Contour modulation is a technique in which a phase-modulated signal waveform and amplifier output impedance work together to maintain higher efficiency over a sizable output power back-off.

The theory research results of one cycle controlled switching power amplifier show that the output signal follow the input signal linearly, and the control performance is decided by output filter.

The term "efficiency" refers to the ability of a speaker system to convert electrical energy (power from the amplifier) into acoustic output.