The Radio Data System (RDS) is intended for application to VHF/FM sound broadcasts in the range 87.5 MHz to 108.0 MHz which may carry either stereophonic (pilot-tone system) or monophonic programs. The main objectives of RDS are to enable improved functionality for FM receivers and to make them more user-friendly by using features such as Program Identification, Program Service name display and where applicable, automatic tuning for portable and car radios, in particular.

The Radio Data System has spawned in Europe around 1980. At present it starts to be used in all the world. It brings some useful possibilities. The listener's receiver can display station name, stations phone number and address, artist and title of actual song playing, traffic announcement, program type and much more.

In USA the system is called Radio Broadcast Data System (RBDS). The RBDS is very similar to RDS. All RDS encoders from our production can be used in USA without any limitations.

The Radio Data System is very flexible and works with many services. The most important ones are described below.

PS - Program service name
This is the label of the program service consisting of not more than eight alphanumeric characters, which is displayed by RDS receivers in order to inform the listener what program service is being broadcast by the station to which the receiver is tuned.

RT - Radiotext
This refers to text transmissions, primarily addressed to consumer home receivers, which would be equipped with suitable display facilities.

RT+ - Radiotext Plus
The RT+ is designed to let the listener take additional benefit from the Radiotext service by enabling receivers to offer direct access to specific elements of Radiotext. Typically the RT+ feature supports song artist and song title elements. These elements anyway carried in the Radiotext, are identified by their class code, length and location within the Radiotext. The receiver must be equipped with the RT+ function (also called "tagging") to take advantage of this feature.

AF - Alternative frequencies list
The list of alternative frequencies gives information on the various transmitters broadcasting the same program in the same or adjacent reception areas. This facility is particularly useful in the case of car and portable radios.
When the PI code indicates local coverage-area, i.e. only one frequency is used, AF list may contain this frequency.

PI - Program identification
This information consists of a code enabling the receiver to distinguish between countries, areas in which the same program is transmitted, and the identification of the program itself. The code is not intended for direct display and is assigned to each individual radio program, to enable it to be distinguished from all other programs. One important application of this information would be to enable the receiver to search automatically for an alternative frequency in case of bad reception of the program to which the receiver is tuned; the criteria for the change-over to the new frequency would be the presence of a better signal having the same PI code.
The PI code consists of four characters. The first two characters have special meaning, second two are used to clearly identify different stations.
The first character identifies country. The second character identifies program type in terms of area coverage:
0 - Local (Local program transmitted via a single transmitter only during the whole transmitting time.)
1 - International (The same program is also transmitted in other countries.)
2 - National (The same program is transmitted throughout the country.)
3 - Supra-regional (The same program is transmitted throughout a large part of the country.)
4 to F - Regional (The program is available only in one location or region over one or more frequencies, and there exists no definition of its frontiers.)

ECC - Extended Country Code
It helps the receiver to recognise the country in cooperation with the PI code. The first most significant bits of the PI code carry the RDS country code. The four bit coding structure only permits the definition of 15 different codes, 1 to F (hex). Since there are much more countries to be identified, some countries have to share the same code which does not permit unique identification. The ECC byte determines the country unambigouesly.

Symbols used for PI and ECC codes for the countries in European Broadcasting Area:

PTY - Program type
This is an identification number to be transmitted with each program item and which is intended to specify the current Program type within 31 possibilities. This code could be used for search tuning. The code will, moreover, enable suitable receivers and recorders to be pre-set to respond only to program items of the desired type. The last number, i.e. 31, is reserved for an alarm identification which is intended to switch on the audio signal when a receiver is operated in a waiting reception mode.

TA - Traffic announcement identification
This is an on/off switching signal to indicate when a traffic announcement is on air. The signal could be used in receivers to:
a) switch automatically from any audio mode to the traffic announcement;
b) switch on the traffic announcement automatically when the receiver is in a waiting reception mode and the audio signal is muted;
c) switch from a program to another one carrying a traffic announcement.
After the end of the traffic announcement the initial operating mode will be restored.

TP - Traffic program identification
This is a flag to indicate that the tuned program carries traffic announcements. The TP flag must only be set on programs which dynamically switch on the TA identification during traffic announcements. The signal shall be taken into account during automatic search tuning, so I recommend to turn this flag on even though you don't transmit any traffic announcements.

DI - Decoder identification
Indicates which possible operating mode is appropriate for use with the broadcast audio.

M/S - Music/speech switch
This is a two-state signal to provide information on whether music or speech is being broadcast. The signal would permit receivers to be equipped with two separate volume controls, one for music and one for speech, so that the listener could adjust the balance between them to suit his individual listening habits.

