REW has many highly sophisticated functions, and they become obvious just in a "less-usual" case of measurements, like these old tape recorders. In case of distortion setting, it is the AES17-2015 standard notch:
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Let me quote from REW manual:
These options are very useful if the fundamental is surrounded by wow/flutter or jitter, like in case of tape recorders.
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The power for THD and THD+N calculation is now derived from power over the octave band around the fundamental frequency. Without this option, the THD+N calculation "sees" high noise just next to the fundamental frequency line and gets crazy, as one can see in post #1 and shows too bad SNR and THD+N (TD+N).
Without AES17-2015 notch, REW shows this:
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