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# 3.8 Summary

Lossy compression is a type of source coding in which information is irretrievably lost. Quantisation is an example, but the term is mostly used in connection with perceptual coding in connection with audio, image and video.

MP3 audio coding uses lossy compression by exploiting frequency masking and temporal masking. The MP3 encoder identifies parts of a sound that are not perceived due to these two masking effects and does not encode them.

Advanced audio coding (AAC) (used in MPEG 4) supports efficient multimedia streaming and is also used with surround sound. It offers a wider range of options than MP3 coding, and can use three key audio quality extensions without excessively increasing the bit rate: perceptual noise substitution, spectral band replication and parametric stereo.

Perceptual coding is used in image files (such as JPEG) and in MPEG video files.

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