New FM-AM Method of Compression, Expansion, and Multiplication

A new method of employing the combination of two signals has been developed for magnitude compression or expansion of audio or video signals and for multiplication or other uses involving modulation with separated components (e.g., instrumentation and remote control). The first signal frequency-modulates a local oscillator, whose output is limited and then amplitude-modulated by the second signal; the combined FM-AM signal is detected in such a way that the first signal is restored to its original form, but modified in amplitude by the second signal. In compression-expansion, the method controls the signal level with great speed and over a wide band without introducing noise, "bounce," or dc shift into the output; no delicate balancing or matched-tube circuits are required. In multiplication, the product of two voltages can be formed by means of a simple circuit that is extremely suitable for analog computers and similar applications.