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Frequency Characteristics Measurements of 78 rpm Acoustic Record Players by the Pulse-Train Method

The authors have been engaged in the research for the restoration of seriously damaged audio signals, employing Generalized Harmonic Analysis (GHA). In this research, it is important to know frequency characteristics of sound reproducing equipments to realize proper sound reproduction. However, frequency characteristics of the ancient acoustic record players such as "Credenza" etc, are significant but not clear: especially, the frequency characteristics when records are actually reproduced. However it can solely be measured with frequency record and vibrator-method is not used any more. In fact, no shellac-made 78 rpm frequency record can be manufactured today: the traditional measurement techniques are inapplicable. On the other hand, one of the authors previously developed Pulse-Train measurement for phonograph cartridges, in order to obtain their frequency characteristics of amplitude and phase performances. This method is applicable so long as any pulse waveform is curved on record surface. Thus the authors employed this method. Radial directional groove was curved on a surface of shellac disc-record, and Pulse-Train response is obtained by reproducing the record with an acoustic record player. Some examples will be exhibited in the report.

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