  This is the first of two papers in which we address the physics of the
low-frequency Quasi-Periodic Oscillation (QPO) of X-ray binaries, in particular
those hosting a black hole. We discuss and repeat the recent analysis and
spectral modelling of the micro-quasar GRO J1655 by Sobczak et al (2000,
hereafter SMR), and compare it with GRS 1915; this leads us to confirm and
analyze in more detail the different behavior noted by SMR, between GRO J1655
and other sources, when comparing the correlation between the QPO frequency and
the disk inner radius. In a companion paper (Varniere et al., 2002, hereafter
Paper II) we will show that these opposite behaviors can be explained in the
context of the Accretion-Ejection Instability recently presented by Tagger and
Pellat (1999).
  We thus propose that the difference between GRO J1655 and other sources comes
from the fact that in the former, observed in a very high state, the disk inner
radius always stays close to the Last Stable Orbit.
  In the course of this analysis, we also indicate interesting differences
between the source properties, when the spectral fits give an anomalously low
inner disk radius. This might indicate the presence of a spiral shock or a hot
point in the disk.
