Many transducers produce an alternating output with a frequency proportional to the transduced quantity. The normal method of converting this output into a suitable digital form is by counting the number of cycles that occur during a fixed time. This paper describes an alternative approach in which the binary number corresponding to the input frequency is continuously available.The system is based on the use of a reversible binary counter. The input frequency is converted into a suitable train of pulses and applied to the ADD input of the counter, and a second train of pulses whose mean frequency is proportional to the number stored into the counter to the SUBTRACT input. When equilibrium is established the number stored in the counter is proportional to the input frequency.