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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.