We analyzed the hard, i.e., most common, state of Cyg X-1,
using RXTE observations that were performed from 1996 to 1999. The
focus of our work lies on parameters and functions characterizing the
short-term variability ( < 1000 sec), like:
a) Power Spectral Densities (PSDs).
b) Linear State Space Models. According to these stochastic models, one typical relaxation timescale can be attributed to the hard state lightcurves of Cyg X-1.
c) Fourier frequency dependent quantities describing the relationship between different energy bands, namely coherence functions and timelags.
We find that the timing properties show a considerable variability, even though the state does not change. This has already been noted for PSDs, but is probably even more pronounced for the timelags. We also show how the changes relate to spectral parameters such as power law indices, coronal temperatures or reflection hump strengths.