Adaptive Differential Pulse Code Modulation(ADPCM) is the process by which analogue voice samples are encoded into high-quality digital signals. The first ADPCM standardised by the CCITT is G. 721 for 32 kbps. Later came the standards G. 726 and G. 727 for 40, 32, 24 and 16 kbps. ADPCM is used to send sound on fiber-optic long-distance lines as well as to store sound along with text, images, and code on a CD-ROM.