Patent GB-1282358-A (GB)
Title: Improvements in magnetic tape read out signal processing systems
IPC: G11B20/20
Filed: 1970-06-01
Abstract: 1282358 Digital data storage COMPAGNIE INTERNATIONALE POUR L'INFORMATIQUE 1 June 1970 [3 June 1969] 26297/70 Heading G4C Digital data is recorded on magnetic tape using phase modulation recording. Means are provided for eliminating non-significant pulses (see below) and correcting and/or detecting errors. Digital data is recorded on as many parallel tracks as there are digits in a character and each track has a twin magnetic head which produces two series of pulses (see below). Phase modulation.-A binary 1 is a transition between a lower and a higher magnetization level and a binary 0 is a transition between a higher and a lower magnetization level. These transitions are known as bit significant transitions. When two successive bits have the same binary value then a transition is required between them and these transitions are known as non- significant transitions. On read out a first output produces a series of pulses 1 + N representing transitions from lower to higher levels while another output produces a series of pulses 0 + N representing transitions from the higher to the lower level. Apparatus.-This comprises a first subassembly A which cancels all non-significant pulses from the two series. A circuit B takes magnetic skew into account (though this may be omitted) and re-conditions the bit-significant pulses for input to a second sub-assembly. The first sub-assembly also comprises a timing control circuit C and a circuit H for detecting any positioning error pulses. The second sub-assembly comprises a shift register R, a trigger circuit D for controlling entry of data into the register and the progression of the data therethrough, a circuit G for detecting and compensating, when possible, for any loss of information in the register, and a circuit F for controlling the transfer of the contents of register R to the outputs supplying the reassembled characters. The second assembly is directly controlled by the first. Also provided is a counter 17 and a parity control (not shown). Operation.-For a perfect track an initial 1 is on the track and this is read off as a pulse in the sequence 1 + N which is transmitted through gates 1 and 3 as the first pulse of series 1 + 0 to bi-stable 4 which is then set to block gate 1. The bi-stable is subsequently reset so that only the non-significant pulses are blocked. Simultaneously the series 1+0 feeds bi-stable 6 and OR gate 5 which enables AND gate 13, the other input of which is connected to a pulse generator (not shown) whose frequency varies as the tape speed. The output of AND gate 13 feeds counter 17 which counts the cycles HFV of the generator and which is reset to zero whenever a pulse in the 1+0 series appears at OR gate 3. When there are no bits missing counter 17 repeats a cycle (Fig. 2, not shown) but, when a bit is missing, outputs P 3  and P 4  are energized and circuit G is energized. If only one track is faulty the parity control device determines whether the parity in one character is erroneous and correction occurs. If more than one track is faulty the tape processing halts and the tape is rejected.