MP3 Digital Sound

MP3 stands for MPEG 1 Layer 3. MPEG is a compression type for digital data. MP3 is a variation of this format that allows sound files to be compressed by 90 percent without major degradation of the quality of the sound. The compressed audio file takes up so much less storage space than on a regular compact disc or tape that it has become very convenient for transfer on the Internet.

The real possibility for sound compression without audible loss comes from the fact that the sampling for CDs contains far more than the necessary data. Sixteen-bit digital sampling at 44.1 kHz gives you a staggering amount of information. From the audio CD you get about 1.4 million bits per second of information, much more information than your ears can process. To create the MP3 signal, the information from the CD format is divided into frequency subbands. Then the signal in each subband is examined in the process of encoding to decide how many bits to allocate to it. The process employs a "psychoacoustic model" to decide which subbands will be recorded most accurately and which will be discriminated against. The idea is that only that which can realistically be heard by the ear is kept.

The favorite visual metaphor is the "polar bear in the snow storm". Against a dark mountain on a clear day, you would have to paint the polar bear with great definition. But if the polar bear is in a snowstorm, you don't have to provide as much detail, because you are not going to see much detail anyway. By analogy, if a sound in a particular subband is going to be masked out by other subbands so that you won't hear it anyway, you might as well save the bits you were going to use to record it. The "psychoacoustic model" makes judgements about which sounds were going to be masked out.

Some model is applied in the encoding of the high-resolution digital sound image to MP3, and that model is inevitably going to take out some audible information. You can improve the model by encoding at a higher bit rate, because you are putting in more information. Typical current bit rates are 128, 160, 192, 256 and 320 kbps. Tests show that the accuracy increases significantly up to 256 kbps with some current decoders, so 256 kbps is perhaps a good comparison standard. At 256,000 bits of information per second, you have reduced the 1.4 Mbps to about 18% - compression by more than five to one. Of course you can get ten to one at 128 kbps, but you can't expect to get it without noticeable loss of sound quality.


Audio signal concepts
HyperPhysics***** Sound R Nave
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