A crucial step in planning a streaming audio project is selecting the format and bitrate for the compressed audio files that will be streamed to users. The quality of the audio produced for your project will be affected by your choices, since some formats produce higher-quality sound than others at identical bitrates. The bitrate will affect the performance of your service, since streaming audio at higher bitrates require greater bandwidth and faster internet connections.
When selecting a format for your streaming digital audio project, the factors that are likely to have the greatest impact on your decision are the ones that, in the end, will probably be invisible to you: the streaming server and the staff in charge of maintaining it. If your project is to be hosted on an existing media server maintained by personnel outside your departmental library, you will probably be expected to work within the limitations of that server, in which case certain decisions will already have been made for you.
A factor of less concern in selecting a streaming audio format is sound quality; all compressed formats, at a sufficient bitrate, will deliver acceptable audio to users. Keep in mind, though, that newer formats--such as AAC and Ogg Vorbis--produce audio of higher quality than older formats--MP3, for example--at an identical bitrate.
The choice of bitrate for the audio files will determine not only the quality of sound but the minimal connection speed for your service. Most users now access the internet using fast broadband connections--ethernet, cable, or DSL--and these can easily handle streams of 128 kbps. If you know that a significant number of your users connect to the internet using slower modem connections, you might consider encoding at two different rates (perhaps 48 kbps and 128 kbps) or use a format (such as RealAudio's Surestream) that can accommodate streams at multiple rates. It is clear, however, that the days of slow, dial-up modems are numbered, so it is better to err on the side of higher bitrates if you want to extend the usability of your compressed files.
Another factor to keep in mind when selecting a streaming audio format is your users' preferences for operating systems. Media players exist for most streaming formats in versions for Windows, MacOS, and Linux, so most streaming services will be compatible with all three operating systems. RealAudio and the MPEG-based formats, because they have a long history and are well established, are safe choices if you want to be sure that your service is accessible on all platforms. On the other hand, if your library uses Macs for public workstations and most of your users own Macs, then selecting format tailored for Windows (such as Windows Media Audio) would be a poor choice. If a strong preference for a specific operating system exists at your institution, then it will probably influence the choice of the streaming server, and the capabilities of the server will, in turn, influence your choice of a streaming audio format.
In the end, a successful streaming audio service can be based on any common compression format. Your decision should be made in reverse. Start by talking with the staff who will manage the streaming server that will host the service. They will tell you which formats are compatible with the server. Once you have a short list of possible formats, try playing audio streams in each format off the web.7.2 If you want to support multiple players or operating systems, be sure to try the streams using every possible combination and take note of whether software or plugins need to be installed to playback the stream. This should give you some idea of what will be required of your users to configure their systems to play back streams in each format. Finally, consider the quality of the audio.
The survey revealed that most libraries use one of two formats for their streaming audio: MP3 (47 percent of the respondents) and RealAudio (39 percent of the respondents). Use of Quicktime (presumably AAC files in a QuickTime wrapper) was reported by 11 percent of the respondents. Also, some of the libraries that use the MP3 format specified that the files are streamed in a QuickTime wrapper. One library (3 percent) bases its audio service on Windows Media Audio.