  Experimental data from an experiment on drift--waves in plasma is presented.
The experiment provides a space--time diagnostic and has a control parameter
that permits the study of the transition from a stable plasma to a turbulent
plasma. The biorthogonal decomposition is used to analyse the data. We
introduce the notion of complex modulation for two--dimensional systems. We
decompose the real physical system into complex modulated monochromatic
travelling waves and give a simple model describing the speed doubling observed
in the data as the control parameter increases.
