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How Does RDS Work?

Date:2019/11/12 14:00:59 Hits:



How to perform sophisticated network following?

This was about the signalling which the broadcaster can add to his program. Let us have a focus on the receiver. To reduce it to one sentence: The receiver should be able to perform the best reception under any conditions. The RDS information is one part of this strategy. Another part is determined by the used hardware concept and how the signal processing of the radio is performed.


The radio normally has different detectors for:

Fieldstrength or level detector

Multipath distortion

Ultra sonic noise detector (USN) (for detecting neighbour channels)

Offset detector

Additional detectors:

Pause detector

Pilot detector


This detector gives an indication of the signal strength of the desired channel at the input of the tuner. The fieldstrength is a good indication for the signal quality, as signal to noise ratio depends on it. Therefore it is a good detector for signal to noise ratio. To assure that the output represents really the signal strength this detector has to be aligned to compensate the spread in analogue components.





The multipath detector measures amplitude fluctuations of the signal. An FM signal is broadcast with a fixed level. Therefore level fluctuations indicate less signal quality. At multipath conditions large level fluctuations can be measured. The multipath detector does not require any alignment.


What is USN? To give an indication about the Ultra Sonic Noise the amplitude of the high frequency content of the MPX signal is measured. This is measured in the bandwidth of approximately 80 kHz up to 150 kHz.


With this detector the misalignment between the modulation and demodulation frequency can be measured. As the misalignment is expected to be small, a large offset indicates disturbance (for example adjacent channel breakthrough).





Radio switches to 98.0MHz for a short AF check (<10 ms). 98.1MHz is detected but offset is detected too therefore no NF took place><10ms). 98.1MHz is detected but offset is detected too therefore no NF took place.


If one or more detectors are triggered, the radio can react with different kinds of strategies to suppress distortion.


One of these strategies is to switch very fast to a frequency where the same program is broadcasted, preferably with no audible distortion. Therefore mainly the PI code is used to determine if the original station is the same as the newly tuned station. This is the reason why the PI code is send at the maximum repetition rate to enable the radio to detect very fast if it has performed a correct network follow.


Sometimes it could be that it took a while to decode the PI code. During this time the radio has two possibilities. The first is to stay muted until the PI code is verified or to enable audio with the risk to be on a different radio program and hear different audio content.


If the reception of the preferred station overall is not so good (no alternatives, low fieldstrength), the radio can activate its concealment strategies. These are:

Mono stereo blend switch the signal between stereo and mono in dependence of distortions. This can be activated by multipath distortion or low fieldstrength.

High cut is a reduction of higher audio frequencies. The most annoying audio distortions are in the higher frequency band, so a low pass filter is activated which reduces the higher frequencies. The cut off frequency and the suppression rate can be set by parameters, which are evaluated during test drives.

Soft mute is a total reduction in the volume of the audio signal. Soft mute mostly is active at low fieldstrength. At low fieldstrength the audio signal is reduced, the noise level is increasing and this is disturbing. When this happens the soft mute reduces the audio level to make this distortion not so annoying. The start and the slope of the soft mute is set by parameters and evaluated during test drives.

Bandwidth control becomes active when the IF filter is not able to suppress adjacent channel breakthrough. In this case an overlap exists between the wanted channel and the adjacent channels. This is often the case in regions with a channel grid of 100 kHz. The selectivity (the bandwidth) of the IF filter need to be adaptive in this case. In this way the bandwidth of the channel filter is reduced if necessary. The bandwidth reduction results in a suppression of the adjacent channel, while keeping the distortion of the wanted signal to a minimum.


Traffic program and enhanced other networks (TP and EON): Besides the network following the second major advantage of the RDS system is the traffic announcement feature. Therefore two bits exist to signal a station with traffic announcements and if the announcement is active or not.


The feature EON normally is related to a complete network chain (e.g. SWR1, SWR2, SWR3 and SWR4). One station is transmitting the announcements (here it is SWR3) and the other programs switch to it when an announcement starts. The customer must not listen to the traffic station he can also listen to his preferred program (e.g. classic music) and will not miss any announcement on the other station.


