MP3 players

Last updated: January 11, 2010.

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?

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 MP3 as another type of computer file and an MP3 player as a special type of computer, dedicated to playing back sounds, and you're halfway to understanding how it all works.

It takes space to store information. 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.

What is compression?

Compression is the secret behind all kinds of digital technologies, including digital photos, music downloading, and a whole lot more, so it's worth going into in a bit more detail before we get back to MP3 players.

Lossy compression

Old-style telegrams are a good example of compression in action. Before telephones were invented, people sent short messages to one another over telegraph wires. The telegraphs were busy and costly, so messages had to be kept short and people compressed their messages into as few words as possible. A message like: "I think I might pay you a visit later this week. I do hope that's alright. Maybe you could reply and let me know if it's convenient?" was compressed into a telegram like: "Visiting later in week. Hope OK. Let me know." Thus, the 27 words of the original message become 9 words in the telegram.

The message is still completely understandable, if a little more terse. We can compress the original message because a lot of the information is "redundant": some of the words are unnecessary and don't really add all that much, so we don't lose the sense of the message when we delete them. We could compress the message even further, but if we take more words, it'll soon stop making sense. In other words, the more we compress a piece of information, the more we reduce its quality. A little information has been lost, however: the telegram is less polite than the original message. And there's no way the receiver can take the 9-word message and figure out what were the other 18 words we deleted, so telegrams are an example of what we call lossy compression: the information we delete during compression is gone for good.

Image compression

If you have a digital camera, you probably know about compression already. Your camera most likely stores photos in a format called JPG (pronounced and sometimes written J-PEG). On most cameras, you can set options so the photos are taken with higher or lower resolution (which just means more or less detail). The higher the resolution, the greater the detail, and the better the photos look—but the more space they take up. Since your camera has a limited memory, you can opt to store lots of low-quality, low-resolution (low-res) images or fewer higher-quality, high-resolution (hi-res) images. The low-res images are compressed more than the high-res ones and the JPG files are correspondingly smaller. However, if you compress photos too much, you start to lose the details very quickly. Underneath, I've compressed a photo of an iPod at different resolutions to show you how the details are rapidly lost.

There's no way of taking one of the low-res photos on the right and going back to the hi-res version on the left: once the information is lost, it's gone for good. That means JPG is also a lossy compression. But note how much we can compress the original photo and still recognize what it is. Even with 95% compression, we can still make out that this is a photo of an iPod. With 50% compression, we hardly lose any detail at all.

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. Why go to a store to buy a CD when you can download the track you want from the Net in a couple of minutes? Well, actually, there is a reason...

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 few 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 ouput (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

Websites

• MP3 from Wikipedia: Quite a detailed technical introduction, but contains a good diagram of the structure of an MP3 file.
• MP3 from the Fraunhofer Institute: A brief history of MP3 from the German laboratory where it was invented.
• MP3 overview and issues: An easy-to-understand introduction by Tania Costanzo, plus a brief look at controversial issues like file-sharing.