Pulse or digital communications – Bandwidth reduction or expansion – Television or motion video signal
Reexamination Certificate
2001-06-15
2004-10-12
Kelley, Chris (Department: 2613)
Pulse or digital communications
Bandwidth reduction or expansion
Television or motion video signal
C375S240000, C375S240010, C375S240120, C375S240020
Reexamination Certificate
active
06804299
ABSTRACT:
BACKGROUND OF THE INVENTION
1. The Field of the Invention
The present invention relates to the field of video processing. In particular, the present invention relates to the field of reducing generational error caused by requantization of a predictive video stream using motion compensation.
2. Background and Relevant Art
Video constitutes a series of images that, when displayed above a certain rate, gives the illusion to a human viewer that the image is moving. Video is now a widespread medium for communicating information whether it be a television broadcast, a taped program, or the like. More recently, digital video has become popular.
An uncompressed digital video stream has high bandwidth and storage requirements. For example, the raw storage requirement for uncompressed CCIR-601 resolution 4:2:2: serial digital video is approximately 20 megabytes per second. In addition, associated audio and data channels also require bandwidth and storage. From a transmission bandwidth perspective, 20 megabytes per second is much faster than conventional transmission techniques can practicably support. In addition, from a storage perspective, a two-hour movie would occupy approximately 144 Gigabytes of memory, well above the capabilities of a conventional Digital Versatile Disk (DVD). Therefore, what were desired were systems and methods for compressing (or coding) digital video in a way that maintains a relatively high degree of fidelity with the original video once uncompressed (or decoded).
One conventional high-quality compression standard is called MPEG-2, which is based on the principle that there is a large degree of visual redundancy in video streams. By removing much of the redundant information, the video storage and bandwidth requirements are significantly reduced.
FIG. 1A
illustrates a display order
100
A of a sequence of pictures. If the video stream represents progressive video, the pictures represent individual progressive frames. If the video steam represents interlaced video, the pictures represent individual interlaced frames containing two fields each.
Under the MPEG-2 standard, there are three classes of pictures, I-pictures, P-pictures and B-pictures. While MPEG-2 allows for a number of display orders, the display order illustrated in
FIG. 1A
is commonly used. In this common display order, there are a series of I-pictures. For clarity, only I-pictures I
1
and I
16
are shown in FIG.
1
A. Each consecutive I-picture pair has four P-pictures interspersed there between. For example, P-pictures P
4
, P
7
, P
10
and P
13
are interspersed between consecutive I-pictures I
1
and I
16
. In addition, two B-pictures are interspersed between each I-picture and each of its neighboring P-pictures. Two B-pictures are also interspersed between each consecutive P-picture pair. For example, B-pictures B
2
and B
3
are interspersed between I-picture I
1
and P-picture B
4
, B-pictures B
5
and B
6
are interspersed between P-pictures P
4
and P
7
, B-pictures B
8
and B
9
are interspersed between P-pictures P
7
and P
10
, B-pictures B
11
, and B
12
are interspersed between P-pictures P
10
and P
13
, and B-pictures B
14
and B
15
are interspersed between P-picture P
13
and I-picture I
16
.
The I-pictures are “intra-coded” meaning that they can be restructured without reference to any other picture in the video stream.
The P-pictures are “inter-coded” meaning that they may only be restructured with reference to another reference picture. Typically, the P-picture may include motion vectors that represent estimated motion with respect to the reference picture. The P-picture may be reconstructed using the immediately preceding I-picture or P-picture as a reference. In
FIG. 1A
, arrows illustrate the predictive relationship between pictures wherein the picture at the head of the arrow indicates the predictive picture, and the picture at the tail of the arrow indicates the reference picture used to reconstruct the predictive picture. For example, the reconstruction of P-picture P
7
uses P-picture P
4
as a reference.
B-pictures are also inter-coded. The B-picture is typically reconstructed using the immediately preceding I-picture or P-picture as a reference, and the immediately subsequent I-picture or P-picture as a reference. For example, the reconstruction of B-picture B
14
uses P-picture P
13
and I-picture I
16
as references.
FIG. 1B
illustrates the decode order
100
B of the pictures. The decode order is similar to the display order except that reference frames are decoded prior to any predictive pictures that rely on the reference picture, even if the reference picture is displayed after the predictive picture. Thus, the arrows in
FIG. 1B
are all rightward facing.
FIG. 2A
illustrates the general process involved with encoding a digital picture
201
using an encoder
200
A that is compatible with the MPEG-2 standard. If the digital picture is to be an I-picture, the digital picture bypasses the motion estimator
202
and is provided to the discrete cosine transformation unit (DCT)
203
, which transforms the digital picture, on a block-by-block basis, from a spatial representation of an image to a frequency representation of the image. The frequency representation is then passed to a quantization unit
204
, which quantizes each frequency, on a macroblock-by-macroblock basis, into definable ranges. A “macroblock” is a 16-pixel by 16-pixel array within the picture. The quantized image is then passed to a variable length coder
205
which performs, for example, variable length Huffman coding on the resulting quantized image. The reduced sized I-picture is then stored or transmitted for subsequent decoding.
If the digital picture
201
is to be a P-picture, the encoding process is similar as for I-pictures with several notable exceptions. If a P-picture, the digital picture is passed first to the motion estimator
202
. For each macroblock (i.e., 16×16 pixel array) in the P-picture, the motion estimator
202
finds a close visual match to the macroblock in the reference picture. The motion estimator
202
then represents the macroblock in the P-picture as a motion vector representing the motion between the macroblock in the P-picture and the close visual match 16×16 pixel array in the reference picture. In addition to the motion vector, a difference macroblock is calculated representing the difference between the macroblock in the P-picture and the close match 16×16 pixel array in the reference frame. A macroblock represented as a difference with corresponding motion vectors is typically smaller than a macroblock represented without motion vectors. Discrete cosine transformation and quantization are then performed on just the difference representation of the P-picture. Then, the difference information is combined with the motion vectors before variable length coding is performed.
B-pictures are encoded similar to how P-pictures are encoded, except that motion may be estimated with reference to a prior reference picture and a subsequent reference picture.
FIG. 2B
illustrates a conventional decoder
200
B in conformance with the MPEG-2 standard. First, a variable length decoder
215
performs, for example, variable length decoding on the picture. The picture (or the difference data of the picture if a P-picture or a B-picture) is passed to the inverse quantizor
214
for inverse quantization on a macroblock-by-macroblock basis. Next, an inverse discrete cosine transformer
213
performs inverse discrete cosine transformation on the frequency representation of the picture, on a block-by-block basis, to reconstruct the spatial representation of the picture. The spatial representation of the picture is passed to the motion compensator
212
where the spatial representation is combined with the motion vectors (if a P-picture or B-picture) to thereby reconstruct the digital picture
201
′. The reconstructed digital picture
201
′ is labeled differently than the original picture
201
to represent that there may be some loss in the encoding p
Moni Shankar
Tardif John A.
Kelley Chris
Nydegger Workman
Wong Allen
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