How MP3 Files Work

If you've 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:

So a CD stores a huge number of bits for each second of music:

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 (or 32 megabytes) of space. Even with a high-speed cable or DSL modem, it can take several minutes to download just one song. Over a 56K dial-up modem, it would take close to two hours.

The MP3 format is a compression system for music. The goal of using MP3 is to compress a CD-quality song by a factor of 10 to 14 without noticeably affecting the CD-quality sound. With MP3, a 32-megabyte song on a CD compresses down to about 3 MB. This lets you download a song much more quickly, and store hundreds of songs on your computer's hard disk.

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're 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's "perceptual" partly because the MP3 format uses characteristics of the human ear to design the compression algorithm. For example:

Using facts like these, certain parts of a song can be eliminated without significantly hurting the quality of the song for the listener. Compressing the rest of the song with well-known compression techniques shrinks the song considerably -- by a factor of 10 at least. When you're done creating an MP3 file, what you have is a "near-CD-quality" song. The MP3 version of the song does not sound exactly the same as the original CD song because some of it has been removed.

Not all MP3 files are equal. Let's take a look at the different ends of the MP3 spectrum in the next section.

The MP3 compression format creates files that don't sound exactly like the original recording -- it's a lossy format. In order to decrease the size of the file significantly, MP3 encoders have to lose audio information. Lossless compression formats don't sacrifice any audio information. But that also means that lossless compression files are larger than their lossy counterparts.

You can choose how much information an MP3 file will retain or lose during the encoding and compression process. It's possible to create two different MP3 files with different sound quality and file sizes from the same source of data. The key is the bit rate -- the number of bits per second encoded in the MP3 file.

Most MP3 encoding software allows the user to select the bit rate when converting files into the MP3 format. The lower the bit rate, the more information the encoder will discard when compressing the file. Bit rates range from 96 to 320 kilobits per second (Kbps). Using a bit rate of 128 Kbps usually results in a sound quality equivalent to what you'd hear on the radio. Many music sites and blogs urge people to use a bit rate of 160 Kbps or higher if they want the MP3 file to have the same sound quality as a CD.

Some audiophiles -- people who seek out the best ways to experience music -- look down on the MP3 format. They argue that even at the highest bit rate settings, MP3 files are inferior to CDs and vinyl records. But other people argue that it's impossible for the human ear to detect the difference between an uncompressed CD file and an MP3 encoded with a 320 Kbps bit rate.

There's no denying that, quality issues aside, the MP3 format is changing music. With music services like Amazon and eMusic, customers can buy music by the song. In a way, the music industry is returning to its roots -- the music single is becoming popular after nearly dying out during the CD era.

In addition, some musicians and audio engineers say that the MP3 format is changing the way music studios mix recordings. They say that the MP3 format "flattens" out the dynamics -- the differences in pitch and volume -- in a song. As a result, much of the new music coming out of the industry has a similar sound, and there's not as much of a focus on creating a dynamic listening experience. Why work that hard on creating a complex sound if no one can detect it [source: Levine]?

From this description, you can see that MP3 is nothing magical. It's simply a file format that compresses a song into a smaller size so it is easier to move around and store on your home computer -- or your portable music player.

Knowing about the MP3 format isn't half as interesting as using it. The MP3 movement -- consisting of the MP3 format and the Web's ability to advertise and distribute MP3 files -- has done several things for music:

That third one was accidental but important. A big part of the MP3 movement is the fact that it has brought an incredible array of powerful tools to desktop computers and given people a reason to learn how they work. Because of these tools, it is now extremely easy for you to:

Let's look at of the many different things you can do with MP3 files and the software that makes it possible.

If you would like to download and listen to MP3 files on your computer, then you need:

If you have recently purchased a new computer, chances are it already has software that can play MP3 files installed on its hard disk. The easiest way to find out if you already have an MP3 player installed is to download an MP3 file and try to double-click on it. If it plays, you're set. If not, you need to download a player, which is very easy to do.

There are literally thousands of sites on the Web where you can download MP3 files. Go to one of these sites, find a song and download it to your hard disk. Most MP3 sites let you either listen to the song as a streaming file or download it -- you'll probably want to download it, if you want to save a copy for later. Most songs range between 2 and 4 MB, so it will take 10 to 15 minutes unless you have a high-speed Internet connection. Once the song has finished downloading, try to double-click on the file and see what happens. If your computer plays it, then you're set.

If you find that you cannot play it, then you need to download an MP3 player. There are dozens of players available, and most of them are free or shareware -- shareware is extremely inexpensive.

You're now ready to begin collecting MP3 files and saving them on your computer. Many people have hundreds of songs they have collected, and they create jukebox-like playlists so that their computer can play them all day long!

Many people who start collecting MP3 files find that they want to listen to them in all kinds of places. Small, portable MP3 players answer this need. These players are like portable cassette players except that they are smaller.

These players plug into your computer's FireWire or USB port to transfer the data. A software application lets you transfer your MP3s into the player by simply dragging the files. See How MP3 Players Work for details.

If you have a CD collection and would like to convert songs from your CDs into MP3 files, you can use ripper and encoder software to do just that. A ripper copies the song's file from the CD onto your hard disk. The encoder compresses the song into the MP3 format. By encoding songs, you can play them on your computer or take them with you on your MP3 player.

If you have a writable CD drive in your computer, there are two ways to save your MP3 files on a CD:

Many MP3 encoders have plug-ins that create full-size WAV files from MP3 files, and some of the encoders will also decode. Once you have the full-size CD tracks, then the software that comes with your CD-R drive will let you create an audio CD easily. Other MP3 encoders and players have similar features. It's good to do a little research before you choose your MP3 application -- some are more reliable than others.

The CD-Recordable FAQ is an excellent source of information on getting data and music onto a CD.

If you are an artist who is recording music at home or in a small studio, you can use MP3 files and the Web to distribute your music to a larger audience. The first step is to create a song, either on a cassette tape, minidisc or CD. If it's on a CD, you can use the ripper and encoder tools described in the previous section to create an MP3 file. If it's on a cassette or other source, you can connect the output of the audio source to the line-in or microphone jack of your sound card and record the music digitally on your computer. Then you can encode that file to create the MP3.

Once you have an MP3 file in hand, you have two distribution options:

Some musicians distribute their music through a blog. Jonathan Coulton, known for his comedic folk songs, uses a blog to keep his fans informed of what's going on in his life. They can also find links to purchase his music or listen to several of his songs for free. Coulton's success with this untraditional approach may inspire other musicians to follow suit.

One good option is to make your MP3 files available on a large Web site and then link to the download area from your band's Web site. This lets you get the best of both worlds, and you can take advantage of the larger site's servers for those big MP3 files.

For more information on the MP3 format, MP3 sites and related topics, check out the links on the next page.