The why and how of a twin-tone distortion test technique, tailored to evaluation of magnetic tapes, is described. The method is basically a special case of the IEC measurement of difference-frequency distortion of the third order (IEC Publ. 268-3)./1: The test signal (twin tone) consists of two frequencies of equal amplitude and a frequency difference on the order of 10% or less. Distortion causes added sidebands which are measured. By doing so, a test is devised which produces an unambiguous distortion criterion, applicable to the entire frequency range of the recorder-tape system. In this and other respects the twin-tone method is superior to the third-harmonic distortion and signal compression criteria. As a universal standard tape distortion test the twin-tone method could well replace these other methods. The relation and difference with these conventional test techniques are given, both in mathematical form and by direct comparison of test results of a cassette tape. A detailed description of simple test equipment, field-proven for about eight years, is also added.
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