It is a 4-bit adpcm. This is a simple illustration of how compression can be achieved more than pcm.

Abstract— We propose a simple speech compression algorithm using subband division and ADPCM (Adaptive Differential Pulse Code Modulation) algorithm. Although speech data are stored in a semiconductor memory device, its capacity and the availa

音频软件的瑞士军刀，里面包含adpcm、fft、fir等音频code和算法工具(A Swiss Army knife for audio software)