The 19.39 Mbps that each DTV station is licensed to transmit can divide this bit-rate up in nearly any manner they please. Depending on codec efficiency and quality, several standard-definition video programs, one or possibly more high-definition programs, or a mixture of HD and SD programs can be carried to viewers. While the video services are heavy bit-rate users, the audio side of these programs requires very little. A stereo program of excellent quality can be carried at 192 kbps, and a very good quality 5.1 channel program can be carried at 384 kbps. Future formats like MPEG Surround could also make better use of the space in a compatible manner.
Even before the robust modes of the ATSC system become widely available, dramatic improvements in terrestrial receivers make the idea of an audio-only service something to think about. Imagine being able to deliver 5.1 channels of full bandwidth, full dynamic range audio to listeners while spending less than 2 percent of the total DTV station data rate on the service. Sending a stereo version would require less than 1 percent per program. In either case, no special equipment is required for reception or reproduction of the audio, it all works with current gear. As enhanced VSB and potentially mobile ATSC become more of a reality, this could provide many useful services for viewers (and listeners), and could provide some interesting business opportunities for television and radio stations that are worth thinking about.
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