The delivery of real-time media is an important factor in the evolution of the Internet. The majority of the Internet users still have access to the Net with a bit rate of 33.6 kbit/s. Even this low bandwidth is often not reached. Therefore an Internet audio streaming application can perform much better when the audio codec quality can be scaled dynamically depending on the load within the Internet. In the MPEG-4 working group the audio activities concentrate on a standard for very low bit rate audio coding tools suitable for multimedia and communication applications. An important aspect of the codecs is quality based scalability. The parametric Harmonic and Individual Line plus Noise codec (HILN) provides an effective method to scale the quality. The transmission of real-time media in the Internet requires specialized protocols. Most multimedia applications are based on the Real-Time Protocol (RTP). RTP extends the transmission with time stamps and control mechanisms for synchronizing different streams. The dynamic scalable audio streaming on the basis of the MPEG-4 codec HILN is realized with a simplified real-time protocol. The audio data is transmitted in frames and planes. The planes represent the audio signal with increasing quality. The transmission of the planes can be dynamically switched on or off. In this way the quality can be scaled in the range of 6 kbit/s to 24 kbit/s. The advantage over existing Internet audio codecs is that the bit rate can be easily adapted to the current load of the net. The probability of gaps in the audio stream is reduced. The audio quality of the HILN performs slightly better than the other low bit rate codecs for the greater number of excerpts. A typical application for audio streaming is a plug in for a WWW browser.
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