April 20, 2005: Basics of MP3 digital audio... its important stuff!

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Got some lil pieces of info to enhance your MP3 knowledge. Learn it well and you won't be stumped when someone asks "Yo buddy, what's the bit rate all about anyways?"

What's the deal about bit rate?
A term widely used in audio compression is bit rate. It is the average number of bits that one second of audio data will consume. The unit used is kbps (1000bits/second).

Who devised mp3?
IIS devised a very powerful algorithm that standardized the ISO-MPEG Audio Layer-3 (IS 11172-3 and IS 13818-3).

How does mp3 compression really work?

MP3 is a "lossy" compression technique. This means that information is actually removed in the process of making the files smaller. Conversely, a "lossless" compression is one in which the redundant information in a file is reduced and is based on a table or special equation. No data is thrown away. MP3 and most other audio formats of its nature such as RealAudio and Microsoft's ASF are perceptually based. They remove information from the sound file that the average person would not be able to hear in the first place. Once it is taken out, it can't be put back in, even if the MP3 is converted to another file format.

What does mp3 compression have to do with human hearing?

MP3 technology came about from the study of psychoacoustics, the study of how the brain interprets sound. When we hear a sound our brains sort out a lot of information. Scientists call this masking. Let's say that in a sound where two parts are close to each other in frequency, one is usually perceived and the other is not. The degree to which this happens depends on the amplitude of the two sounds. MP3 removes the sound we can't hear and alters the dynamic through gain reduction algorithms.

Can you tell me more about mp3 and my ears?

Certainly. Here are some specifics about how the MP3 standard is actually created as it pertains to how we hear things.

Audition thresholds

The minimal audition threshold of the ear is not linear. It is represented, according to the law of Fletcher and Munsen, by a curve dug between 2Khz and 5Khz. It is not therefore necessary to code sounds situated under this threshold, because they will not be perceived. Often, some passages of musical pieces cannot be coded to a given rate without altering the musical quality. The MP3 then uses a short reservoir of bytes that acts by using passages that can be coded to an inferior rate to the given flow as a buffer. Under a given frequency the human ear is no longer able to locate the spatial origins of sounds. The MP3 format can therefore (in option) exploit this trick by using what is called joint stereo. Some frequencies are then recorded as a monophonic signal accompanied by some information so as to restore a minimum of spatialization.

Huffman is cool.

The MP3 also uses the classic technique of the Huffman algorithm. It acts at the end of the compression to code information; this is not therefore an algorithm of compression in itself but a coding method. This coding creates variable length codes on a whole number of bits. Higher probability symbols have shorter codes. Huffman codes have the property to have a unique prefix; they can therefore be decoded correctly in spite of their variable length. The decoding is very rapid (via a correspondence table).

Write your comments. If you think there is more to learn about MP3s, feel free to share the knowledge. Cheers.

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