Abstract: Disclosed is a digital modem using continuously variable sloped delta modulation and adapted for digitizing voice signals in a telephone system. In the analog to digital direction (A/D), an analog input signal is converted into a digital output, the output logic condition being indicative of the direction of change in amplitude from the last signal input. The signal pulse indicating positive change or absence of pulse indicating negative change is passed to a shift register for transmission of the first pulse signal and storage of a plurality of bits. A feedback network receives the pulse bits from the shift register, and the bits are converted to a summed signal for resolution into a reconstructed or simulated signal against which the incoming signal is checked. For the receiving or D/A direction, a somewhat similar circuit in reverse is used to reconstruct an analog signal from the received data bits.