Principle of RDS broadcasting

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Spectrum of the signal transmitted by a FM radio

The frequency spectrum range of a signal transmitted by an emetteur of radio stereo FM contains several different parts :

Mono Transmission	The range of frequency < 15Khz is used for mono transmission corresponding to the sum of channels Left and Right. This allows to make compatible a stereo emission with mono receivers, because the complete signal is available in this range and the upper frequencies are not processed. The theoretical modulation rate of the signal mono is of the order of 45 %
19KHz Pilot tone	The pilot tone 19KHz is used for the demodulating of the upper spectrum.This occurence of this fréquency indicates to the receiver a stereo emission The theoretical modulation rate of this frequency is of the order of 10 %
Stereo Transmission	The Left and Right channels difference is transmitted under the shape of double side band with carrier deleted. This carrier correspond to the subcarrier (38KHz). The theoretical modulation rate of the stereo signal is of the order of 22.5 %
38KHz subcarrier	The stereo subcarrier is exactly the double of the pilote tone 19KHz. This carrier is deleted to limit the depth of modulation, it is regenerated by the receiver from the pilot tone.
RDS Transmission	The transmission of RDS signal used the same principle of stereo transmission (sdouble side band with carrier deleted). The carrier used is 57KHz and the modulation rate is much low than stereo signal. The high part spectrum using allow to not disturb mono and stereo transmission, keeping compatibility with No RDS receiver
57KHz subcarrier	THe RDS subcarrierr is exactly triple of pilot tone (19KHz). This carrier is deleted to limit the depth of modulation,it is regenerated by the receiver from the pilot tone.