A new principle to avoid undesirable oscillations in electro acoustic loops application to the design of a hands free telephone set without voice switching

A new general principle is described which makes possible the avoidance of undesirable oscillations in feedback systems. The design of a hands free telephone set is taken as an example. This principle, whose theory and practical limitations are given, makes use of a sustained auxiliary electroacoustic oscillation, created intentionally in an auxiliary feedback loop containing a narrow band pass filter, to regulate the gain of an AGC amplifier inserted in the main loop in order that the modulus of the overall gain along the main loop might be always less than one. A suitable realization of the narrow band pass filter in the form of a special mechanical filter is also described.