The information for the change to the traffic station is sent in the 14A groups, including the PI code of the traffic program and all alternative frequencies of it. When an announcement takes place the radio knows it has to switch to the other program. The best frequency is selected out of the AF list. When the announcement is finished, the radio goes back to the original program.



RDS - The Software

RDS is the most difficult technology to receive analog FM stations. There are different ways to use this technology.

The easiest way is to decode the program station name and display it. In this case the software is very small and simple but it does not use all the possibilities of RDS and gives the customer only a little benefit.

The next stage is the implementation of all RDS features according the standard. And make some additional improvements by optimising thresholds during test drives. Now the software becomes larger and a little bit more complex. This is the way manufacturer use, who does not deliver radios to the European car industry.

The best solution is to implement the standard. In addition several quality parameters are defined. These are used in specific developed algorithm. All this enables the software to switch fast to the best alternative frequency in critical reception areas.This way requires a lot of know how and knowledge of problems in the field. This is the only possibility to develop RDS software which will be accepted by the European car manufacturers. To achieve this kind of performance the RDS software becomes very big and complex. To explain what kind of improvements are possible three examples of main features of RDS are chosen.



NF-Following

Target of network following is to tune automatically to the frequency with the best quality without any notable mutes, miss change and sound effects.

The old RDS software controls continuously the field strength, multipath and noise of the alternative frequencies. In the background the AFs are maintained according field strength, history of the PI code and the neighbourhood relation to the actual frequency. Up to 100 alternatives frequencies are stored even if the field strength is below the threshold. The software switches to an AF, if the field strength, multipath or noises of the tuned frequency reach a certain threshold. The AF with the best field strength is selected.


This implementation has some weakness:

It can occur that the selected AF has a lot of multipath and/or noise. Therefore the sound is worse than current frequency.

It can also happen that it switches to an AF but not on the exact right frequency but 100 kHz beside. In this case the sound quality is also not good.

If the actual frequency has very low field strength high cut and stereo blend are activated.

If the alternative frequency has strong field strength than a change is audible because high cut and stereo blend are switched off immediately.

The main problem occurs in weak signal areas. The software starts every few seconds an AF search to find a better AF which leads to a mute. So there is alternating noisy sound and mute which is very annoying.





The new RDS software controls continuously more than eight different quality indicators of 35 alternative frequencies. They are continuously updated in the background. This process is not audible. In this table the AFs are sorted in order of the value of the quality parameters and it is kept up to date continuously.


A change is initiated if one of the quality indicators reaches a certain threshold. A change is also started if the quality indicators of an alternative frequency are better than the actual one.


Before a change is executed both frequencies are compared. Therefore TCN has invented an algorithm which uses the quality parameters.


The result of this calculation is directly related to the sound impression. A high value guarantees a good sound impression. Only if this value is higher than the actual frequency, software will switch to the alternative frequency. The software is able to switch up to 20 times per minutes between different alternative frequencies.


To prevent that a change between a weak actual frequency to a strong alternative frequency is audible the new software control during the switch high cut and stereo blend. Therefore the change even in this case is nearly not audible.



The Improvements

The new software always switches very fast to the best alternative frequency because it compares the total quality of the signal and not only the field strength.

It switches always to the center frequency as the offset indicator is used.

There is no miss switch as the table with the AFs is the continuously updated in order of the quality parameter. There is only a change if the quality parameters are better.

There is no alternating switching between mute and noisy signals in weak signal areas as the software stays on actual frequency as long as there is no frequency with better quality parameters available.

・In weak signal areas the sound impression is optimized by using high cut stereo blend and bandwidth control. ・During switching between a bad actual frequency and a good alternative frequency the audible impression is reduced by controlling high cut, stereo blend and bandwidth control.



You may also like:

What is Radio Data System(RDS)?

What Does RDS Stand For?

The Structure of Encoder RDS?



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