A power transmitter (101) for inductively transferring power to a power receiver (105) comprises a resonance circuit (201) comprising a transmitter coil (103) for generating a power transfer signal. A sampler (511) samples a current through, or voltage over, the transmitter coil (103). A message receiver (509) receives messages load modulated onto the power transfer signal based on the samples. A driver (203) generates a drive signal for the resonance circuit (201) and a resonance modification circuit (505) reduces the resonance frequency of the resonance circuit (201) by slowing a state change for a resonating component of the resonance circuit (201) for a fractional time interval of the cycles of the drive signal. A sample time controller (513) controls the sample times in response to at least one of start-times and end-times of the fractional time intervals, and specifically may set the sample times to be within the fractional time intervals.