Radio Data System


Radio Data System is a communications protocol standard for embedding small amounts of digital information in conventional FM radio broadcasts. RDS standardizes several types of information transmitted, including time, station identification and program information.
The standard began as a project of the European Broadcasting Union, but has since become an international standard of the International Electrotechnical Commission.
Radio Broadcast Data System is the official name used for the U.S. version of RDS. The two standards are only slightly different.
Both carry data at 1,187.5 bits per second on a 57 kHz subcarrier, so there are exactly 48 cycles of subcarrier during every data bit. The RBDS/RDS subcarrier was set to the third harmonic of the 19 kHz FM stereo pilot tone to minimize interference and intermodulation between the data signal, the stereo pilot and the 38 kHz DSB-SC stereo difference signal.
The data can be sent with error correction, but using error correction is not mandatory. RDS defines many features including how private or other undefined features can be "packaged" in unused program groups.

Development

RDS was inspired by the development of the Autofahrer-Rundfunk-Informationssystem in Germany by the Institut für Rundfunktechnik and the radio manufacturer Blaupunkt. ARI used a 57-kHz subcarrier to indicate the presence of traffic information in an FM radio broadcast.
The EBU Technical Committee launched a project at its 1974 Paris meeting to develop a technology with similar purposes to ARI, but which was more flexible and which would enable automated retuning of a receiver where a broadcast network transmitted the same radio programme on a number of different frequencies. The modulation system was based on that used in a Swedish paging system and the baseband coding was a new design, mainly developed by the British Broadcasting Corporation and the IRT. The EBU issued the first RDS specification in 1984.
Enhancements to the alternative frequencies functionality were added to the standard and it was subsequently published as a European Committee for Electrotechnical Standardization standard in 1990.
In 1992 the U.S. National Radio Systems Committee issued the North American version of the RDS standard, called the Radio Broadcast Data System. The CENELEC standard was updated in 1992 with the addition of Traffic Message Channel and in 1998 with Open Data Applications and, in 2000, RDS was published worldwide as IEC standard 62106.

RDS2

The RDS-Forum decided at its annual meeting in Glion/Montreux to bring the new standard RDS2 on the way. The standard will be created in close collaboration with U.S. colleagues from NRSC RBDS-Subcommittee and should offer a unified platform for FM broadcasting and data services worldwide.
;Key features:
The following information fields are normally contained in the RDS data:
;AF : This provides the receiver with a list of frequencies that allows a receiver to re-tune to a different frequency providing the same station when the first signal becomes too weak. Before performing the switch, a radio will check for a matching PI code to ensure the AF is the same station. This is often used in car stereo systems, allowing the head unit to automatically tune into the stronger signal on the move, optionally with the same regional code.
;CT : Can synchronize a clock in the receiver or the main clock in a car. Due to transmission vagaries, CT can only be accurate to within 100 ms of UTC. CT is not usually transmitted if a broadcaster has no way to regularly synchronise the clock within the RDS encoder.
;EON : Informs the receiver about other networks or stations, linked to the one being listened to, for dynamically changing data such as the TA flag turning on for a particular station of the network in a particular moment due to a traffic programme being broadcast, and automatically and temporarily allows the radio to tune into that station.
;PI : This is the unique 4 character hexadecimal code that identifies the station. Every station in a country should use a unique 3 character code with the correct country prefix character. In the US, PI is determined by applying a formula to the station's call sign. The US NRSC is seeking to add an ancillary traffic type to lessen confusion. The PI code is the most important RDS parameter and the most frequently transmitted within the RDS data structure. The RDS standard for non US use defines country codes for all countries so that no where with common borders has the same code. This removes the need to coordinate codes between different countries. Any transmission that carries the same code is considered by receivers to be the same and can be switched to as an alternative frequency to improve reception.
;PS : 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 PI code, frequency and other details associated with that preset. In some countries, stations use the PS to dynamically send other information. This is prohibited in some countries and was not its intended use within the RDS system.
;PTY : This coding of up to 31 pre-defined programme types allows users to find similar programming by genre. PTY31 is reserved for emergency announcements in the event of natural disasters or other major calamities.
;REG : This is mainly used in countries where national broadcasters run "region-specific" programming such as regional opt-outs on some of their transmitters. This functionality allows the user to "lock-down" the set to their current region or let the radio tune into other region-specific programming as they move into the other region.
;RT : This function allows a radio station to transmit a 64 character free-form text message that can be either static or in sync with the programming.
;RT+ : An enhancement of the original RT which allows Artist, Title and some other metadata to be sent to receivers.
;TA, TP : The receiver can often be set to pay, taking advantage of the EON linkage if it's available, special attention to this flag and, for example, pause a 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 signal an actual traffic bulletin in progress, with radio units perhaps performing other actions such as pausing a CD/MP3 or raising the volume during the traffic bulletin.
;TMC : Digitally encoded traffic information. Not all RDS equipment supports this, but it is often available for automotive navigation systems. In many countries only encrypted traffic data is broadcast, and so an appropriate decoder, possibly tied to a subscription service, is required to use the traffic data. The subscription is often paid by the vehicle manufacturer and is therefore transparent to the user.
;US NRSC FM Translator Announcements: The National Radio Systems Committee has introduced a unique Radio Data System Program Identification code for US FM translators. One type of metadata transmitted by RDS subcarrier is the PI code, which is used by the receiver to uniquely identify the audio program being broadcast by the FM station. In the U.S., the PI code has historically been derived from a radio station’s call sign, which can become complicated when used in conjunction with FM translators. A new algorithm just for FM translators has been created that assigns a unique PI code to each FM translator. This algorithm has been implemented using a web-based tool and a list of all known PI codes for all FM translators in the US.

