Looking at the need for a Pilot Filter
Pilot
To ensure a clean 19KHz pilot reference signal a bandpass filter is employed. Car has to be taken to ensure the phase reference is not disturbed.
The Pilot filter details
The pilot filter takes the output from the DAC second channel, that has the pilot reference. This output is filtered by the multi-feedback bandpass filter to generate a pure 19KHz signal.
The 158k resistor should be replaced by a 150k + 10k multi-turn trimmer combination to allow adjustment of the filter resonant frequency. Even with precision components the component tolerance will cause the max output frequency to vary. This must be adjusted in productioon to the peak value to ensure a consistent phase shift through the filter. The pilot reference produced by the DSP must take into account any phase shift in the filter so that the pilot reference phase is consistant with the pilot phase produced by the MPX, otherwise any external RDS that uses this pilot reference will not generate RDS with the correct phase relationship with the MPX.
The pilot reference level can be adjusted and a buffer amplifier ensure any load capacitance will not effect the filter response.