Monitoring radio broadcasts is a growth industry. Advertisers want to make sure that the adverts they've paid for are being transmitted. The Innes Corporation, based in New South Wales, claims it has made a major improvement in automatic tracking of broadcasts (WO 99/63688). The company's solution is to create a "notch" in the sound frequency spectrum which is rapidly switched on and off to create a digitial signal that conveys an identifying code. The result is that automatic receivers can pick up the code, but because the sound peak is the same it does not offend listeners' ears.