[1] Developed jointly by IRT and Blaupunkt,[2] it indicated the presence of traffic announcements through manipulation of the 57kHz subcarrier of the station's FM signal.
[4] The SK signal is actually the 57 kHz subcarrier that is transmitted by the ARI-compliant FM station for this functionality.
In the Radio Data System environment, the TP signal is equivalent to this basic function.
Radios that used the "classic" mechanical push-button preset system would have one of these buttons set aside as the VF switch.
Blaupunkt even made attempts to roll it out into the US market since 1982 by gaining support from selected FM broadcasters in the big US cities, but it did not catch on.