  We have compared simultaneous Ryle Telescope radio and Rossi X-Ray Timing
Explorer X-ray observations of the galactic microquasar GRS 1915+105, using the
classification of the X-ray behaviour in terms of three states as previously
established. We find a strong (one-to-one) relation between radio oscillation
events and series of spectrally hard states in the X-ray light curves, if the
hard states are longer than ~100s and are "well separated" from each other. In
all other cases the source either shows a low-level or a high level radio
emission, but no radio oscillation events. During intervals when the source
stays in the hard spectral state for periods of days to months, the radio
behaviour is quite different; during some of these intervals a quasi-continuous
jet is formed with an almost flat synchrotron spectrum extending to at least
the near-infrared. Based on the similarities between the oscillation profiles
at different wavelengths, we suggest a scenario which can explain most of the
complex X-ray : radio behaviour of GRS 1915+105. We compare this behaviour with
that of other black hole sources and challenge previous reports of a relation
between spectrally soft X-ray states and the radio emission.
