CBR control strategies provide constant or relatively constant bitrate
output with variable quality. The control strategies include various
techniques and tools, which can be used in combination or independently.
For example, an audio encoder uses a trellis in two-pass or
delayed-decision CBR encoding. The trellis nodes are states derived by
quantizing buffer fullness values. The transitions between nodes of a
previous stage and nodes of a current stage depend on encoding a current
chunk of audio at different quality levels. When pruning the trellis, the
encoder uses a cost function that considers smoothness in quality as well
as quality in absolute terms. The encoder may store compressed data at
different quality levels, then output the compressed data after
simplification of the trellis to a suitable point. If the two-pass or
delayed-decision CBR encoding fails, the encoder uses one-pass CBR
encoding for the sequence or part of the sequence.
