A driver circuit comprising: output circuitry for connecting the circuit to a load, the output circuitry comprising one or more energy-storing components; switching circuitry arranged to supply power from a power supply to the load by supplying a current through at least one of the energy-storing components that resists a change in current, or applying a voltage across at least one of the energy-storing components that resists a change in voltage; and control circuitry arranged to control the switching circuitry, to cause the above-mentioned current or voltage to oscillate between an upper envelope and a lower envelope. The control circuitry is configured to modulate data into this current or voltage by shifting the upper envelope between at least a first amplitude level and a second amplitude level, and by shifting the lower envelope by the same amount in the same direction at the same time.