  We present a digital algorithm for measuring the phase difference between two
sinusoidal signals that combines the modified fringe-counting method with
two-sample zero crossing to enable sequential signal processing. This technique
can be applied to a phase meter for measuring dynamic phase differences with
high resolution, particularly for heterodyne interferometry. The floor noise
obtained from a demonstration with an electrical apparatus is $5\times10^{-8}
\mathrm{rad/\sqrt{Hz}}$ at frequencies above approximately 0.1 Hz. In addition,
by applying this method to a commercial heterodyne interferometer, the
floor-noise level is confirmed to be $7\times10^{-14} \mathrm{m/\sqrt{Hz}}$
from 4 kHz to 1 MHz. We also confirm the validity of the algorithm by comparing
its results with those from a standard homodyne interferometer for measuring
shock-motion peak acceleration greater than 5000 m/s^2 and a 10 mm stroke.
