RDS - Radio Data System

Radio Data System, or RDS, is a standard to add a small amount of data to a normal FM radio transmission. RDS can be used to distribute all kinds of information. This information can be related to the transmitted radio program, such as the name of the station or the name of the track or the artist, but it can be used for other purposes as well. RDS is in use in Europe and Latin America since the early 1990s.

The RDS data is transmitted on a sub-carrier together with the audio. RDS uses a sub-carrier of 57 kHz. This is the 3^rd harmonic of the pilot tone for FM stereo. Stereo information is centered around the second harmonic of the pilot tone. RDS can carry 1187.5 bits/s. An error correction code is used in the transmission of the data. This brings the netto data rate down to 730 bit/s, ca. 100 characters per second.

The RDS standard specifies a number of different types of information, such as:

• AF
  Alternative Frequencies. This gives information about other frequencies on which the same station broadcasts. This allows a receiver to re-tune to a different frequency of the same station when the first signal becomes too weak (e.g. when moving out of range). This is often utilised in car stereo systems.
• CT
  Clock Time. The clock time can be used to synchronise a clock in the receiver or the main clock in a car. Due to transmission fluctuations, the time can only be accurate to within 0.1 second.
• EON
  Enhanced Other Networks. Allows the receiver to monitor other stations for traffic broadcasts.
• PI
  Programme Identification. This is the unique code that identifies the station. Every station receives a specific code with a country prefix. In the US, PI is determined by applying a formula to the station's call sign.
• PS
  Programme Service. This is simply an eight-character static display that represents the call letters or station identity name. Most RDS capable receivers display this information and, if the station is stored in the receiver's presets, will cache this information with the frequency and other details associated with that preset.
• PTY
  Programme Type. This coding of up to 31 pre-defined programme types – e.g. (in Europe): PTY1 News, PTY6 Drama, PTY11 Rock music, – allows users to find similar programming by genre. PTY31 seems to be reserved for emergency announcements in the event of natural disasters or other major calamities.
• RT
  Radio Text. This function allows a radio station to transmit a 64-character free-form textual information that can be either static e.g. station slogans or in sync with the programming such as the title and artist of the currently-playing song.
• TA, TP
  Travel Announcements, Traffic Programme. The receiver can often be set to pay special attention to this flag and e.g stop the CD or retune to receive a Traffic bulletin. The TP flag is used to allow the user to find only those stations that regularly broadcast traffic bulletins whereas the TA flag is used to stop the CD or raise the volume during a traffic bulletin.
• TMC
  Traffic Message Channel. A specific channel for traffic information. Requires a RDS-TMC decoder. This channel is also used by e.g. car navigation systems.

RDS is standardized by the European Broadcasting Union. The official name for the U.S. version of RDS is called Radio Broadcast Data System. The two standards are nearly identical. The main difference is in the classification of the program type that is transmitted, such as news, sports, drama, pop music and jazz music.