Abstract: A narrowband phase modulation communication system is disclosed which eliminates thresholding. At the transmitter, quadrature modulation which produces very large maximum phase deviation is employed in conjunction with a constant amplitude limiter. The constant amplitude resultant contains the carrier and voice signal components in quadrature and is transmitted, at a suitable frequency, in this form. At the receiver, a narrowband phase locked loop is employed for demodulation. This demodulating loop coherently tracks the carrier component and eliminates the thresholding problem normally encountered. In a modification, the carrier component is used to transmit data either alone or in combination with the voice signal.