Statistical Compression: CBR vs. VBR

Currently, most digital broadcasting systems use MPEG-2 as a compression standard to encode video within a constant bit rate CBR (Constant Bit Rate). This is precisely the biggest advantage of CBR coding because it makes it easy to transport through conventional communication channels. Since CBR controlled encoding aims to offer balance to the desired bitie averages over a specific time period, this implies that the number of bits used to encode the different segments of a video program are the same. The result is the variation of the quality of the encoded video with respect to the content. For many broadcast or streaming applications, a higher bit rate is frequently used to ensure acceptable video quality needed for the most complex segments of video content. At the same time, more bitios are thrown away unnecessarily by simple segments of video content.

To improve information across digital broadcast networks, migrating digital video encoding technology from CBR to a variable bit rate VBR (Variable Bit Rate) is the next step that could achieve substantial bit savings without sacrificing encoded video quality. The high compression ratio of the VBR encoding system has become evident in applications such as DVDs, VCDs, and video servers. Depending on the application, configuration and encoder used, the efficiency in coding achieves improvements of up to 40%.

The principle of VBR encoding is to encode video content as much as possible to consistent quality. The number of bits used to encode each box is a function of spatial and temporal complexity of the content of the box. This results in a VBR bit stream.

The MPEG-2 video encoding algorithm supports two modes of operation for the output of its bit rates. One is CBR which has been studied in detail and used as the default mode of operation for the most well-known applications. The other mode is VBR coding which has been syntactically supported but has not been worthy of detailed study.

Among the benefits of the VBR (Variable Bit Rate) encoding mode is that this encoding has a minimum expenditure of reserved bits that can be used to encode fixed sequences or any other class of simple sequences. It is mainly modulated by the quality parameters of the encoding to thus achieve the goal with respect to the quality of the encoding.

By comparing the average rate of bites encoded in VBR mode, in contrast to what is offered by CBR encoding, the former can achieve higher encoding quality since it can use higher rates to encode more complex content.

Assuming that the transport channel is of constant bandwidth, there are three meanings for the efficiency of VBR flows:

It is a simple flow transport mode where one or more VBR streams are directly multiplexed without any intelligent processing. The multipleser has a bit output rate that is equal to the sum of the highest bit rate of all VBR input streams.

An open-loop intelligent multiplezer is defined as one that can understand the syntax of input packets to the video stream layer. It has a protection element for large packets of information so that the multiplezer has enough time to analyze the input bit streams and make a distribution of the output packets with the redundancy and time planning mechanisms.

For utilization of bit-saving capabilities for video services, a closed-loop intelligent multiplezer can be used where the highest rate of each VBR encoder is coordinated by a statistical multiplexing rate control agent. This agent will change the distribution of the higher rate in the frequency so that the sum of this is always equal to the percentage of the multiplex output of the multiplescreator.

One of the most important features of VBR encoding is its ability to achieve video qualities similar to or better than the random complexity of video content when using fewer bits as in the case of CBR encoding. This savings figure is a significant gain for any digital communications system.

This makes it possible to increase the number of video programs per stream, improve encoding using the same average bit rate, use the available bit rate ABR (Available Bit Rate) for asynchronous data transmission services defined by ATSC and DVB specifications, statistically pre-compressed video for remote feeds or from an MPEG-2 video server, statistical re-multiplening of bitstreams from multiplexed video streams, direct encoding to DVD players that can offer MPEG stream output without the need for encoding and decoding processes, statistical multiplexing for MPEG stream-based camcorders for news and live event applications.