MP3 players

by Chris Woodford. Last updated: July 11, 2011.

Technology can be absolutely astounding! MP3 players, such as iPods, are a great example. Smaller than a pack of cards and only a little heavier, they can store thousands of music tracks, photos, or videos so you can take them with you wherever you go. A typical 20GB (gigabyte) iPod has enough memory to store about 500 CDs—rather more than you can fit in your pocket! So what exactly is "MP3" and how does it work?

Photo: Quite possibly the best thing I have ever bought in my life: my 20GB Apple iPod. I still can't quite believe that this tiny little beauty holds (at the last count) 3,717 tracks on 401 albums by 250 artists—and yet fits in my pocket!

What is MP3 technology?

MP3 files

An MP3 player gets it name from the MP3 files that you store on it. Just as DOC is a type of computer file used by the Microsoft Word word-processing program, and PDF is another type of file for storing printable documents, so MP3 is a particular file type used for storing music. Think of MP3s as computer files and an MP3 player as a special type of computer, dedicated to playing back sounds stored in coded format inside those files, and you're halfway to understanding how it all works.

Sampling

MP3 is an example of digital technology, which means sounds you hear are stored in numerical form. CDs are digital too, but older music formats (including LP records and cassette tapes) used analog technology. That means music was stored as a physical or magnetic representation of the original sound, without using any numbers at all. A sound twice as loud as normal might have been stored by a groove on a plastic record that was twice as deep as normal, so the stored information was a faithful "analog" of the original sound.

The key to storing music (or any other kind of sound) in digital format is a process called sampling—and it's a kind of "music by numbers." When you were younger, you might have learned to play a simplified piano, xylophone, or other musical instrument with numbers stuck to the keys or bells. Instead of reading a musical score, you would have simply read a list of numbers and pressed the corresponding key or bell to generate each note in turn. (Jingle Bells, for example, looks a bit like this written in digital form: 3 3 3—3 3 3—3 5 1 2 3.) MP3s and CDs work in a similar way. At the time of recording, a computer "listens" to the music track that's being recorded and "samples" the volumes and frequencies of the sounds: about 44,000 times each second, it analyzes all the sounds it can hear and converts them into a number. This process is carried out by an electronic circuit called an analog to digital converter, which turns sounds (analog) into streams of numbers (digital), which are then stored in sequence in an MP3 file or on a CD. When the file or CD is played back later, the reverse process happens: a digital to analog converter turns the numbers back into analog electrical signals that become sounds when they're fed into a loudspeaker. The faster the computer samples (the higher the sampling rate, the more closely the digital file resembles the original sounds and the higher the quality of the recording. That's why higher sampling rates give better quality MP3 files. The downside of a higher sampling rate is that it produces more digital information that has to be stored—a bigger file size, in other words.

Advantages of digital technology

What's so good about digital technology? One big advantage is that you can store more information in less space. If you've got some encyclopedias on CD-ROMs or DVDs, you'll know that computers are particularly good at cramming large amounts of information into pretty tiny spaces. The Encyclopedia Britannica, whose 20-odd volumes fill a whole shelf in your local public library, fits comfortably onto a couple of CDs or a single DVD. Tricks like this are possible because computers use a technique called compression—a way of squeezing information so it takes up much less room.

How is music stored inside an MP3 file?

Normal sound files stored on a computer take up huge amounts of space. Consider: you can fit the Encyclopedia Britannica onto a couple of CDs, but one CD will normally hold only about an hour's worth (maybe a dozen or so tracks) of music. That means each track on a normal CD must be taking up a huge amount of space—equivalent to one or two volumes of an encyclopedia! MP3 is a mathematical trick for taking the same musical information and squeezing it into about one twelfth as much space. You can make MP3 files that are smaller or larger by compressing them by different amounts, but the more you compress them the worse they'll sound. Just like telegrams and JPGs, MP3 is a lossy compression.

Inside an MP3 file, music is stored as long strings of binary numbers (zeros and ones) in a series of chunks called frames. Each frame starts with a short header (a kind of table of contents), followed by the music data itself. At the start of an MP3 file there is a kind of "index card" that stores details of the track name, artist, genre, and so on. This information is called metadata and each part of it (artist, track, and so on) is stored in what's called an ID3 tag. Many MP3 programs have an option that lets you "edit the ID3 tags." It sounds technical and complex, but it's simply a way to change the "index card" at the front of the MP3 file.

Artwork: A CD track takes up about 12 times as much room as the same track converted into MP3 format.

The great thing about an MP3 file is that it takes up so little room. A typical music track takes up only about five megabytes or so when you turn it into MP3 form, compared to the 60 megabytes or so it would take up on a CD. That means you can send an MP3 file over the Internet twelve times more quickly and cheaply than the same information stored in CD format. You can also store an awful lot more MP3 files on your music player. The relatively small size of MP3 files and the speed with which they can be downloaded has revolutionized the music business since the mid-1990s.

How does an MP3 player work?

If MP3s are computer files, it follows that MP3 players must be computers. It's absolutely true! The iPod in your pocket is a far more powerful computer than the ones people had on their desks 20 years ago.

All computers, which are machines that process information (data), have a four basic components. They have an input device (for getting the data in), a memory (for storing data), a processor (for working on the data), and an output device (for getting the data back out again). Think of an iPod or MP3 player and you'll see that it has all these things. It has an input (probably a USB docking lead that hooks it up to your computer), a memory (either a small hard drive or a flash memory that can store MP3 files), a processor (something that can read the MP3 files and turn them back into music), and an output (a socket where you plug in your headphones). Most MP3 players have another output also: a little LCD display that tells you what's playing.

Switch on your iPod to play your favorite track and it works just like a computer. The processor chip loads an MP3 file, reads the ID3 index cards, and displays the artist and track name on the display. Next, it works its way through the MP3 file reading each frame in turn. It reads the header, followed by the data, and turns the digital information (the binary ones and zeros) back into sound frequencies that your ears and your brain decode as music. That's pretty much all there is to it. But remember this: the real secret of a digital music player is not the plastic gadget in your hand but the clever technology behind the MP3 files it's playing!

Photo: A Sony Network Walkman MP3 player uses flash memory to store songs instead of a hard drive, so it's much smaller and lighter than a traditional iPod. This is quite an old model with a 512MB memory, so it can store only about 8-10 CDs worth of music. That may not sound much compared to an iPod, but it's just right for keeping in your pocket or bag for those long, tedious journeys.

Further reading

On this website

• Analog versus digital technology
• Compact discs (CDs) and DVDs
• LP record players

On other sites

• MP3 from Wikipedia: Quite a detailed technical introduction, but contains a good diagram of the structure of an MP3 file.
• The Anatomy of an MP3 file by Scot Hacker. An extract from Scot's book about MP3 (see below), which explains how an MP3 file is structured internally into a header and frames.

Books

• MP3: The Definitive Guide by Scot Hacker. O'Reilly, 2003. Great technical introduction to the basic technology of storing sounds in MP3 files.
• Understanding MP3 by Martin Ruckert. Birkhäuser, 2005. A fairly technical guide to how sounds are encoded in MP3s. A bit too complex for general readers.