Digital audio applications to short wave broadcasting

Digital audio is becoming prevalent in different broadcasting media. In the US, developments include in band digital systems for the FM and AM bands, while in Europe and elsewhere, the Eureka 147 system is undergoing large scale trials. The minimization of data rate is very important in broadcasting because of bandwidth and power constraints. Audio compression is therefore used to reduce the data rate required to transmit and reproduce the audio at the receiver. The data rate that is needed depends on the audio bandwidth that is to be maintained, whether it is monaural or stereo, and the distortion level that can be tolerated by the audience. A great deal of effort has been expended to bring down the data rate needed to reproduce CD quality audio, on the high end of the audio compression performance scale, and on speech compression for telephony, on the low end of the scale. Less work has been done in the medium bandwidth speech and music area, which is most appropriate for short wave broadcasting. The bandwidth of a short wave channel is 10 kHz. The narrow bandwidth and the very difficult propagation characteristics of the HF bands severely limit the data rate that can be transmitted. It will take a very efficient compression scheme to achieve the desired voice and music quality. Attention must be paid to the interaction of the receiver and audio decoder in minimizing undesirable audio artifacts when the received signal approaches threshold. This paper describes the unique requirements of audio compression systems suitable for HF broadcasting from the perspective of the overall short wave broadcasting system design