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== '''MP3 Basics''' == | == '''MP3 Basics''' == | ||
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== Background == | == Background == | ||
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== Perceptive Coding == | == Perceptive Coding == | ||
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| − | A detailed model of human perception must be developed so only | + | There is much more information in a signal than the human ear and mind process. This extra information can be eliminated to make gains in compression. Also, sounds that are perceived, but not as well as other sounds, can be further compressed. A detailed model of human perception must be developed so only unnecessary information is eliminated or harshly compressed. Psychoacoustics, the study of how humans perceive sound, is used to do this. |
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| + | Some facts about human perception that perceptive coding uses: | ||
| + | * Humans can only perceive sound between 20 Hz and 20 kHz. | ||
| + | * Our sensitivity to sound is different at different frequencies. | ||
| + | * When a high energy signal is within a certain frequency range of a low energy signal, only the high energy signal will be perceived. | ||
| + | * When two sounds are very close in time and one is much louder than the other, we only perceive the louder sound. | ||
Revision as of 06:28, 11 December 2009
MP3 Basics
Background
- MPEG (Moving Picture Experts Group) was established in 1988 by the ISO (International Standards Organization) and the IEC (International Electrotechnical Commission).
- In 1993, standards for audio compression were released. 3 layers of complexity were established, known as MPEG-1 Audio Layers 1, 2, and 3 respectively.
- Layers 1 and 2 were based on the MUSICAM technology. Layer 3, the most complex, was based on ASPEC (Adaptive Spectral Perceptual Entropy Coding). MPEG-1 Audio Layer 3 is now known simply as MP3.
- ASPEC was developed by the Fraunhofer Institute for Integrated Circuits, who with their partners Thompson Mulltimedia, still hold many patents for MP3 encoding and decoding technology.
Basic Information
Goal: To compress files as much as possible while retaining the same perceived audio quality as the original signal.
- Uses lossy audio coding, also known as perceptive coding, to take advantage of the imperfections in human perception. Also uses traditional compression techniques, such as Huffman Coding.
- MP3 is a standard. It suggests encoding methods and standardizes file formatting and decoding, but there are many different encoding schemes.
- Very popular form of compression for music, especially downloading over the internet, playing on portable music players, and storing large amounts of music.
- Other types of audio files, such as WAV, AAC, WMA, and OGG are also popular.
Perceptive Coding
There is much more information in a signal than the human ear and mind process. This extra information can be eliminated to make gains in compression. Also, sounds that are perceived, but not as well as other sounds, can be further compressed. A detailed model of human perception must be developed so only unnecessary information is eliminated or harshly compressed. Psychoacoustics, the study of how humans perceive sound, is used to do this.
Some facts about human perception that perceptive coding uses:
- Humans can only perceive sound between 20 Hz and 20 kHz.
- Our sensitivity to sound is different at different frequencies.
- When a high energy signal is within a certain frequency range of a low energy signal, only the high energy signal will be perceived.
- When two sounds are very close in time and one is much louder than the other, we only perceive the louder sound.
