1. The simplest of these levels would refer to a low complexity coder such as PCM or DM.
2. The next level refers to a medium complexity coder such as DPCM with non adaptive prediction or sub-optimal adaptive prediction or
SBC ( Sub-band Coding) with non-adaptive bit allocation, which offers a significantly lower bit rate than PCM for a given quality.
3. The last level denotes a higher complexity coder such as APC ( Adaptive Predictive Coding ) or ATC ( Transfer Coding with adaptive bit assignment, an encoding complexity comparable to that of fully adaptive APC ),
which provides the very best result in bit rate reduction by utilizing highly adaptive algorithms and/or extensive use of long-term waveform memory.
notes on audiocoding