Pulse or digital communications – Bandwidth reduction or expansion – Television or motion video signal
Reexamination Certificate
2000-12-12
2003-07-15
Britton, Howard (Department: 2613)
Pulse or digital communications
Bandwidth reduction or expansion
Television or motion video signal
C370S395620
Reexamination Certificate
active
06594316
ABSTRACT:
FIELD OF THE INVENTION
The invention relates in general to the transmission of variable-rate bit streams and more particularly to detecting and preventing imminent bandwidth overflow. In particular, the invention relates to adaptive, high accuracy bit rate control of a coded bit stream in an asynchronized encoding system.
BACKGROUND OF THE INVENTION
In recent years, the transmission of data, and in particular video data, has exposed a problem related to the fact that video data often requires a high bandwidth, is bursty, and has temporal constraints. Traditionally, data transmission has been done on the public switched networks provided by the telephone companies or on packet networks. The public switched networks are designed for interactive voice applications and so provide relatively low-bandwidth circuits that satisfy stringent temporal constraints. The packet networks are designed for the transfer of data between computer systems. The only constraint is that the data eventually arrive at its destination. The bandwidth available for a transfer depends on the degree of congestion in the network. The packet networks thus typically make no guarantees about when or even in what order the data in a burst of data will arrive at its destination.
It may thus be appreciated that neither the telephone network nor the packet network is well adapted to handle high-bandwidth, bursty data with time constraints. An example of such data is digital television which has been compressed according to the Motion Picture Experts Group (“MPEG”) MPEG-2 standard, as set forth in ISO/IEC 13818-1 and 13818-2.
The MPEG-2 standard defines an encoding scheme for compressing digital representations of video. The encoding scheme takes advantage of the fact that video images generally have large amounts of spatial and temporal redundancy. There is spatial redundancy because a given video picture has sections where the entire area has the same appearance; the larger the areas and the more of them there are, the greater amount of spatial redundancy in the image. There is temporal redundancy because there is often not much change between a given video image and the ones that precede and follow it in a sequence. The less change between two video images, the greater the amount of temporal redundancy. The more spatial redundancy there is in an image and the more temporal redundancy there is in the sequence of images to which the image belongs, the fewer the bits of information that will be needed to represent the next successive image.
The MPEG-2 compression scheme presents a sequence of video images as a sequence of compressed pictures, each of which must be decoded at a specific time. There are three ways in which pictures may be compressed. The first way is called intra-coding, in which the compression is done without reference to any other picture. This encoding technique reduces spatial redundancy but not time redundancy, and the pictures resulting from it are generally larger than those in which the encoding reduces both spatial redundancy and temporal redundancy. Pictures encoded in this way are called I-pictures. A certain number of I-pictures are required in a sequence, first, because the initial picture of a sequence is necessarily an I-picture, and second, because I-pictures permit recovery from transmission errors.
Time redundancy is reduced by encoding pictures as a set of changes from earlier or later pictures or both. In MPEG-2, this is done using motion compensated forward and backward predictions. When a picture uses only forward motion compensated prediction, it is called a Predictive-coded picture, or P picture. When a picture uses both forward and backward motion compensated predictions, it is called a bi-directional predictive-coded picture, or a B picture for short. P pictures generally have fewer bits than I-pictures and B pictures have the smallest number of bits. The number of bits required to encode a given sequence of pictures in MPEG-2 format is thus dependent on the distribution of picture coding types mentioned above, as well as the picture content itself. As will be apparent from the foregoing discussion, the sequence of pictures required to encode the images of the news anchorperson will have fewer and smaller I-pictures and smaller B and P pictures than the sequence required for the MTV song presentation, and consequently, the MPEG-2 representation of the images of the news anchorperson will be much smaller than the MPEG-2 representation of the images of the MTV sequence.
The MPEG-2 pictures are typically received by a receiver such as a consumer electronics device, namely a digital television set or a set-top box provided by a cable television (“CATV”) service provider. The device constraints may limit the amount of memory available to store the MPEG-2 pictures. Moreover, the pictures are being used to produce moving images. The MPEG-2 pictures must consequently arrive in the receiver in the right order and with time intervals between them such that the next MPEG-2 picture is available when needed and there is room in the memory for the picture that is currently being sent. In the art, a memory that has run out of data is said to have underflowed, while a memory which has received more data than it can hold is said to have overflowed. In the case of underflow, the motion in the TV picture must stop until the next MPEG-2 picture arrives, and in the case of overflow, the data that did not fit into memory is simply lost.
It is also important to understand a distinction that MPEG-2 draws between a variable, bursty video data stream and a variable bit rate encoded stream. Indeed under MPEG-2, there is defined a constant bit rate (“CBR”) and a variable bit rate (“VBR”) stream. In an encoded CBR video sequence, a relatively motionless television anchorperson will still be encoded at a specified bit rate. I, P, and B pictures will all still be relatively large because the constraints of the CBR system will mandate fullness of the video “pipe”. In VBR, the transmitted picture is only as large as it needs to be as there is no systemic requirement to fill the video pipe.
FIG. 1
is a representation of a system
10
including digital picture source
12
and a television
14
that are connected by a channel
16
that is carrying a MPEG-2 bit stream representation of a sequence of TV images. The digital picture source
12
generates uncompressed digital representations (“UDR”) of images
18
, which go to variable bit rate (“VBR”) encoder
20
. VBR Encoder
20
encodes the uncompressed digital representations to produce a variable rate bit stream (“VRBS”)
22
. Variable rate bit stream
22
is a sequence of compressed digital pictures
24
(a . . . n) of variable length. When the encoding is done according to the MPEG-2 standard, the length of a picture depends on the complexity of the image it represents and whether it is an I-picture, a P picture, or a B picture. Additionally, the length of the picture depends on the encoding rate of VBR encoder
20
. That rate can be varied. In general, the more bits used to encode a picture, the better the picture quality.
The VRBS
22
is transferred via channel
16
to VBR decoder
26
, which decodes the compressed digital pictures
24
(a . . . n) to produce uncompressed digital pictures
28
. These in turn are provided to television
14
. If television
14
is a digital television, they will be provided directly; otherwise, there will be another element that converts uncompressed digital pictures (“UDP”)
28
into standard analog television signals and then provides those signals to television
14
. There may of course be any number of VBR decoders
26
receiving the output of a single encoder
20
.
In
FIG. 1
, channel
16
transfers VRBS
22
as a sequence of packets
30
. The sequence of packets, in the context of the MPEG-2 standard, are known as the Packetized Elementary Stream or “PES”. The compressed digital pictures
24
thus appear in
FIG. 1
as varying-length sequences of packets
30
. Thus, picture
24
(
a
) may have “n” packets while pictu
Huang Si Jun
Olson Michael L.
Barnhardt III Hubert J.
Britton Howard
Couturier Shelley L.
Massaroni Kenneth M.
Scientific-Atlanta, Inc.
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