Rate Distortion Optimization

The compressed bitstream will vary the size depending upon the contends of the frames. For example, a slow moving movie will generated very less compressed data, where as a fast moving movie will generate significantly large compressed data, for the same resolution & fps. This characteristics may not be welcoming in most of the situations. The rate control mechanism will keep the output bitrate within the requirement.

Figure:  Rate Control

Rate control is possible by adding a buffer on the output of the VLC and a feedback of a rate control from the buffer.  This feedback will control the strength of quantization coefficients in the quantizer. If we need constant bitrate (CBR) video, then the buffer output bitrate must be constant. That means its input bitrate must be controlled so as to avoid overflow or underflow. The common way of controlling the bitrate is to monitor buffer fullness, and then feed back this information to the quantizer. Usually the step-size of the quantizer is adjusted to keep the buffer around the midpoint, or half full.