Pulse or digital communications – Bandwidth reduction or expansion – Television or motion video signal
Reexamination Certificate
2000-10-05
2004-09-21
Philippe, Gims (Department: 2613)
Pulse or digital communications
Bandwidth reduction or expansion
Television or motion video signal
C370S395100, C348S390100
Reexamination Certificate
active
06795506
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to systems and methods for transmitting data. More specifically, the present invention relates to systems and methods for efficiently scheduling and multiplexing multiple compressed bitstreams including video data using bit rate information describing the bit rate of the video data. In addition, the present invention relates to systems and methods for producing bit rate information describing the bit rate of video data and transmitting bitstreams including compressed video data and the bit rate information.
Video services are provided by a wide array of video content suppliers. For example, residential digital video services may include digital television, video on demand, Internet video, etc.—each service having hundreds of programs. A program refers to one or more bitstreams that are used to represent the video content and associated audio content. A target receiver for the programs, such as a set-top box (STB) located in a residential home, receives video programs from a number of different video content suppliers via assorted transmission channels. Typically, the ‘last mile’ of transmission between the video content suppliers and the target receiver is along the same transmission channel, requiring the channel to carry multiple video programs from the wide array of suppliers—and often simultaneously.
There are presently a variety of different communication channels for transmitting or transporting video data. For example, communication channels such as coaxial cable distribution networks, digital subscriber loop (DSL) access networks, ATM networks, satellite, terrestrial, or wireless digital transmission facilities are all well known. In fact, many standards have been developed for transmitting data on the communication channels. For the purposes herein, a channel is defined broadly as a connection facility to convey properly formatted digital information from one point to another. A channel includes some or all of the following elements: 1) physical devices that generate and receive the signals (modulator/demodulator); 2) medium that carries the actual signals; 3) mathematical schemes used to encode and decode the signals; 4) proper communication protocols used to establish, maintain and manage the connection created by the channel 5) storage systems used to store the signals such as magnetic tapes and optical disks. The concept of a channel includes but is not limited to a physical channel, but also logical connections established on top of different network protocols, such as xDSL, ATM, IP, wireless, HFC, coaxial cable, Ethernet, Token Ring, etc.
The channel is used to transport a bitstream, or a continuous sequence of binary bits used to digitally represent compressed video, audio and/or data. A bit rate is the number of bits per second that is required to transport the bitstream. A bit error rate is the statistical ratio between the number of bits in error due to transmission and the total number of bits transmitted. A channel capacity is the maximum bit rate at which a given channel can convey digital information with a bit error rate no more than a given value.
Since transmission of video data with existing communication channels is often excessive, compression is an approach that has been used to make digital video images more transportable. Digital video compression allows digitized video data to be represented in a much more efficient manner and makes it possible to transmit the compressed video data using a channel at a fraction of the bandwidth required to transmit the uncompressed video data. For example, a digitized video data having an uncompressed bit rate of roughly 120 million bits per second (Mbps) can be represented by a compressed bitstream having a bit rate of 4-6 Mbps. Compression represents in significant data savings which results in much more efficient use of channel bandwidth and storage media.
When the digital video is first compressed, the encoder assumes a particular bit rate profile, whether it is constant bit rate (CBR) or a variable bit rate (VBR). The word “profile” refers to the fact that transport bit rate may not be constant, but variable under certain constraints, such as peak bit rate, average bit rate, minimum bit rate, etc. For example, a constant bit rate stream at 4 Mbps does not have the same bit rate profile as a variable bit rate stream at an average of 4 Mbps but has larger maximum bit rate and smaller minimum bit rate, respectively.
The VBR representation of compressed video data allows a video encoder to generate compressed bitstreams that, when decoded, produce consistent video quality. However, as a result of the compression process, the number of bits required to represent the compressed data differs widely from picture to picture. The specific VBR characteristics of the compressed bitstream depends on the complexity of the video image, amount of motion in the video sequence, as well as changes made in post-generation such as scene cuts, fades, wipes, picture-in-picture, etc. As channel capacities are often expressed as constant bit rates, the variable nature of VBR compressed bitstream often poses a problem for video transmission.
One potential consequence of exceeding channel capacity for a VBR compressed bitstream on a particular channel is compromised video quality. Commonly, if one or more bitstreams is too large to fit within a channel, video data may be dropped from the bitstream or simplified to allow transmission; thus sacrificing end user video quality. Due to the real-time nature of compressed video transmission, dropped packets are not re-transmitted. Also, it is important to point out that compressed bitstreams are usually generated by either real-time encoders or pre-compressed video server storage systems. Both are likely to be in a remote site, away from the network itself. This increases the difficulty in encoding the video signal with a resulting bit rate profile sensitive to the connection bandwidth available for a particular channel or target receiver.
To further reduce the excessive amount of video transmission, bitstreams are frequently combined for transmission within a channel to make digital video data more transportable. A multiplex is a scheme used to combine bitstream representations of multiple signals, such as audio, video, or data, into a single bitstream representation. A re-multiplex is a scheme used to combine multiple bitstream representations of multiplexed signals into a single bitstream representation.
One important benefit of the VBR compression is achieved through the so-called ‘statistical multiplexing’. Statistical multiplexing is an encoding and multiplexing process which takes advantage of the VBR nature of multiple compressed video signals. When a statistical multiplexer combines multiple bitstreams, an algorithm may be used to adapt the bit rate of each VBR video signal but the total bit rate of the output multiplex is kept at a constant value. Statistical multiplexing encompasses multiplexing architecture having a reverse message path from the multiplexer to the encoders. This is also often referred to closed-loop statistical multiplexing.
FIG. 1A
illustrates a high level architecture for a conventional closed-loop statistical multiplexer (statmux)
10
. The closed-loop statmux
10
has a closed-loop signal path
11
between a statmux rate controller
12
and program encoders
14
and
16
. The signal path
11
provides the rate controller
12
with a global view of the bit rate requirements for each of the program encoders
14
and
16
and allows communication between the rate controller
12
and each encoder. The encoders
14
and
16
provide compressed video, audio and data bitstreams to a multiplexer
15
, which schedules the compressed bitstreams to output a multiplexed compressed bitstream
18
. Each of the encoders
14
and
16
does not have knowledge of the bandwidth requirements of data being encoded by the other encoder and hence relies on messages sent by the rate controller
12
. Based on the
Liu Humphrey
Zhang Ji
Beyer Weaver & Thomas LLP
Cisco Technology Inc.
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