ZyXEL Communications Omni 56K II - Manual (Page 41)

Omni 56K II and Omni 56K Plus II User’s Guide
Specifications and Functions 3-13
3.18.2 4-bit Voice Data Compression
The main issue in the digitized voice mode is the amount of storage required. A relatively simple ADPCM
algorithm can reduce the speech data rate to half the rate and maintain about the same voice quality. This
algorithm can also be used to reduce the speech data rate to 1/3 or 1/4 of the original rate, but with voice
quality degradation. 4-bit ADPCM is used in the Omni 56K II and Omni 56K Plus II modems.