  We study a large sample of RXTE PCA/HEXTE observations of Cyg X-1. We
characterize the spectra by soft and hard X-ray colours (which define the
spectral states), and fit them with a physical model of hybrid,
thermal/non-thermal Comptonization. We then fit the power spectra by a sum of
Lorentzians. We show the resulting correlations between the spectral colour
(defining the hard, intermediate and soft spectral states), the Comptonization
amplification factor, the fractional Compton reflection strength, and the peak
frequency of the Lorentzians. We also calculate the fractional variability
(rms) as a function of photon energy. We fit the obtained rms dependencies by
physical models of varying accretion rate, a varying soft-photon input from an
outer disc to a hot inner plasma, and a varying dissipation rate in a hot
corona above a disc.
