The paper is concerned with the measurement of subjective thresholds of the unwanted pitch and amplitude fluctuations which occur in the reproducing output of all types of sound recording systems. The pitch fluctuations are commonly known as -wow- or -flutter-, and new equipment is described for generating controlled fluctuations of this type in musical or other program signals. The threshold measurements thus made possible are more realistic than those hitherto available, which have been related only to frequency fluctuations in test tones. The generation and threshold measurement of controlled amplitude fluctuations are also described, since amplitude and frequency fluctuations commonly occur together. The results obtained enable frequency weighting characteristics to be defined which can be incorporated into instruments measuring the magnitude of the unwanted amplitude and frequency fluctuations in practical systems. It is shown that such instruments may then approximate to an ideal subjectively weighted measuring device, which, when threshold value is reached, will give the same indication irrespective of the type and frequency of the fluctuation measured.
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