  A technique for calculation of variability coherence at different timescales
performed directly in the time domain is introduced. Simulations are made to
compare the coherence spectrum derived by the time domain technique with the
coherence function by Fourier analysis. The results indicate that in comparison
with the Fourier analysis the time domain technique is more powerful for
revealing signal coherence in noisy data. We apply the time domain technique to
the real data of the black hole binaries Cygnus X-1 and GX 339-4 and compare
the results with their Fourier coherence spectra.
