A signal conversion system in which an analog signal of a certain magnitude can be encoded as a digital signal making full use of the available number of bits of an A/D converter. The system includes amplitude adjustment apparatus with an up-down counter actuated by the maximum available binary value of the digital signal to count UP one step and set an amplitude selection circuit to effectively attenuate the amplitude of the analog signal by a predetermined amount. The counter may have more than one memorizable count level to control a corresponding number of amplitude selection levels. The counter counts DOWN when the most significant bit of the digital signal drops from 1 to 0. The invention includes a corresponding analog signal reconstruction circuit except that it includes a D/A converter that produces an analog signal at maximum value that must be attenuated for low-amplitude values.