GB1484256A - Arrangements for sensing recorded information signals - Google Patents

Abstract

1484256 Capacitative record playback apparatus EMI Ltd 14 Nov 1974 [22 Nov 1973] 54210/73 Heading G5R An arrangement for playback of signals recorded as surface deformations 5a on amedium 5 has a probe electrode 1 and a co-operating electrode 4 on opposite sides of medium 5, and an alternating current bridge including a pair of closely coupled inductive ratio arms 10 and arranged to sense changes in capacitance 9 between the electrodes 1, 4. The A.C. bridge may be of the type disclosed in Specifications 587,878, 581,164 and 581,161. The bridge earth or neutral point is connected to a screen 2 of probe lead la, screen 2 forming part of an enclosure containing inductive ratio arms 10. The bridge output is derived from toppings across all or part of arms 10 through a double screened transformer 14. The wanted output may be taken directly from the output of a phase sensitive rectifier of an amplifier 15, in which case the bridge must be made to balance at a rate capable of following the highest recorded modulation frequency. Alternatively, the bridge may be made to balance at a rate accommodating relatively slow capacitance changes, e.g. due to scanning motion, the output of transformer 14 then additionally being taken to a further amplifying and rectifying device 16 which processes the information to give an output indicative solely of rapid changes in capacitance 9 and not of its magnitude. The bridge is automatically balanced by a T-network comprising capacitors 12, a second pair of inductive ratio arms 13 across A.C. source 11, and a varactor 17 the capacitance of which is controlled via a resistor 18 from the output of amplifier 15. A similar arrangement may be provided for balancing the quadrature or loss component of capacitance 9. The medium 5 may be a plastics disc having a spiral record track.