An adaptive version of the basic delta modulator employing full-width pulses andRCintegration is described. A digital technique is used to sense the slope of the input signal and to control the amplitude of the pulses supplied to theRCnetwork in the feedback loop. Subjective testing with speech signals and a modulator clock rate of 56 kbit/s has indicated a useful volume range of 40 dB for commercial telephony-grade performance. At a clock rate of 19.2 kbit/s a signal-quantization noise ratio of 16 dB has been obtained over a dynamic input range of 20 dB for an 800-Hz sine wave. Also described is an application of the adaptive delta modulator known as the Adaptifon system in which the compression and expansion circuits of Lincompex are realized by the delta modulation technique. Speech is transmitted in analog form at constant amplitude, which, together with an FM syllable rate channel, occupies the conventional 3-kHz bandwidth. The receiving system has the capability of removing fading from signals transmitted over an HF path.