A testing method or apparatus utilizes a filter or modifier to measure time varying or dynamic harmonic, intermodulation, cross modulation, and or N-Beat or triple distortion from a device. With a stairstep or arbitrary signal and another signal, Nth order harmonic, cross modulation, triple beat, N-Beat, and or intermodulation distortion is measured via the filter or modifier at different offsets or time provided by an arbitrary low frequency signal. For example, an amplifier with crossover distortion will show increased (e.g., time varying) harmonic, cross modulation, triple beat, and or intermodulation distortion near the zero crossing while providing less distortion in other portions of the transfer curve of the amplifier. One or more distortion signals from the device (e.g., audio device) may be measured for a phase and or frequency modulation effect.