MP3, a computer file format used for speeding downloads of online music, has found widespread popularity among music fans and has challenged the music industry's traditional business model. Many Internet sites distribute songs, popular and obscure, that can be downloaded for free or for a charge. Some of the sites distribute bootleg copies of music; the recording industry has an ongoing effort to prosecute the people running these illegal sites.

MP3 technology takes music and converts it to a digital format that is more easily managed by computers. When one listens closely to an MP3 song on a good stereo, some tones might sound tinny and other sections a little unnatural. But on small computer speakers these defects might not be as noticeable. Also, more sophisticated MP3 conversion techniques can improve the sound. In any event, for many fans the convenience of the smaller file size compensates for the loss of quality.

-- Robert Thomason

* To make an MP3 file from a CD album, a music fan would put the album in the CD drive of his computer. He would then instruct a software program to extract the song and copy it to the computer's hard drive. The program encodes a new file that is considerably smaller and usually bears the .mp3 file extension. These programs are called `ripper/encoders.`

* A three-minute song, when recorded digitally, can require 40 megabytes of computer memory, the equivalent of about 36 floppy disks. If it is converted to an MP3 file, the song would take up only 4 megabytes, or about four floppies. MP3 software eliminates the frequencies that humans do not hear well. Some frequencies in songs are played louder than others and they drown out, or mask, the quieter frequencies. The MP3 algorithm analyzes the song it is converting and keeps only the frequencies that constitute the sounds humans hear best.

* Once the MP3file is encoded, it is stored on any computer memory medium large enough to store it. (Floppies won't work because you can't split up the file among numerous floppies.) Using a computer with a sound card and a software player -- Winamp, Musicmatch, RealJukebox or Windows Media Player -- the music fan can play the song back. In computer lingo, the compressed file has been decoded back into analog soundwaves recognizable to the human ear. The file can be transferred from the computer to a portable listening device such as the Rio, the Nomad, the raveMP or the Lyra. These types of players are priced in the $100 to $250 range. The industry estimates that from 600,000 to 800,000 were sold last year, with some marketing planners estimating that as many as 2 million may be sold in 2000.