What are constant and variable bitrate streams?
Constant bitrate streams are buffer regulated to allow continuos transfer of coded data across a constant rate channel without causing an overflow or underflow to a buffer on the receiving end. It is the responsibility of the Encoder’s Rate Control stage to generate bitstreams which prevent buffer overflow and underflow. The constant bit rate encoding can be modeled as a reservoir: variable sized coded pictures flow into the bit reservoir, but the reservoir is drained at a constant rate into the communications channel. The most challenging aspect of a constant rate encoder is, yes, to maintain constant channel rate (without overflowing or underflow a buffer of a fixed depth) while maintaining constant perceptual picture quality. In the simplest form, variable rate bitstreams do not obey any buffer rules, but will maintain constant picture quality. Constant picture quality is easiest to achieve by holding the macroblock quantizer step size constant, e.g. quantiser_scale_code of 8 (linea