CT - Clock-Time and Date
Time and date codes should use Coordinated Universal Time and Modified Julian Day. The listener, however, will not use this information directly and the conversion to local time and date will be made in the receiver's circuitry. CT is used as time stamp by various RDS applications and thus it must be accurate.

EON - Enhanced Other Networks information
This feature can be used to update the information stored in a receiver about program services other than the one received. Alternative frequencies, the PS name, Traffic Program and Traffic Announcement identification as well as Program Type and Program Item Number information can be transmitted for each other service. The relation to the corresponding program is established by means of the relevant Program Identification.

IH - In House Applications
This refers to data to be decoded only by the operator. Some examples noted are identification of transmission origin, remote switching of networks and paging of staff. The applications of coding may be decided by each operator itself.

PIN - Program-Item Number
The code should enable receivers and recorders designed to make use of this feature to respond to the particular program item(s) that the user has preselected. Use is made of the scheduled program time, to which is added the day of the month. The transmitted Program Item Number code will be the scheduled broadcast start time and day of month as published by the broadcaster.

PTYN - Program Type Name
The PTYN feature is used to further describe current PTY. PTYN permits the display of a more specific PTY description that the broadcaster can freely decide (e.g. PTY=4: Sport and PTYN: Football). The PTYN is not intended to change the default eight characters of PTY which will be used during search or wait modes, but only to show in detail the program type once tuned to a program. If the broadcaster is satisfied with a default PTY name, it is not necessary to use additional data capacity for PTYN.

TDC - Transparent Data Channels
The transparent data channels consist of 32 channels which may be used to send any type of data.

This is the most frequent case. Fill your FM frequency to AF and make sure the second digit in PI is 0 (zero), for example 20A6.

For each station in the same location unique PI (Program Identification) must be assigned. Stations that carry different program must be unambiguously identified by the last two PI digits. In other case they are recognised as one station by car radios, regardless of any other service settings. If the broadcaster hasn't received the 4-digit PI from regulatory office, he must choose such number that is not in conflict with other stations in the location.
When you receive the RDS encoder, the PI is usually set to factory default value FFFF and it's needed to change it as soon as possible.

By reason of higher safety during driving, some car radios suppress showing of dynamic text information in PS. This is also the reason why car radios don't support Radiotext (RT).
To get the dynamic PS on all car radio displays, set Low speed of the scrolling PS transmission or use another mode, for example Word alignment. Please take into consideration that using of dynamic PS may be restricted in your country so you should not get round the car radio safety measures.

This question is concerned only with the event that you transmit in stereo. In this case it's preferable to synchronise the RDS subcarrier with the 19 kHz pilot signal, rather than leave these two signals in complete independence. In fact substantial intermodulation may be produced between the 3rd harmonic of the pilot signal and the RDS signal, generating a unwanted beat which may be audible.

The RDS signal level on FM transmitter input should be as small as possible, but RDS must be displayed properly on a receiver. If you don't have any special measuring instrument, compare it with commercial radio stations. The right level should be between 3 and 11 % of the audio signal, measured in peak-to-peak values. Recommended value is about 6 %, which results in 4 kHz deviation of the FM carrier. Don’t forget that maximum FM carrier deviation with RDS and audio signal is 75 kHz.

The RDS signal must respect a phase criterion (in phase or in quadrature) with the 19 kHz pilot tone. In order to achieve this characteristic, the RDS encoder integrates a digital PLL with software phase control.
The oscillograms below show the appearance of RDS signal, pilot tone and a RDS signal added to the pilot tone. In real operation the phase difference can be easily measured for example using the PIRA75 FM Analyzer.

Conditions: Pilot deviation: 6.8 kHz, RDS deviation: 3.4 kHz, no audio input. Measured at the FM transmitter input.

  
1. RDS signal, 2. Pilot tone 19 kHz.

  
3. RDS and pilot in-phase, 4. RDS and pilot in quadrature.

Many cheap stereo transmitters have no input provided for the RDS encoder connection. In this case it's not possible to connect any RDS encoder without internal modification of the transmitter. First of all, you should ask the transmitter's vendor how to modify the transmitter to be able to accept RDS signal. If you have received no response or if you are not familiar with electronics, forget about this transmitter! The RDS system exists for more than 25 years but some transmission equipment producers still look over it. Pira CZ can't study individual requests and suggest any modifications. As a general guide you may use this picture.

To cover actual market needs we produce two different RDS encoder systems - the PIRA32 and the MiniRDS. Following table introduces the main differences between them.