The present invention relates generally to systems and methods for transmitting data. More specifically, the present invention relates to efficient transmission of video data over shared transmission of channels.
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. 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 video, audio and/or data. A bit rate is the number of bits per second 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. Compression represents significant data savings which results in much more efficient use of channel bandwidth and storage media.
When 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. 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.
To further reduce the transmission load, bitstreams are frequently combined for transmission within a channel. 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 multiplexer then refers to an apparatus or software designed or configured to multiplex multiple bitstreams using a suitable multiplex scheme. Two functions of the multiplexer are a) scheduling packets including the video data and b) rate control of the video data. When there is sufficient bandwidth on the outgoing channel to send all the video data, the multiplexer simply schedules all the packets to send out.
If the channel bandwidth is not enough, rate reduction is used to decrease the bandwidth required to send the video data. The rate reduction may include re-quantization with a larger quantization step value or complete re-encoding, for example. After suitable rate reduction, the multiplexer then schedules the packets for transmission. Rate reduction often diminishes end-user video quality. Since rate reduction often leads to picture quality degradation, it should be minimized. However, many conventional transmission systems overestimate the amount of rate reduction necessary, leading to diminished output video quality.
Based on the foregoing, improved methods and systems for transmitting video data over transmission channels would be desirable.