RDS support

As far as implementation is concerned, most car stereos will support at least AF, EON, REG, PS and TA/TP.
There are a growing number of RDS implementations in portable audio and navigation devices thanks to lower-priced, small-footprint solutions.

RDS compatibility

The RDS sub-carrier at 57 kHz occupies ±2 kHz of the composite spectrum which in theory keeps it above the upper cutoff of the stereo subcarrier at 53 kHz. However the 53 kHz cutoff is entirely dependent on the performance of the 15 kHz low pass filters used before the stereo encoder. In older equipment, these filters were only designed to protect the 19 kHz pilot and sometimes did not provide sufficient protection to the RDS subcarrier when a significant amount of stereo information was present. In this situation, stereo enhancement devices combined with aggressive audio processing could render the RDS subcarrier unreceivable.
Composite clipping systems may also degrade the RDS sub-carrier because of the harmonics created by the clipping. More modern composite clippers include filtering to protect the RDS subcarrier.
The RDS subcarrier typically uses 2–4 kHz of carrier deviation. Therefore, the deviation available for the program material is reduced by this amount, assuming the usual 75 kHz deviation limit is not exceeded.

Program types

The following table lists the RDS and RBDS program type codes and their meanings:
The later RBDS standard implemented in the U.S. made no attempt to match the original RDS plan, therefore several identical radio formats were given different numbers, including jazz, weather, sports, and rock. Other similar formats such as varied/college and phone-in/talk are also mismatched. This is mainly a problem for Americans taking portable radios out of their country.

RDS Technical Specification

The RDS standard as specified in EN 50067:1998 is separated into these sections according to the OSI model.
  1. Data Channel
  2. Baseband Coding
  3. Message Format

    Data Channel (Physical Layer)

The physical layer in the standard describes how the bitstream is retrieved from the radio signal. The RDS hardware first demodulates the 57 kHz RDS subcarrier signal to extract a Biphase encoded signal which contains both the bitrate clock and the differentially encoded bitstream. This allows for retrieving the RDS bitstream via a differential decoder, which requires a synchronised clock and a differentially encoded bit-stream.

Baseband Coding (Data-Link Layer)

The data-link layer describes the baseband coding of which the largest element in the structure is called a "group" sized 104 bits wide. Within each group are 4 blocks sized 26bits wide. Each block contains a 16bit data word and a 10 bit checkword. In the case of the FM Tuner RDA5807M IC, it displays a group in separate 16bits blocks over four i2c registers. All group is sent most significant byte first, with no gap between groups or blocks.

Synchronisation of the baseband coding structure from the RDS bitstream

The identification of a RDS message blocks and groups is done via a 10-bit offset table containing offset word: A, B, C, C', and D.

Message Format (Session And Presentation Layer)

A RDS message group consist of two versions of the group structure, designated in the standard as type A, and type B.

Shared Structure

Within Block 1 and Block 2 are structures that will always be present in both group versions, for fast and responsive identifications. First block of every group, will always be the program identification code. The second block dedicates the first 4 bits for Application/Group Type.
Meaning of Block 2 Bits
Message Version B
Block 3 is used for repeating program identification code.

Program Identification Code (PI Code)

This allows for quick identification of radio program type, based on country, coverage area, and program reference number. While the country code is specified by the standard, bit 11 to bit 0 is specified by each country local authorities.

Group Type

This is a short list of the full group type. Each group type may have a secondary version available

RDS Message Examples

These are non comprehensive examples that covers just the simple messages likes station name, radio text, and date time.

Group Type 0 - Version B - Station Name

As we have already described previous fields above, these dot points below show just the application specific fields.
The station name and decoder identification code is sent progressively over 4 groups, where the offset is defined by bit C1 and C0.

Group Type 2 - Radio Text

As we have already described previous fields above, these dot points below show just the application specific fields.
The station name and decoder identification code is sent progressively over 4 groups, where the offset is defined by bit C1 and C0.

Group Type 4 - Version A - Clock Time and Data

When group type 4A is used, it shall be transmitted every minute according to EN 50067.
The clock time group is inserted so that the minute edge will occur within +/- 0.1 seconds of the end of the clock time group.
Time and date are packed as these:
Note: The local time offset is expressed in multiples of half hours within the range -12h to +12h
The following three images illustrate how RDS can be used on an FM radio station; the latter two were taken when the radio was tuned to Nottingham radio station Trent FM. All the images are of the display on the Sony XDR-S1 DAB/FM/MW/LW portable radio.
's Save a Prayer - the bottom line scrolls to reveal the rest of the text.

RDS chipsets

Companies such as ST Microelectronics, Silicon Labs in Austin, Texas and NXP Semiconductors offer single-chip solutions that are found in these devices.