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Streaming audio: A technology that lets you listen to music and other types of sound files in real time over the Internet without lengthy downloads.

Anyone who uses the Web is used to waiting for files to download. But you can skirt the download delay by tuning in to streaming audio content--audio files that begin playing seconds after you click a link. Hear radio stations located across town or across the globe. Missed National Public Radio's Morning Edition? Listen to it after dinner with on-demand programming. Or audition tracks from music CDs before you buy from Web sites such as CDNow.com. Since 1995, when RealPlayer became the first widely available streaming player, real-time sound has been only a click away.

• Free or inexpensive streaming players deliver good sound quality, even over modem connections.
  
  
• Content is proliferating, with thousands of sites providing live and on-demand content.
  
  
• Competing technologies mean you'll need to install multiple players to hear the full range of offerings.
  
  
• Hardware add-ons and forthcoming Internet radio appliances let you listen away from your PC.
  
  

Streaming audio's appeal is its immediacy. With traditional downloads such as MP3 files, you have to save the entire file to your hard drive before listening to it. Streaming audio begins playing seconds after you click a link.

The files reside on special Web servers. When you click a link to begin playback, your streaming player downloads several seconds' worth of audio into an area of memory called a buffer. When the buffer is full, it spoons data into the player portion of the streaming software and you hear sound. Meanwhile, the software continues to download data into the buffer. This download-while-playing process allows streaming to offer near-immediate gratification.

Most of the time, anyway. Internet congestion or other connection problems, such as static on the line, can interrupt the incoming stream. As a result, the buffer may empty completely and stop the playback until it refills.

Slimming the Signal

The key to streaming audio technology is file compression. Most music files are far too large to squeeze through the narrow pipelines modems use to connect most of us to the Internet. Compression removes portions of the original audio signal that most of us can't perceive anyway--a process called perceptual encoding. Very high and very low frequencies are discarded, for example.

This creates a file that is small enough to download via a modem, but still sounds much like the original. Sharp-eared music lovers will hear a difference: Heavily compressed audio lacks brilliance and can have blips in the sound, similar to the "swirly" sound of a shortwave radio. But the latest compression schemes--known as codecs--do a far better job than first-generation streaming technologies, such as Real Audio 1.0.

Compression is only part of what makes streaming possible; special data-transfer schemes, also called streaming protocols (such as RealTime Streaming Protocol), do the rest. With streaming audio, timing is everything. Streaming protocols make sure you get the notes of the song in the right order and at exactly the right time.