Patent GB-917750-A (GB)
Title: System for reading magnetically recorded information signals
IPC: G11B20/10
Filed: 1961-11-24
Abstract: 917,750. Transistor pulse circuits. INTERNATIONAL BUSINESS MACHINES CORPORATION. Nov. 24, 1961 [Jan. 24, 1961], No. 42112/61. Class 40 (6) [Also in Group XIX] In reading binary data magnetically recorded by the non-return-to-zero method in which a reversal of flux is induced in the magnetic medium to record a one bit, signals from the transducer are differentiated, amplified and squared, and integrated, a further circuit producing output pulses when the output of the integrator reaches a predetermined voltage level. The method of recording requires that zero cross-over points of flux be detected to sense binary ones. Signals from the transducer (not shown) after being differentiated at C1, R1 are amplified and clipped, by push-pull circuits 5, 8 and 11. The waveforms are shown in Fig. 3 and are referenced by the conductors on which they appear. Amplifier 11 uses NPN transistors and from the common emitter resistor R21 is taken conductor 12 on which appears the rectified mixed output of the amplifier. When signals to the bases of the transistors are both at earth potential, currents through the transistors are equal, and their parallel resistance is such that the current through R21 causes conductor 12 to be at a negative potential. When one of the signals to the transistors goes positive and the other goes negative, their resistance becomes less and the current through R21 is greater, raising the potential of conductor 12. The signals on conductor 12 are input to emitter follower T6 and are integrated in circuit 13. When conductor 12 rises in potential C13 discharges and the voltage on conductor 14 rises, T7 being kept non-conductive by C12. When conductor 12 then drops in potential, T6 conducts more heavily and T7 is caused to conduct, charging C13. In the reference voltage comparator circuit 15, an output appears on line 20 only when the output of the integrator 13 reaches a reference voltage. Normally, T9 and T10 are conducting, T8 is off and the potential on conductor 20 is down. T8 becomes conductive if the voltage on conductor 14 reaches near earth potential, T9 then passes much less current and cut-off T10. The potential of line 20 then rises to near earth voltage. The amplifier outputs of circuit 11 are applied to a second mixer 18 (Fig. 2b), of much the same form as circuit 11 but using PNP transistors. The output is taken from the common emitter resistor R34 and is the inverse of the output of circuit 11. And circuit 21 produces a negative going signal in response to positive going outputs from the second mixer 18 and reference voltage comparator circuit 15. There is a delay 27 (Fig. 2a) between the collector of T9 and the base of T10 to prevent the trailing edge of the pulse from circuit 15 from ending until the output from mixer 18 appears. The output of and circuit 21 is delayed and compared with the output of the second mixer 18 in NPN and circuit 25 which outputs a negative going pulse, repre senting a binary one in response to coincidence of negative going inputs. There is also described an embodiment in which second mixer 18 is replaced by two positive peak clippers, the output of one being anded together with the delayed output of the other (Fig. 4b, not shown).