A method is proposed for measuring nonlinear distortion in audio networks meaningfully up to the top edge of the passband. It reads the crossmodulation between two frequencies whose ratio is nearly, but not quite, 3:2, such as 50:33. Two beat products, one each for even- and odd-order curvature of the nonlinearity being tested, namely, f(1) - f(2) and 2f(2) -f(1), then fall close together inside the band and can be read through a narrow bandpass filter, thus minimizing noise. Purity of the testing waveforms is of little importance. distortion at middle frequencies is read similarly. For readings at the bottom of the band, the testing frequencies must be in a ratio of nearly 2:1, such as 11:5. Two beat products again fall close together inside the band, but now the third harmonic of the lower testing frequency lies close by and must be minimized. Means are discussed for generating the testing signals digitally from a crystal source. Design considerations for band-pass measuring filters are discussed also. Readings with an average-reading meter are compatible with those for total harmonic distortion.
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