motion vector. Simplistically speaking, frames are broken into even and odd fields, and the content of each field is predicted based on the information contained in both the odd and the even fields of the past and/or future frames (depending on the frame type, P or B-frames, respectively). More specifically, the content of P- and B-frames are predicted by analyzing the even and odd fields of past and/or future anchor frames. A typical field prediction process is depicted in FIG.
4
.
With reference to
FIG. 4
, two frames
402
and
410
are depicted broken into their constituent even (
404
and
412
) and odd (
406
and
414
) fields, respectively. In this example, frame
402
is an I-frame, while frame
410
is a B-frame. In accordance with the prior art, the even field
412
of B-frame
410
is predicted from the even
404
and odd
406
field of the prior I-frame
402
.
Those skilled in the art will appreciate that, although the computationally intensive video encoding associated with the MPEG-2 standard provides high resolution video imagery, its implementation typically requires one or more powerful, dedicated processor(s) (e.g., a microcontroller, an application specific
Keith Michael
McVeigh Jeffrey S.
Blakely , Sokoloff, Taylor & Zafman LLP
Le Vu
Senfi Behrooz
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