Chapter 14 discusses techniques for varying the quantization so as to minimize visible distortion for a given bit rate. Spatial and temporal masking effects tend to reduce the visibility of artifacts in more active areas areas with strong temporal and/or spatial variations and these effects can be used to improve the overall perceptual quality of a video sequence. Variable quantization is also, however, the most important mechanism for controlling MPEG bitrates. In this chapter the discussion turns to the methods by which variable quantization and other techniques are used to control the bitrate of MPEG data streams. In most cases the two uses of variable quantization for improving quality and for controlling the bitrate are complementary. ^[1]

A number of other aspects of encoder decision making will be considered in this chapter. These include GOP structure, bit-allocation between I-, P-and B-pictures, macroblock type selection, feedback to control bitrate, and buffer overflow and underflow.

This chapter starts with a review of literature relating to MPEG rate control. Then, buffer overflow and underflow, the idealized video buffering verifier (VBV) decoding model for regulating decoder buffer fullness, and proportional rate control feedback systems are discussed. Finally, some measures an implementer can take to obtain a robust rate control system that can handle unexpected extremes in picture complexity are described.