The paper describes the why and how of a double-tone distortion test technique, tailored to evaluation of magnetic tapes. The method is basically a special case of the CCIF intermodulation test. The test signal consists of two frequencies of equal amplitude and a frequency difference in the order of 10% (called a twin-tone), of which the generated sidebands 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 THD and signal-compression criteria which could be replaced to advantage by the twin-tone technique as universal standard tape distortion test. The relation and differences 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 as many as 7-8 years, is given.
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