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The need to improve transmission through routine testing of program circuits for the effect of nonlinear distortion has long been recognized. Equipment has been developed for making such an evaluation on a "one-shot" basis, consistent with standards set up by the Video Transmission Engineering Advisory Committee (VITEAC) and Bell Telephone Laboratories. This equipment is a pair of special purpose test instruments, a signal generator, and a distortion detector. The signal generator provides three precisely controlled sinusoidal audio signals, a low-frequency and a highfrequency tone, either singly or simultaneously, and a reference tone. The distortion detector is a two-tone selective voltmeter that measures harmonic distortion directly in decibels below the received reference tone level. Among the parameters that can be measured are harmonic distortion, line loss, line noise, and flatness of transmission.