Motorola m25 Guide de l'utilisateur télécharger pdf (Page 58)

m25 DIGITAL AUDIO PLAYER 58

Appendix A – The MP3 Format

The MP3 format has vastly expanded the capabilities of music distribution. Able to compress digital sound to a fraction

of the size of conventional WAV sound, MP3 lends itself ideally to Internet distribution, and to new listening device

technologies. Your Motorola m25 is a leading edge player able to take advantage of the incredible flexibility afforded

by this format.

To understand how the MP3 format works, it is helpful to know something about how CDs store music. A CD stores a

song as digital information. Data on a CD-ROM uses an uncompressed, high-resolution format that splits the sound

into separate samples for the left and right speakers, thus creating stereo. Music is sampled 44,100 times per second,

with each sample being 2 bytes (16 bits) long. A CD, then, stores a huge number of bits for each second of music:

44,100 samples/second * 16 bits/sample * 2 channels = 1,411,200 bits per second

Specifically, 1.4 million bits per second equals 176,000 bytes per second. If an average song is three minutes long,

then the average song on a CD consumes about 3.2 megabytes of space. That limits the number of songs that can be

carried on a small portable device and makes for burdensome download times, especially if you are not using high-

speed Internet connections.

Codec is the MP3 Fuel

The key to the compact nature of MP3 formatted music is compression. MP3 is actually a digital audio codec, which is

a method of compressing and decompressing digitized sound. A standard CD audio format uses roughly 10 MB for

just one minute of music. MP3 uses just 1 MB for that same minute of music.

The way that MP3 compresses is essentially by removing those sounds deemed inaudible to the human ear. There

are highs and lows in any sample that are beyond, or nearly beyond, our aural comprehension, and MP3 effectively

eliminates them to produce the compact files necessary to provide the ease and flexibility the format delivers. While

there is undeniably a loss of overall sound quality, it is generally near-CD quality, and acceptable for portable listening.

It is a compromise between high fidelity and the advantages of transferability and storage ease.

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Motorola m25 Guide de l'utilisateur Page 58