  The low-frequency power spectra of the X-ray and radio emission from four
microquasars suggest that two distinct modes of energy output are at work: (i)
the `coupled' mode in which the X-ray and radio luminosities are closely
coupled and vary only weakly, and (ii) the `flaring' mode, which dramatically
boosts the radio luminosity but makes no impact on the X-ray luminosity. The
systems are in the flaring mode only a few percent of the time. However, flares
completely dominate the power spectrum of radio emission, with the consequence
that sources in which the flaring mode occurs, such as GRS 1915+105 and Cyg
X-3, have radio power spectra that lie more than an order of magnitude above
the corresponding X-ray power spectra. Of the four microquasars for which we
have examined data, in only one, Cyg X-1, is the flaring mode seemingly
inactive. While Cyg X-1 is a black-hole candidate, one of the three flaring
sources, Sco X-1, is a neutron star. Consequently, it is likely that both modes
are driven by the accretion disk rather than black-hole spin. Radio imaging
strongly suggests that the flaring mode involves relativistic jets. A typical
microquasar is in the flaring mode a few percent of the time, which is similar
to the fraction of quasars that are radio loud. Thus there may be no essential
difference between radio-loud and radio-quiet quasars; radio loudness may
simply be a function of the epoch at which the source is observed.
