Multiplex communications – Communication techniques for information carried in plural... – Adaptive
Reexamination Certificate
1999-05-28
2002-08-20
Chin, Wellington (Department: 2664)
Multiplex communications
Communication techniques for information carried in plural...
Adaptive
C370S477000
Reexamination Certificate
active
06438139
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention has to do with the transmission of variable-rate bit streams generally and more particularly with the efficient time multiplexing of several such bit streams onto a transmission medium.
2. Description of the Prior Art:
FIGS. 1-3
A new problem in data transmission is the transmission of data that requires a high band width, is bursty, and has temporal constraints. Traditionally, data transmission has been done on the public switched networks provided by the telephone companies and 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 amount of bandwidth available for a transfer depends on the degree of congestion in the network. The packet networks thus typically make no guarantees whatever about when or even in what order the data in a burst of data will arrive at its destination. As may be seen from the foregoing, 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 MPEG-2 standard. For details on the standard, see
Background Information on MPEG
-1
and MPEG
-2
Television Compression.
FIG. 1
shows those details of the MPEG-2 standard that are required for the present discussion. The standard defines a 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 areas 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 the amount of 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 that will be needed to represent the image.
Maximum advantage for the transmission of images encoded using the MPEG-2 standard is obtained if the images can be transmitted at variable bit rates. The bit rates can vary because the rate at which a receiving device receives images is constant, while the images have varying number of bits. A large image therefore requires a higher bit rate than a small image, and a sequence of MPEG images transmitted at variable bit rates is a variable-rate bit stream with time constraints. For example, a sequence of images that shows a “talking head” will have much more spatial and temporal redundancy than a sequence of images for a commercial or MTV song presentation, and the bit rate for the images showing the “talking head” will be far lower than the bit rate for the images of the MTV song presentation.
The MPEG-2 compression scheme represents a sequence of video images as a sequence of pictures, each of which must be decoded at a specific time. There are three ways in which pictures may be compressed. One way is 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 Bidirectional predictive-coded picture, or a B picture in 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 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 “talking heads” 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 talking heads will be much smaller than the MPEG-2 representation of the images of the MTV sequence.
The MPEG-2 pictures are being received by a low-cost consumer electronics device such as a digital television set or a set-top box provided by a CATV service provider. The low cost of the device strictly limits 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 which is currently being sent. In the art, a memory which 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 which did not fit into memory is simply lost.
FIG. 1
is a representation of a digital picture source
103
and a television
117
that are connected by a channel
114
that is carrying a MPEG-2 bit stream representation of a sequence of TV images. In system
101
, a digital picture source
103
generates uncompressed digital representations of images
105
, which go to variable bit rate encoder
107
. Encoder
107
encodes the uncompressed digital representations to produce variable rate bit stream
109
. Variable rate bit stream
109
is a sequence of compressed digital pictures
111
of variable length. As indicated above, 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
107
. That rate can be varied. In general, the more bits used to encode a picture, the better the picture quality.
Bit stream
109
is transferred via a channel
114
to VBR decoder
115
, which decodes the compressed digital pictures
111
to produce uncompressed digital pictures
105
. These in turn are provided to television
117
. If television
117
is a digital television, they will be provided directly; otherwise, there will be another element which converts uncompressed digital pictures
105
into standard analog television signals and then provides those signals to television
117
. There may of course be any number of decoders
115
receiving the output of a single encoder
107
.
In
FIG. 1
, channel
114
transfers bit stream
109
as a sequence of packets
113
. The compressed digital pictures
111
thus appear in
FIG. 1
as varying-length sequences of packets
113
. Thus, picture
111
(
d
) has n packets while picture
111
(
a
) has k packets. Included in each picture
Birch Christopher H.
Huang Si Jun
Barnhardt III Hubert J.
Chin Wellington
Couturier Shelley L.
Duong Frank
Massaroni Kenneth M.
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