Hey, Bob,
Thanks for that citation, a very clean DSP implementation of the classic stereo demod and a good example of how to do a PLL in Sigma. My 25kHz pilot did have amplitude perturbations, which I cleaned up, first with a clipper and follow-up filter, and second with an amplitude leveler with a somewhat unique character.
I never cease to be amazed at what can be done in Sigma by using very basic arithmetic elements, sometimes surpassing the performance of the more complex 'building blocks.'