The history of the homodyne and synchrodyne

The Homodyne and Synchrodyne are systems of demodulation for amplitude-modulated signals; they use a local oscillator, synchronized in frequency to the carrier of the wanted signal, to modulate the received wave. Thus the wanted signal is obtained immediately as an audio output without further detection, and unwanted signals are left on carriers of the difference-frequency between their original carrier and that of the wanted signal. There is no detection of the unwanted signals since the whole signal path can be kept free of non-linearity. Unwanted signals can thus be completely rejected by a low-pass filter the audio output. The receiver consequently has the advantages of high quality combined with high selectivity, and is inherently a demodulator of precision. The principles can be extended to permit separation of signals whose sidebands overlap, and they also lead to improved detection of pulse signals in the ¿coherent detector.¿ The history of the systems over the last 30 years is outlined in this paper. Although they are of great interest, they appear never to have had any commercial exploitation until recent application in colour television.