Patent GB-853202-A (GB)
Title: An electrical system for reproducing information from a record medium such as a magnetic tape
IPC: G11B20/14
Filed: 1957-07-30
Abstract: 853,202. Digital electric calculating and storage apparatus. SPERRY RAND CORPORATION. July 30, 1957 [July 30, 1956], No. 24063/57. Class 106 (1). In a system for reproducing binary information pulses recorded on a record medium according to a phase modulation method, a circuit including an oscillator produces from information signals, obtained in response to relative movement between the record medium and a transducer, regular sprocket pulses which control the sampling of the information signals to produce read-out signals indicative of the phase of each of the recorded pulses. As described, binary information read from a magnetic tape or wire is passed via a pair of alternately operating buffer shift registers, e.g. to a computer. Tape-reading circuit.-Digits " 1 " and " 0 " are represented on the tape 10, Fig. 2, by square waveforms, Fig. 1 A-C, 180 degrees out of phase with each other. If the information (such as that shown in Fig. 1 C) is closely packed, the play-back waveform from head 16 will be as shown in Fig. 1 E. This waveform after amplification, is passed to a sampling gate 18 and also to a differentiating circuit 19 whose output, Fig. 1 G, after being amplified and shaped in circuit 20 (see Fig. 1 H) is passed to a further differentiator 21 whose output is rectified at 22 so as to provide a series of pulses, Fig. 1 I, one for each change in state of the recorded waveform, Fig. 1 C. These pulses are used to control a flywheel oscillator 23, e.g. as described in Specification 808,247, so that it produces regularly spaced timing pulses on line 24 which are halved in frequency by binary counter 25 whose output supplied via pulse former 28 to line 27 provides sprocket pulses, Fig. 1 F, for sampling the playback waveform (see Fig. 1 E) at the mid-point of each digit interval. The sampling gate 18 comprises a transformer circuit (Fig. 3, not shown) which provides a pulse on the " 0 " or " 1 " output 34 or 35 according to whether the play-back waveform is positive or negative during the sampling period, the "1" pulses being supplied to output terminal 43. Blocks of information on the tape, such as 13, are separated by single "1" sentinel pulses 12, 14, and groups of " 0 " control pulses 11, 15 which serve to lock the oscillator 23 in frequency and phase. An information block is assumed to comprise 720 digits which are counted at 42. Supposing that the tape is initially at rest with head 16 opposite a control pulse group, tape movement is begun by a start signal which is applied also at 29 to trigger a one-shot multivibrator 30 which then opens gate 32a. " 0 " signals should then be read from the tape; if a pulse is obtained on " 1 " output line 35 indicating an incorrect phase setting of the binary counter 25, this pulse is passed to the counter via gate 32a and delay 32 thus bringing the sprocket pulses on 27 correctly into phase with the play-back waveform. When multivibrator 30 flips back it opens gate 33 and zeroizes counter 42 through line 52. The next " 1 " pulse on line 35, viz. a sentinel pulse, will set bi-stable flip-flop 37 to condition a in which it opens gate 39 via line 38 and buffer 49. The counter 42 then receives both the " 0 " and the "1" pulses from 18 via buffer 41. When 720 such pulses have been counted, the counter applies and end-of-block signal on line 47 which resets flip-flop 37 but keeps gate 39 open via line 47a. After the next pulse (which should be a " 1 " sentinel pulse) is received, the counter applies a signal to line 50 which still holds gate 39 open and also opens gate 44; if the pulse represented a "1" it will therefore pass from line 35 (after being delayed by means not shown) through this gate to block check output 44a. This output may be used to stop the tape, and the next pulse from 18 (which should be a " 0 " control pulse) steps on the counter so as to remove the signal on 50 and close gate 39. Lines 34, 35 are connected to inhibiting inputs of check gate 45, so that only if no output pulse appears on either line in response to a sprocket pulse on line 27, the latter pulse is allowed through the gate and a gate 40 (opened by output a of 37 while information pulses are being read) to error terminal 46. Input to computer.-The "1" pulses at output 43, Fig. 2, are applied via terminal 61, Fig. 4, and transistor switch 57 or 75 to a shift register Y or X (not shown) respectively which is stepped by the sprocket pulses on line 27 applied via terminal 63 and a corresponding switch 58 or 74. When flip-flop 37, Fig. 2, is set to a the pulse on line 38 is applied via pulse former 52 and buffer 53 to set dynamic flip-flop 54 to condition b; transformer winding 55 is then energized thereby energizing windings 59 and 60 and rendering conductive transistors 57 and 58. The information pulses are therefore passed initially to register Y; when Y is full, e.g. after receipt of 12 digits, a signal at 65 resets 54 to a thus producing an output pulse which is passed via gate 66 (opened by the signal on 38) to set dynamic flip-flop 70 to b thus energizing the switch circuits for register X. When X is full, a signal at 78 similarly causes a switch back to register Y. While one register is being filled, further similar switching circuits (Figs. 5 A and B, not shown) cause the information to be stepped out of the other register under control of computer clock pulses. Multi-channel readout.-Where the tape has more than one channel, the read-out circuits are duplicated, and a coincidence gate (Fig. 6, not shown) ensures that the information on parallel lines is entered simultaneously into the computer even if, e.g. owing to mechanical skew, it is entered non-simultaneously into the corresponding shift registers.