For digital audio projects, servers provide two basic functions: storing audio files and delivering them to users. There are several ways a server may deliver digital audio to a client. The two most common are (1) using a web server to make a digital audio file available for downloading and (2) using a streaming server to deliver a stream of digital audio data to a digital audio player.
The first option is the easiest, since web servers are ubiquitous, and the question becomes simply one of storage space. When a user keys the appropriate URL into a browser or clicks a link to the URL on a web page, the web server delivers the file to the browser, and once the file is transferred completely, it can either be played back or stored on the local computer. Some audio formats--such as Microsoft's Advanced Streaming Format (ASF)--provide ``progressive playback,'' which allows playback of a file to begin before it is fully downloaded.5.8
Delivering audio as a stream requires a streaming server, which is simply a server running streaming software. If your institution already maintains a streaming server, you might investigate the possibility of adding your digital audio service to the existing server. In fact, most digital audio projects end up sharing a streaming server with other streaming media projects. Seventy-nine percent of the respondents to our survey reported that their service shares a server with other applications.
As mentioned earlier, the choice of server operating system can dictate the choice of streaming server software, which will in turn dictate the type of audio files you can deliver through your service. According to our survey, the two most popular streaming servers are RealNetwork's Helix Server (along with its predecessor, RealServer) and Apple's QuickTime Streaming Server. The Helix Server software is available for the Windows 2003 Server, Linux, and Solaris operating systems, and the server can deliver files in RealAudio, Windows Media, Quicktime, MP3, and AAC formats. The QuickTime Streaming Server software runs on Apple's OS X and can stream audio contained in QuickTime (.mov) files as well as AAC (.mp4) files.
Some smaller libraries have even set up a digital audio service by ripping CD tracks in iTunes and making the resulting library available on the library's network.