How MP3 Files Work

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How MP3 Files Work
by Marshall Brain

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The MP3 movement is one of the most amazing phenomena that the music industry has ever seen. Unlike other movements -- for example, the introduction of the cassette tape or the CD -- the MP3 movement started not with the industry itself but with a huge audience of music lovers on the Internet. The MP3 format for digital music has had, and will continue to have, a huge impact on how people collect, listen to and distribute music.

If you have ever wondered how MP3 files work, or if you have heard about MP3 files and wondered how to use them yourself, then this edition of How Stuff Works will be fascinating. In this article you will learn about the MP3 file format and also learn how you can start downloading, listening to and saving MP3 files onto CDs!

The MP3 Format
If you have read the HSW article entitled How Compact Disks (CDs) Works, then you know something about how CDs store music digitally. A CD stores songs as digital information. The data on a CD uses an uncompressed, high-resolution format. Specifically here's what happens to create a CD:

Music is sampled 44,100 times per second

The samples are 2 bytes (16 bits) long

Separate samples are taken for both the left and right speakers in a stereo system

Therefore 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

1.4 million bits per second is 176,000 bytes per second. If an average song is 3 minutes long, then the average song on a CD consumes about 32 million bytes of space. 32 Mbytes is a lot of space for one song, and it is especially large when you consider the bandwidth most people have available for their Internet connections. Over a 56 kbit modem it would take something close to 2 hours to download one song.

The Name
MPEG is the acronym for Moving Picture Experts Group. This group has developed compression systems used for video data. For example, DVD movies, HDTV broadcasts and DSS satellite systems use MPEG compression to fit video and movie data into smaller spaces. The MPEG compression system includes a subsystem to compress sound, called MPEG audio Layer-3. We know it by its abbreviation - MP3.

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 losing the CD quality of the sound. With MP3, a 32 megabyte song on a CD compresses down to 3 megabytes or so. This lets you download a song in minutes rather than hours, and it lets you 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. The "perceptual" part in the name means that 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.

Using facts like these about the human ear, certain parts of a song can be eliminated without hurting the quality of the song for the listener. Compressing the rest of the song with well-known compression techniques shrinks the song considerably - a factor of 10 at least. [If you would like to learn more about the specific compression algorithms, see the links at the bottom of this article.]

From this discription you can see that MP3 is nothing magical. It is simply a file format that compresses a song into a smaller size so it is easier to move around on the Internet and store.

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

It has made it easy for anyone to distribute music at nearly no cost
It has made it easy for anyone to find music and access it instantly
It has taught people a great deal about manipulating sound on a computer

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:

download an MP3 file from a web site and play it
rip a song from a music CD and play it directly or encode it as an MP3 file
record a song yourself, convert it to an MP3 file and make it available to the world
convert MP3 files into CD files and create your own audio CDs from MP3 files on the web
rip songs off of various music CDs and recombine them into your own custom CDs
store hundreds of MP3 files on data CDs
load MP3 files into tiny portable players and listen to them wherever you go

To do all of these things all that you need is a computer with a sound card and speakers, an Internet connection, a CD-R drive to create CDs and an MP3 player. If you simply want to download MP3 files from the web and listen to them then all that you need is a computer with a sound card and speakers and an Internet connection - something you probably already have!

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

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

A computer
A sound card and speakers for the computer (if your computer has speakers it has a sound card)
An Internet connection (if you are browsing the web to read this article, then you have an Internet connection and it is working fine)
An MP3 player (a software application you can download from the web in 10 minutes)

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 are 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. Here's three of the biggest:

MP3.com
Scour - looks across a number of MP3 sites for songs and software
Riffage

Go to one of these sites, find a song and download it to your hard disk (most MP3 sites let you listen to the song as a streaming file or download the song -- you want to download the song). Most songs range between 2 and 4 megabytes, 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 are 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 so they are extremely inexpensive. One of the most popular is WinAmp, which you can download from www.winamp.com. For a complete list of all of the available players, this page from MP3.com offers a very nice selection. Once you download and install a player, double-click on the MP3 file that you downloaded and it will play.

You are 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 juke-box like playlists so that their computer can play them all day long!

Taking Your MP3 Files with You
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. Here are three typical examples:

These players plug into your computer's parallel port to transfer the data, and a software application lets you transfer your MP3 files into the player by simply dragging the files.

Creating Your Own MP3 Files
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 convert your favorite songs. 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.

AudioCatalyst is a popular ripper/encoder that you can use to do the job. This page from MP3.com contains an extensive list of other rippers and encoders.

Writing MP3s to CDs
If you have a writable CD drive in your computer, there are two ways to save your MP3 files on a CD:

You can write the MP3 files themselves onto a data CD in order to save them and clear some space on your hard disk. You can then listen to the files on any computer.
You can convert (decode) your MP3 files into full-size CD tracks and then save them to an audio CD. This allows you to listen to your favorite MP3 files on any CD player.

WinAmp has a plug-in that will create full-size WAV files from an MP3 file, 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.

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

Distributing Original Music
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 an extremely large audience. The first step is to create a song, either on a cassette tape, mini-disc or on a CD. If it is on a CD then you can use the ripper and encoder tools described above to create an MP3 file. If it is on a cassette (or minidisc), you can connect the output of your cassette (or minidisc) deck 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 2 distribution options:

You can go to a site like MP3.com and let them distribute your music. The advantage of this approach is that MP3.com gets millions of visitors every month, so the potential audience you can reach is very large. To sign up with MP3.com go to the artist area. You will need at least one MP3 file, a photograph of your band and/or album cover and some information about the band in order to sign up. The process is simple. MP3.com has other programs for artists as well.
You can create your own web site for your music or band and promote your website yourself. This gives you more control and individuality, but requires you to get the word out on your site. See How Web Pages Work for details on creating and hosting your own web site for your band.

One good option is to make your MP3 files available on MP3.com and then link to the MP3.com download area from your band's web site. This lets you get the best of both worlds and also lets you take advantage of MP3.com's servers for the large MP3 files.

Links
Popular MP3 sites

MP3.com
Scour - looks across a number of sites
Riffage

MP3 Software

MP3.com Software list

MP3 Format Details

MP3 FAQ - a very nice intro to MP3
Lots of MP3 Information
Layer-3 FAQ
MPEG audio and video FAQ
http://www.dolby.com/tech/highqual.html - nice technical description of how psychoacoustic models help compress sound.
A free audio compression format? - lists a number of patents having to do with MP3 audio compression, including:
CD-Recordable FAQ

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