A simple vibration recorder is developed to analyze the vibrations produced by the micro-tremour of the earth or building. Its covering frequency ranges 1-30c/s. A magnetic tape recording method is adopted instead of a smoked paper recording method, for the latter is not appropriate to process the recorded data. As the frequency characteristic of an ordinary magnetic tape recorder is very poor in the range of these vibrations, the pulse width modulation system is used to record and play back the vibrations. The fundamental pulse frequency is 500c/s, and the wow and flutter due to the recording and play-back mechanism offer no problem in practical use. Some improvements have been made in the wave shaping circuits.
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