Apparatus and methods for data demodulation in FM-FSK communication systems may include comparing the power spectral density (PSD) of the received frequency spectrum with that of the previously received samples using digital signal processing on a multi-sample message. A narrow band FM-FSK receiver may include a filter configured to pass FM signal components of a predetermined signal band, a memory configured to store the filtered signal component, and a DSP operably connected to the filter and the memory. The DSP may be configured to output a digital signal based upon a comparison of successive DSP calculated frequencies associated with a peak power of a power spectrum density (PSD) of successive samples of the filtered multi-sample message.