Patent GB-902163-A (GB)
Title: Improvements in systems for reading information from a magnetic record
IPC: G11B20/12
Filed: 1959-06-26
Abstract: 902,163. Reading magnetic tapes. INTERNATIONAL BUSINESS MACHINES CORPORATION. June 26, 1959 [June 30, 1958], No. 21955/59. Class 106 (1). [Also in Group XL (c)] Errors arising from skew of a multichannel magnetic tape are obviated by interspersing synchronizing bits regularly among the data on each channel of the tape, causing a set of data bits on one channel to be read into a data store, on the occurrence of a synchronizing bit gating the store contents to use, and reading the next set of data bits on that channel into an alternate data store. Errors due to speed variations of the tape are overcome by using a variable frequency clock for each channel controlled by the data read from the tape and errors due to tape jitter are overcome by broadening the data pulses from the read circuits and using narrow pulses to gate the information to the data store. Tape reading.-The system of recording preferably utilized is non-return-to-zero in which a 1 is indicated by changing the magnetic condition of the record medium from one state to another. This results in the production of a read pulse to denote the presence of a 1. The information is sensed by a magnetic coil 2 and is fed to a sensing circuit 62 which produces a positive pulse which is inverted by transformer 63 and passed to flip-flop 64 and delay 66. The flip-flop is thus switched on for the period of the delay and the resultant output is passed to phase inverter 68 which produces a positive signal on line 6 representative of a 1 bit and signal on line 8 180 degrees out of phase with that on line 6. The variable frequency clock (described in more detail below) is adjusted at the beginning of a record by a burst of sixteen 1's. Detection of the first 0 indicates start of information and thereafter synchronizing 1 bits appear after every five information bits. The signals on line 6 pass to a blocking oscillator 70 which converts the broad pulse from phase inverter 68 to a very sharp positive pulse at the position of the leading edge of the broad pulse. The output of oscillator 70 passes to a gate 76 and to the clock 74. Gate 76 is opened by a positive D.C. signal on the diamond input whereon a pulse on the arrowed input produces a pulse at the output. Flip-flop 78 is initially set with a positive D.C. output from its 0 side (off), thus gate 76 is open and the first 1 bit of duration of thirteen of the synchronizing 1 bits. Clock 74 emits on line 189 clock pulses, and on line 194 half-period clock pulses, all of which are delayed by lines 90, 106 to place the sharp clock pulses at the middle of the time interval to which they refer. After the first 1 bit pulses from delay line 90 pass through gate 86 to gate 84. When multivibrator 80 stops, gate 84 is opened and clock pulses subsequent to the thirteenth one bit of the synchronizing burst condition the A.C. input of gate 94. The first 0 bit, signalled by high potential on line 8, causes gate 94 to emit a pulse switching on flipflop 100. This opens gates 96 and 108. Thereafter half-period clock pulses are passed to a counter 110 which produces an output every sixth input pulse and on the first 1 bit after the 0 bit which signals the end of the synchronizing burst. An output from the counter switches flip-flop 112 off to block transfer of a synchronizing pulse to a register. Information pulses on line 6 switch on flip-flop 120, the output of which is gated to flip-flop 112 to switch it on. Flipflop 112 off, conditions gate 128, while the flipflop on, conditions gates 148, 150 and 152. There are two shift registers A and B (Fig. 2c, not shown) for each information channel, having a capacity of five bits. Flip-flop 130 determines which register is used and is switched by the output from gate 128. This output is also gated to flip-flop 142, the output of which indicates that the register is full. While flipflop 112 is on signals on line 6 indicating 1 bits are gated through gates 154 or 156 dependent on whether A or B register is in use and signals on line 8 indicating 0 bits are gated through gates 162 or 164 to shift the register. Variable frequency clock.-The frequency of a multivibrator 182 (Fig. 3) which produces clock the synchronizing burst operates single shot multivibrator 70 and flip-flop 78 to switch it on, closing gate 76 and opening gate 86. Multivibrator 80 operates to close gate 84 for the pulses and half-period pulses on lines 189 and 194 respectively is varied by a D.C. signal on line 184. The signal derives from a time discriminator 198 to which the inputs are information and synchronizing pulses on line 72 and a sawtooth waveform from generator 196 driven by the half-period pulses. If the pulse is at the centre of a sawtooth wave, the output of the discriminator is a zero voltage. Variations in the relative positions of the pulse and sawtooth result in variations in the output voltage which are transmitted to the screen of the multivibrator through the stabilizing network 200 and D.C. amplifier 202. Detailed description of the circuits which in part utilize transistors is given. Specifications 748,996, 802,657, 844,308, 857,313 and U.S.A. Specification 2,954,528 are referred to.