OKI (Dialogic) ADPCM encoding and decoding

# What does it do?

OKI ADPCM is widely used in the CTI industry because it is the principal format supported by Dialogic. As the market leader, they tend to define "common practice". It offers a good balance of simplicity and quality at rates of 24kbps or 32kbps. 32kbps is obtained by ADPCM compressing 8k samples/second linear PCM. 24kbps is obtained by resampling to 6k samples/second and using the same ADPCM compression algorithm on the slower samples.

The algorithms for this ADPCM codec can be found in "PC Telephony - The complete guide to designing, building and programming systems using Dialogic and Related Hardware" by Bob Edgar. pg 272-276.