MP3 Format

I

Introduction

MP3 Format, type of data-file format used for storing sound recordings in compressed digital form. The format’s full title is MPEG-1 Audio Layer 3. MP3 format is widely used for compressing music recordings so they take up much less hard disc space than files of compact disc (CD) quality. The compressed files can be stored and played on many digital devices, including home computers, laptop computers, portable MP3 players, satellite radio players, and cell phones. The files can also be easily copied and transferred with computers and downloaded over the Internet.

The MP3 files have become widely accepted by consumers as a way to store and listen to music. MP3 players can hold hundreds of songs in a small, lightweight package. Sparked by the release of the Apple iPod in 2001, MP3 players have almost completely supplanted personal cassette-tape players and have largely replaced portable CD players as convenient devices for listening to recordings on the go.

II

How Files Are Compressed

In MP3 format the large data files of CD quality recordings are greatly reduced in size. The complex process uses special digital audio encoding and a compression format, along with an algorithm (sequence of mathematical steps) that causes loss of particular parts of the original data. This data is discarded on purpose and in a way intended to make the loss only minimally noticeable to an ordinary listener. A four-minute song on a CD would create a file of 42 MB (megabytes), while the same song encoded to MP3 would be reduced to just 3.84 MB. This reduction means that the file requires one-tenth of the hard disc space, allowing 10 times as much music to be stored in the same space.

The process involves the sound being first encoded (compressing the data) to create a bitstream of data that is the MP3 format. This bitstream is then decoded (decompressing the data), at which point it can be converted to an electrical current and sent to earbuds, headphones, or loudspeakers to be heard.

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How the sound gets encoded to the MP3 file format can vary greatly. It is an interesting and unusual process. There is no precise specification for an MP3 encoder while the decoding algorithm and the file format of the bitstream are well defined. This contrast between encoding and decoding is intentional. When a new encoding algorithm is developed next week or in three years, an iPod will still be able to play that new MP3 file. One consequence, though, is that not all MP3 recordings will have the same quality because of the quality differences between encoders.

MP3 encoding is a “lossy” process. This means some information of the original sound is “lost.” In fact, the data is purposefully discarded. MP3 actually uses two compression techniques to reduce files sizes: one lossy and one lossless. The encoder will first throw away portions of the sound that are most readily perceived by humans, making what was determined to be an “acceptable” reduction in sound quality by the designer of the encoder. Then it encodes redundancies (repeating information) in the bitstream to achieve further compression. It is the first part of the process that makes the biggest difference between encoders and the quality of the MP3 files they create.

The study of how humans perceive sound—the ear/brain connection--is called psychoacoustics. The designers write and program compression algorithms that mimic psychoacoustic patterns more or less accurately. Psychoacoustic models are complex algorithms that take into account sounds outside the human hearing range (too high or low, or too soft), simultaneous and temporal masking of sounds (when one sound causes another to not be perceived), and sound changes (loudness or pitch) that are too subtle for the typical listener to hear.

The encoder will analyze the incoming signal, reduce it to set mathematical patterns, and compare those patterns to psychoacoustic models that are stored in the encoder. The encoder might then discard much of the data that does not match the stored models, and keep the data that does match.

Encoders also differ by the sample rates (times a second that the signal is captured) and bitrates (how many bits should be allotted to storing each second of music) used or available to the consumer. The encoder has been designed (that is, its software has been written) to work best at certain sample and bitrates. Higher rates will result in higher quality sound, all other things being equal.

Considering the MP3 files have only 10 percent of the original sound-information, they sound amazingly good to most people. The typical consumer is probably not concerned with the lost data, if the person is even aware of it. The loss in sound quality is less than what is encountered in moving from a CD to FM radio. However, audiophiles—people who care deeply about the sound quality of recordings—readily notice differences. Also, people in the business of making recordings can quickly tell whether or not they are hearing an original file or one that has been compressed.

III

Creating MP3 Files

MP3 files can be created of any sound using an encoder. The encoder usually exists as software embedded in computer programs. The encoder may give choices of bitrate, sample rate, modeling algorithms, and more.

The bitrate—how many bits should be allotted to storing each second of music—can be specified in encoding. The lower the number of bits, the more data will be discarded, and the worse the resulting music will sound. MP3 files can be encoded at bitrates from 32Kbps to 320Kbps (thousand bits per second), in eight-bit increments. 128K is often considered acceptable for most consumer uses. Some encoders offer variable bit rates (VBR), where sections of the music use higher bit rates than others—allowing higher bit rates where needed to capture more data more accurately.

The MP3 format allows for sampling rates of 32, 44.1 and 48 KHz. A user must balance file size and sound degradation to determine which sampling rate to use. The higher the sampling rate, the more data sampled, the larger the final file, and the more details of the original sound are preserved.

Encoders are designed to do some functions better than others. Their software might be written to make the smallest file size possible while maintaining a reasonably acceptable sound quality, or perhaps it was written with the goal that files created at the highest bitrate would achieve a near-CD quality (although the files would be much larger than the first example). No two encoders are the same in how they work and in their objective. A consumer can shop for particular levels of sound quality: small file sizes at generally adequate quality, medium-size files at good quality, or large files of high quality.

IV

Portability and Uses of MP3 Files

The encoder leaves an MP3 file it has created on a computer or a recording device. Home computers make creating, downloading, and distributing MP3 files easy. Users can copy and encode songs from CDs, DVDs, digital radio broadcasts, or other digital formats.

The files created are ready to be copied to a recordable CD or DVD, or they can be stored in and played on an MP3 player, a portable device that contains a small computer hard drive or flash memory. A person’s entire music collection of hundreds of CDs can be condensed to fit in a single, small MP3 player--giving choice of music and portability. The MP3 files can also be played directly on a computer, and transferred to other devices. A smaller MP3 file can be used for a ringtone on a cell phone, for example.

The files readily travel through the Internet by design. They can be sent to friends, incorporated into a podcast (a term derived from Apple’s iPod device), attached to an e-mail, incorporated into a MySpace page, and much more. A consumer can search the Web for MP3 files, download files, and purchase songs at iTunes and other services. MP3 files and compressed audio files of many formats are widely used for music sharing, playing, and storing, and they have many other uses, as well.

All kinds of audio recordings are now often available as podcasts—MP3 and other kinds of media files that can be downloaded from a Web site. Podcasting brings sound and visuals together, with MP3 and similar video technologies. The uses for podcasts span entertainment of all sorts, and new uses are flourishing and being discovered. Radio programs, television programs, and news stories are among the most popular types of podcasts. In education, classes can be recorded as MP3 files. Lectures can be posted to a class Web site, demonstration and instructional videos created, verbal instructions provided to students, and more. eb

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