* (speedup) maybe replace high-order FIR downsampling filter with 2nd order butterworth followed by lower order FIR filter
* figure out why we sometimes lose stereo lock
* (feature) implement RDS decoding
* (quality) consider FM demodulation with PLL instead of phase discriminator
* (speedup) consider downsampling of IF signal before FM detection
            (work in progress in branch "filterif")