Continuously Variable Slope Delta modulation is a voice coding method. It is a delta modulation with variable step size (adaptive delta modulation), first proposed by Greefkes and Riemens in 1970
M4A is an audio file format that is very similar to MP4. It is a proprietary file format of Apple. iTunes store contains the audios as M4A format. It uses MPEG-4 codec to contain audio files. One of the basic differences between M4A and MP4 is M4A does not contain videos like MP4.