PURPOSE:To realize the transmission of an audio signal with high fidelity by providing a logarithmic converter comparing an analog signal with a reference signal in a delta modulation communication system. CONSTITUTION:A transmitted analog signal is inputted to a terminal 10 of the logarithmic converter 12 and compared with the reference signal inputted to a terminal 14. After a prescribed DM bit is transmitted, the step size is transmitted and it is decided by the converter 12. The converter 12 compares the analog signal with the current value of a digital code signal and the step size is increased as the difference between the sampled analog signal and the digital signal is increased. The output of the converter 12 is digital quantity having the corrected step size and it is used as the step size of the DM bit transmission of prescribed number. A larger amplitude change is attained at a comparatively narrow band width in this way.
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