Abstract: In this controlled decay envelope detector, a capacitor is used to hold the momentary peaks of the input signal. Its only discharge current path is controlled by a circuit which simultaneously monitors the capacitor voltage and the input signal such that the discharge current from the capacitor is proportional to the voltage difference between the capacitor voltage and a conventional exponential RC decay voltage provided in the control circuit. The resulting discharge from the capacitor, in response to a negative-going step input voltage, is an increasing current starting from zero at the time of the transient. The resulting detector output describes an upwardly convexing voltage decay curve which holds its level essentially equal to that of the initial peak input voltage until the next estimated peak input occurs. Only when the next peak input does not come, or is lower than the last one, will the output start dropping at an increasing rate, which provides a correspondingly precise detection of the envelope.

Abstract: A digital circuit for generating a linearly increasing signal is provided for controlling the level of a recording A. C. bias signal. The digitally controlled bias signal is mixed with an analog signal of a known frequency and constant level and recorded. The circuit detects and stores a recording bias level corresponding to a reproduce signal envelope level, equal to an obtained peak level adjusted with respect to a known overbias level. The circuit compensates the detected recording bias level with respect to distance between the recording and reproducing heads to obtain an optimum recording bias level. The detected bias level remains stored and unchanged during power supply interruption.