Patent GB-812934-A (GB)
Title: Improvements in or relating to information storage systems
Inventor: SHARP JOHN JOSHUA
IPC: G11B20/14
Filed: 1956-01-26
Abstract: 812,934. Electrical digital data-storageapparatus. BRITISH TABULATING MACHINE CO. Ltd. Jan. 11, 1957 [Jan. 26, 1956], No. 2598/56. Class 106 (1). Binary signals are recorded on a magnetic medium such as tape, drum or disc by generating for each signal to be recorded a group of at least three consecutive spaced pulses of alternately opposite polarity, means responsive to an applied signal of one binary significance to select a consecutive pair of pulses having one polarity sequence and responsive to an applied signal of the other binary significance to select a consecutive pair of the opposite polarity sequence and means for applying a selected pulse pair to a recording head. The purpose of making a record in this manner is to avoid the necessity for having a track of clock pulses which particularly on a magnetic tape are subject to variations of tape characteristics. The signals recorded as magnetic areas arranged in pairs of opposite polarity are read by a reading head and applied in antiphase to the inputs of a gate which is opened by a pulse of one polarity to pass a pulse of the same polarity so that an output is obtained only when the first occurring pulse of a pulse pair is of said polarity. Recording.-The signals to be recorded are generated in binary form by an input device 2, Fig. 1, a high voltage being generated for " zero " and a low voltage for " one ". Clock pulses from source 1 trigger a test pulse generator circuit 3 to produce output pulses of 4 Á secs. duration which are applied through cathode follower 3a to a zero test gate 4 which opens only when a " zero " signal is applied from device 2. The output from the cathode follower 3a is also applied to delay units 5, 6, 7, which are provided to give the spacing between the two pulses used for recording each signal. This spacing is approximately equal to the time taken for a point on the record medium to pass the air gap in the recording head. The spacing used is about 2 Á secs., each recording pulse having a duration of about 1 Á sec. Unit 5 gives a delay of 2 Á secs., and units 6 and 7 give delays of 3 Á secs. each, the test pulse from cathode follower 3a being accordingly applied in turn to each of the three recording pulsegenerator circuits 8, 9 and 10. These circuits are triggered by the test pulse to produce a recording pulse of 1 Á sec. duration. The pulses from circuits 8 and 10 pass through cathode followers 8a, 10a to gates 11, 12 which are governed by a strobe pulse-generating circuit 15. A zero signal from the input device 2 allows the test pulse from the pulse generator 3 to pass through gate 4 to a differentiating circuit 4a, the output of which causes the circuit 15 to produce a pulse of 10 Á secs. duration. This pulse opens gate 11 and closes gate 12. The recording pulse from circuit 9 passes through cathode follower 9a and inverter 13 to one half of the centre-tapped primary of a transformer 14, the outputs of gates 11 and 12 being applied to the other half. The transformer energizes a recording head 48 to impress the signals on the magnetic tape. Operation.-When the input device emits a signal corresponding to a binary " zero " gate 4 is opened to pass a clock pulse from source 1. 2 Á secs. later a first recording pulse emitted from circuit 8 passes through gate 11 to the recording head circuit. This only happens in the case of a " zero " signal. After 3 Á sees. a second recording pulse is emitted from circuit 9 which is inverted and fed to the recording head transformer 14. After another 3 Á secs. a third pulse is emitted from circuit 10 and passes to the transformer 14 through gate 12 if a binary " one " signal is supplied or is suppressed if there is a binary " zero " signal. By this means the second pulse is always fed to the transformer 14 and if the input signal is a " zero " it is preceded by the first recording pulse or if the input signal is " one " it is followed by the third recording pulse. A binary " zero " gives rise to a recording head input of a negative pulse of 1 Á secs., followed after 2 Á secs. by a positive pulse of 1 Á sec. The binary " one " gives rise to a pair of pulses in the opposite order. Reading.-The voltage induced in the reading head 45, Fig. 3, is a positive going wave followed by a negative going wave or vice versa. This signal is passed through amplifier 16 to the primary of a transformer 17 having a centretapped secondary connected to two squarewave generators 18, 19 which accordingly receive the signal wave forms in anti-phase. Each signal picked up by the reading head causes the generators 18, 19 to produce a positive pulse, the pulse from generator 18 coming first for a " one " and that from generator 19 coming first for a " zero ", The pulse from generator 18 is applied through a differentiating circuit 20 to a strobe pulse generator 21 the output of which is a pulse lasting 10 Á secs. which is applied to open a gate 22. The output pulse from generator 19 passes through the gate if it has been opened by an earlier signal from the generator 18 but if the pulse from generator 19 occurs first it is suppressed. There is accordingly only an output signal in the case of a binary " one ".