Patent GB-1109942-A (GB)
Title: Improvements in systems for exploiting information signals
IPC: G11B20/14
Filed: 1966-05-25
Abstract: 1,109,942. Data readout from moveable 'magnetic medium. SOCIETE INDUSTRIELLE BULL-GENERAL ELECTRIC. 25 May, 1966 (1 June, 1965], No. 23496/66. Heading G4C. A device for interpreting read signals derived from a magnetic recording, these signals oscillating from one to the other of two voltage levels at the end of bit cells, and having an inversion in the centre of a bit cell if a '1' is stored and no inversion in the centre of a bit cell if an " 0 " is stored (s in Fig. 5), comprises means for producing first and second sequences of pulses (r 1 , r 2 ) respectively 0À75T and 1À25T after the termination of a bit cell, T being the nominal duration of a bit cell, a first flipflop connected to change state upon receipt of a first sequence pulse if the read signal is at a first level, a second flip-flop connected to change state upon receipt of a second sequence pulse if the read signal is at its second level, a device to compare the states of the first and second flip-flops, and a digit store having separate inputs connected to the outputs of the comparator device and a common input which receives the second sequence of pulses such that the output of the store supplies at each bit period a signal of a first value when the data detected is a " 1 " or a signal of a second value if the data detected is a " 0 ". Amplified read pulses s and inverted read pulses s<SP>1</SP> from a magnetic tape or drum are fed to a Signal Shaper 10 comprising transistor elements forming a symmetrical emitter feedback amplifier (Figs. 2A, 2B, not shown) and which forms pulses Z<SP>1</SP> (Fig. 5) and their inversion Z<SP>0</SP>. These pulses are then differentiated in circuit 11 to form control pulses for a time base circuit 12. The saw-tooth time base waveform is passed to a time measuring device causing a non-conducting transistor (T12, Fig. 2A, not shown) to conduct at time 0À75T and to stay conducting at the end of a bit period and to produce a waveform w<SP>1</SP> (Fig. 4, not shown). A pulse generator 14 supplies a first sequence of pulses r 1  which feed a mono-stable flip-flop 15 to produce square waves v commencing at the termination of the pulses r 1 . A second pulse generator 16 produces the second sequence of pulses r 2  each pulse commencing at the termination of each pulse v. A mono-stable trigger circuit 17 produces pulses r 3  from each of the pulses r 1  and r 2 , the leading edge of each pulse r 3  coinciding with the trailing edge of a pulse r 1  or r 2 . The various signals so formed are connected to AND gates and flip-flops as shown in Fig. 3. Flipflops M1, M2 produce signals m 1 , m<SP>1</SP> 1 , m 2 , m<SP>1</SP> 2  which are connected to the comparator C1 as shown, VP being a control system. Normally m 2  is the inverse of m1 and shifted by a bit halfperiod. CN normally has a negative voltage output during the bit half-period that the two signals m 1 , m 2  have negative voltages. The signals CN<SP>1</SP>, CN are fed to flip-flops MI with signals r 2  and the output mi of the flip-flop indicates the value of the bit in the previous bit period. Error detection.-A flip-flop MI is used to signal an error reading such as that shown during times t5 to t7 where due to poor contact between the magnetic material and the reading head, the signal does not pass through zero at the centre of a " 1 " reading or at the end of a bit period. This causes a pulse r 1  to occur when a " 1 " occurs on signal CN and indicates an error.