MP3 Fomat

The MP3 FormatIf you have read How CDs Work, then you know something about how CDs store music. A CD stores a song as digital information. The data on a CD uses an uncompressed, high-resolution format. Here's what happens when a CD is created:

****Music is sampled 44,100 times per second. The samples are 2 bytes (16 bits) long.
****Separate samples are taken for the left and right speakers in a stereo system.
****So a CD 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

Let's break that down: 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 32 million bytes of space. That's a lot of space for one song, and it's especially large when you consider that over a 56K modem, it would take close to two hours to download that one song.

The MP3 format is a compression system for music. The MP3 format helps reduce the number of bytes in a song without hurting the quality of the song's sound. The goal of the MP3 format is to compress a CD-quality song by a factor of 10 to 14 without noticably affecting the CD-quality sound. With MP3, a 32-megabyte (MB) song on a CD compresses down to about 3 MB. This lets you download a song in minutes rather than hours, and store hundreds of songs on your computer's hard disk without taking up that much space

Is it possible to compress a song without hurting its quality? We use compression algorithms for images all the time. For example, a GIF file is a compressed image. So is a JPG file. We create Zip files to compress text. So we are familiar with compression algorithms for images and words and we know they work. To make a good compression algorithm for sound, a technique called perceptual noise shaping is used. It is "perceptual" partly because the MP3 format uses characteristics of the human ear to design the compression algorithm. For example.

****There are certain sounds that the human ear cannot hear.
****There are certain sounds that the human ear hears much better than others.
****If there are two sounds playing simultaneously, we hear the louder one but cannot hear the softer one.

What it mp3.

The name of the file extension and also the name of the type of file for MPEG, audio layer 3. Layer 3 is one of three coding schemes (layer 1, layer 2 and layer 3) for the compression of audio signals. Layer 3 uses perceptual audio coding and psychoacoustic compression to remove all superfluous information (more specifically, the redundant and irrelevant parts of a sound signal. The stuff the human ear doesn't hear anyway). It also adds a MDCT (Modified Discrete Cosine Transform) that implements a filter bank, increasing the frequency resolution 18 times higher than that of layer 2.
The result in real terms is layer 3 shrinks the original sound data from a CD (with a bit rate of 1411.2 kilobits per one second of stereo music) by a factor of 12 (down to 112-128kbps) without sacrificing sound quality.
Because MP3 files are small, they can easily be transferred across the Internet.

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