MP3 is a lossy compression format. It provides a representation of pulse-code modulation-encoded (PCM) audio data in a much smaller size by discarding portions that are considered "less important" to human hearing (similar to JPEG, a lossy compression for images).
A number of techniques are employed in MP3 to determine which portions of the audio can be discarded, including psychoacoustics. MP3 audio can be compressed with different bit rates, providing a range of tradeoffs between data size and sound quality.
The MP3 format uses, at its heart, a "hybrid transform" to transform a time domain signal into a frequency domain signal:
- 32 band polyphase quadrature filter
- 36 or 12 tap MDCT; size can be selected independent for sub-band 0...1 and 2...31
- aliasing reduction postprocessing
MP3 Surround, a version of the format supporting 5.1 channels (see surround sound), was introduced in December 2004. MP3 Surround is backward compatible with standard stereo MP3, and file sizes are similar.
In terms of the MPEG specifications, AAC (Advanced audio coding) from MPEG-4 is to be the successor of the MP3 format, although there has been a significant movement to create and popularize other audio formats. Nevertheless, any 'succession' is not likely to happen for a significant amount of time due to MP3's overwhelming popularity (MP3 enjoys extremely wide popularity and support, not just by end-users and software but by hardware such as DVD and CD players).
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