area identification
In theARI information(Autofahrer-Rundfunk-Information), a regional identifier, the area identifier (BK), is broadcast in addition to the station identifier( SK).
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For the area identifier, the subcarrier, the 57 kHz signal, is amplitude modulated with one of six low frequencies between 23 Hz and 54 Hz. With this identifier, traffic messages can be assigned to a specific regional traffic area.
The BK frequencies are derived from the 19 kHz frequency of the pilot tone via frequency multiplication.