 DCT is the most widely used form of lossy compression, for popular image compression formats (such as JPEG), video coding standards (such as MPEG and H.264/AVC) and audio compression formats (such as MP3 and AAC).In the case of audio data, a popular form of transform coding is perceptual coding, which transforms the raw data to a domain that more accurately reflects the information content.For example, rather than expressing a sound file as the amplitude levels over time, one may express it as the frequency spectrum over time, which corresponds more accurately to human audio perception.