Introduction

Audio is the first streaming media to gain public acceptance. The first applications of streaming media that have been widely used by consumers are the audio-only webcasts, or Internet radio . The other driver for interest in PC audio has been the universal acceptance of the MP3 format for low-data-rate audio files. Although not a streaming format (it is for download then play), most popular codecs can be used to wrap MP3 files for streaming. There are the audio-only codecs: Shoutcast, Winamp, Audion, and the multimedia architectures: Windows Media, QuickTime and Real.

To stream audio you have to go through three processes:

• Digitization

• Encoding (or compression)

• Packetization

The professional digital audio format, AES-3, is often the basis for digitization. The AES standard has a data rate of over 3 Mbit/s for stereo audio sampled at 48 kHz. To transmit this over a 28 kbit/s dial-up circuit, the data would have to be reduced by a factor of 100:1, and that excludes the overheads of packetization and error resilience.

The compression schemes split between the speech or voice coding developed by the telcos and the military, and the general audio or waveform encoders developed for multimedia applications. The speech coding algorithms generally use a model of the vocal tract and transmit voicing parameters for reconstruction by the decoder into intelligible speech. Much of the Internet radio delivers music over a 28 or 56 kbit/s voice circuit, but there is a big difference in fidelity between voice telephony and an acceptable system...