Drop=out recognition circuit for video tape recorder - has control voltage derived from periodically recurring frequency and used for threshold stabilisation

摘要

The carrier for a visual display unit is modulated by the useful signal with a specified swing, and is demodulated in an f.m. demodulator which delivers a drop-out marking pulse when the carrier frequency exceeds a certain threshold, or drops below it. A control voltage (UR) derived from a frequency periodically recurring in the carrier, is used for stabilisation of the threshold (f1, f2, UT). The feedback loop producing the control voltage passes through an integrator (5) and an amplifier (6). In a more elaborate arrangement, a return signal from the control voltage line (UR) is passed through to a Schmitt-Trigger, in series with the frequency demodulator on the output side of the line.