  The "broad iron spectral features" are often seen in X-ray spectra of Active
Galactic Nuclei (AGN) and black-hole binaries (BHB). These features may be
explained either by the "relativistic disc reflection" scenario or the "partial
covering" scenario: It is hardly possible to determine which model is valid
from time-averaged spectral analysis. Thus, X-ray spectral variability has been
investigated to constrain spectral models. To that end, it is crucial to study
iron structure of BHBs in detail at short time-scales, which is, for the first
time, made possible with the Parallel-sum clocking (P-sum) mode of XIS
detectors on board Suzaku. This observational mode has a time-resolution of
7.8~ms as well as a CCD energy-resolution. We have carried out systematic
calibration of the P-sum mode, and investigated spectral variability of the BHB
GRS 1915+105. Consequently, we found that the spectral variability of GRS
1915+105 does not show iron features at sub-seconds. This is totally different
from variability of AGN such as 1H0707--495, where the variation amplitude
significantly drops at the iron K-energy band. This difference can be naturally
explained in the framework of the "partial covering" scenario.
