Interpolation of signed values A and B is efficiently performed by simple circuitry. A sound data encoding apparatus comprises a block dividing means that divides the sound data into blocks according to predetermined rules, and an encoding means that encodes the blocks in groups of a plurality of consecutive blocks. The monitor transfers a process from one processing unit to another in response to the monitor values from the processing units. Having multiple inefficiency thresholds allows for a more dynamic response to a diverse audience of callers and serves different needs of a caller during various parts of a call.