Variable Bit Rate
DVD specifies a Constant Linear Velocity (CLV) of 3.49 m/s, giving a transfer rate of 11.08 Mbits/sec--although 9.8 Mbits/sec is the top usable limit. But, MPEG-2 implements variable bit rate encoding (often called variable rate bitstream encoding), because it helps optimize quality in the program video stream while providing for maximum compression. Essentially, the sequences with high degree of motion are allowed higher data rates than the sequences with lower degrees of motion. It uses track buffers, intermittent reads, and even a complex ‘double pass’ process to achieve a high quality data stream within the allocated bandwidth (variable bit rate decoding for the video, the audio streams, and the subtitles). Obviously, the average bit rate in DVD-Video depends on the complexity of the video application, and can be anywhere up to the maximum of 9.8 Mbits/sec (higher rates up to the 11.08 Mbits/sec will not play in DVD-drives and players). An average bit rate of 4.7 Mbits/sec is generally used along with 133 minutes of high quality video to describe DVD-Video performance from a single layer disc (capacity up to 4.7 gigabytes of data).
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