  We analysed the long-term variability of four microquasars (GRS 1915+105, Cyg
X-1, Cyg X-3, and Sco X-1) in radio and X rays. The results of our analysis
indicate the existence of two distinct modes of energy output, which we refer
to as the `coupled' mode and the `flaring' mode. The coupled mode is
responsible for mildly fluctuating, flat-spectrum radio emission, coupled with
the X-ray emission; the flaring mode produces powerful, steep-spectrum radio
flares, with no significant counterpart in X rays. We find that the fraction of
time spent by a typical microquasar in the flaring mode is similar to the
fraction of quasars that are radio-loud. This is consistent with the hypothesis
that radio-loudness of quasars is a function of the epoch at which the source
is observed.
