For physical processes which express themselves as a frequency, forexample magnetic field measurements using optically-pumpedalkali-vapor magnetometers, the precise extraction of the frequencyfrom the noisy signal is a classical problem. We describe herein afrequency measurement system based on an inexpensive commerciallyavailable computer sound card coupled with a software single-toneestimator which reaches Cramér–Rao limited performance, a featurewhich commercial frequency counters often lack. Characterization ofthe system and examples of its successful application to magnetometryare presented.