Let us now move from the technology of digital audio to three of its network applications. Our first one is streaming audio, that is, listening to sound over the Internet. This is also called music on demand. In the next two, we will look at Internet radio and voice over IP, respectively.
The Internet is full of music Web sites, many of which list song titles that users can click on to play the songs. Some of these are free sites (e.g., new bands looking for publicity); others require payment in return for music, although these often offer some free samples as well (e.g., the first 15 seconds of a song).The process starts when the user clicks on a song. Then the browser goes into action. Step 1 is for it to establish a TCP connection to the Web server to which the song is hyperlinked. Step 2 is to send over a GET request in HTTP to request the song. Next (steps 3 and 4), the server fetches the song (which is just a file in MP3 or some other format) from the disk and sends it back to the browser. If the file is larger than the server’s memory, it may fetch and send the music a block at a time.
Using the MIME type, for example, audio/mp3, (or the file extension), the browser looks up how it is supposed to display the file. Normally, there will be a helper application such as RealOne Player, Windows Media Player, or Winamp, associated with this type of file. Since the usual way for the browser to communicate with a helper is to write the content to a scratch file, it will save the entire music file as a scratch file on the disk (step 5) first. Then it will start the media player and pass it the name of the scratch file. In step 6, the media player starts fetching and playing the music, block by block. In principle, this approach is completely correct and will play the music. The only trouble is that the entire song must be transmitted over the network before
the music starts. If the song is 4 MB (a typical size for an MP3 song) and the modem is 56 kbps, the user will be greeted by almost 10 minutes of silence while the song is being downloaded. Not all music lovers like this idea. Especially since the next song will also start with 10 minutes of download time, and the one after that as well.
