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
AAC is an audio file format that works similar to MP3 but compresses more than MP3. It significantly reduces the audio file size without losing the quality. To reduce the file size it uses the MPEG-4 standard to cut off the low frequency sounds that cannot be heard in a human’s ear.