Automatically calibrated rf envelope detector circuit

Abstract

A method and apparatus is disclosed for automatically maintaining a transducing head on the proper track and is particularly adapted for a helical scan recording and reproducing apparatus that is capable of providing special motion effects, such as slow, still and fast motion, and other effects. The apparatus is of the type which utilizes transverse positioning of the transducing head to accurately follow a track during reproducing and, at the completion of the reproduction from the track, to properly position the head in position to either reproduce the next adjacent successive track, reproduce the same track or reproduce another track so that the appropriate special motion effect is achieved. To provide accurate head tracking, a small oscillator motion (dither) is applied to the head to cause it to vibrate laterally between two limits relative to the track. This causes amplitude modulation of the reproduced signal, which is in the form of an RF envelope of frequency modulated carrier. A feedback servo circuit is responsive to the amplitude modulated reproduced signal to provide continuous adjustment of the transducing head position so as to maintain the magnetic head in optimum reproduce relationship with respect to the track. An automatically calibrated RF envelope detector circuit includes a reference level setting feedback loop which takes a reference level from the input signal to the detector circuit to control the gain of the latter circuit regardless of tape RF level differences, operating conditions, component characteristics, etc.