A simple acousto-optic system is described that is capable of demodulating frequency- and phase-modulated signals using a bicell photodetector. A summary of an analytical study of the system is provided, describing its main performance characteristics. Results from a Mathcad model of the system are presented showing that, by taking the sum and difference of the outputs from the two bicell elements, it is possible to identify and demodulate a wide range of different types of modulation without any a priori information. Thus the technique is well suited for surveillance receiver and electronic support measures (ESM) applications. In particular, it is shown that biphase shift keying (BPSK), quaternary phase shift keying (QPSK), offset-keyed QPSK, differential QPSK, continuous phase frequency shift keying (FSK) [including minimum shift keying (MSK)], and ordinary FSK all give unique and easily identifiable signatures. Experimental results are presented, which are in good agreement with both theoretical and modeled results. © 1998 Society of Photo-Optical Instrumentation Engineers.