Presenting symbolic modulation as a method of generating RDS signals.
The alternative RDS encoder as shown in this diagram directly generates the RDS modulation given a sequence of symbol information.
There is a sample clock at 342KHz, this is locked to the 19KHz pilot reference by taking samples through an ADC. These sample triggers are used to select symbol information from a table. This information, represents an output level from the DAC.
The choice of symbol instructions are generated from the RDS frames and placed in an array. Since each RDS symbol takes exactly 48 cycles of the 57KHz carrier, then as each symbol is selected from the table each of the 288 samples for the symbol are sent to the DAC. Thus the RDS modulated carrier waveform is directly synthesised by the DAC without further need for filtering apart from a simple LPF to remove DAC harmonics.