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The three most common methods of measuring non-linear distortion in audio equipment are the harmonic method in which harmonics of a single sinusoidal input signal are measured; the SMPTE, or modulation, method in which the modulation of a high audio frequency by a low frequency is measured; and the CCIF, or difference frequency, method in which the beat note between two closely spaced frequencies is measured. These methods are discussed with particular reference to the behavior which may be expected in applications where the equipment under test includes pre-emphasis or de-emphasis networks, or where the distortion is symmetrical. It is shown that in a number of cases a more satisfactory method for use at the higher audio frequencies would be the measurement of the third-order component, rather than the second, produced in the CCIF method. A modification of the CCIF method using a sharp-cutoff low-pass filter is described which is capable of measuring both second- and third-order components, it is then shown that an instrument for this new method and the standard SMPTE method have many elements in common and that one instrument could be devised to make both forms of tests. The discussion is illustrated with the results of measurements made on a simulated distortion generator and on actual samples of audio equipment.