Abstract: Analog-voltage samples to be transmitted in digital form over a communication channel, ranging between a lower and an upper limit V.sub.min and V.sub.max, are quantized in 2.sup.n levels with 2.sup.n/2 major quantum steps each subdivided into 2.sup.n/3 minor quantum steps represented by n-bit words, a group of n/2 bits of each word representing the major quantum steps and another group of n/2 bits representing the minor quantum steps. A converter at the transmitting end includes a first and a second constant-current generator, of high and low rate respectively, which are sequentially activated to discharge and to charge an input capacitor temporarily storing an analog-voltage sample to be digitized. The first generator rapidly discharges the input capacitor while an associated first pulse counter receives a series of forward-counting clock pulses, each pulse corresponding to a major quantum step, until the absolute value of the capacitor voltage drops below that of a predetermined reference voltage V.sub.