Radio Data System, or RDS, is a communications protocol standard for embedding small amounts of digital information in conventional FM radio broadcasts. The RDS system standardises several types of information transmitted, including time, station identification and programme information.
Radio Broadcast Data System (RBDS) is the official name used for the U.S. version of RDS. The two standards are nearly identical, with only slight differences.
The standard began as a project of the European Broadcasting Union (EBU), but has since become an international standard of the International Electrotechnical Commission (IEC).
Both use a 57 kHz subcarrier to carry data at 1,187.5 bits per second. The 57 kHz frequency was chosen for being the third harmonic of the pilot tone for FM stereo, so it would not cause interference or intermodulation with it, or with the stereo difference signal at 38 kHz (the second harmonic). The data format utilizes error correction. RDS defines many features including how private (in-house) or other undefined features can be "packaged" in unused program groups. -Dan