Patent GB-986645-A (GB)
Title: Improvements in or relating to digital recording systems
IPC: G11B20/14
Filed: 1962-05-30
Abstract: 986,645. Digital data storage. POTTER INSTRUMENT CO. Inc. May 30, 1962 [June 2, 1961], No. 20832/62. Heading G4C. In a method of recording digital data, each item is recorded as a predetermined combination of transitions and absences of transitions, each item occupying n possible transition positions, there being for each data item at least one transition; and in that sequence of data items giving the longest sequence of absences of transitions there are at least two but not more than 2n-2 successive absences of transitions. In one recording method a cell (left hand side of Fig. 1), for recording on a drum, disc or tape a data item has two possible transition positions, at the initial cell boundary, and at the cell mid-point. Such a method can represent binary data in this way (marked "Barber binary", "Gabor binary") or can represent ternary data with digits 0, 1, 2 being represented by the illustrated combinations of transitions (solid line) and absences of transitions (wavy line). In a second recording method, "two bit binary", right hand of Fig. 1, there are three possible transition positions in a cell, digits 0, 1, 2, 3 being represented as shown. In an arrangement, Fig. 3, for recording 6-bit binary numbers in the range 0-63, a number to be recorded is supplied by a source 1, as a 6-bit parallel number of lines 2 via a first diode matrix converter 3 (Fig. 4, not shown) which provides a 1-outof-64 output to a second diode matrix converter 5 (Fig. 5, not shown) whose output represents the input in a "4-digit binary coded ternary plus 1" form, right hand of Fig. 2, this code being derived by coding numbers O-63 in 4-digit ternary code as shown in Fig. 2 and coding each ternary digit in 2-bit binary form with one added, so that ternary digits 0, 1, 2 are represented as 01, 10, 11 respectively. The four ternary digits are recorded in parallel by four heads 19, Fig. 3, each ternary digit being recorded in the form shown at bottom left of Fig. 1, timing pulses being provided by a clock source 29 at twice the cell rate to a single-stage binary counter 25 so that pulses on output lines 27, 24 represent cell-boundary and mid-cell times for controlling AND gates such as 8, 11 in the inputs to the recording heads 19. Read-out circuits. Data recorded in the "binary coded ternary plus 1" form may be read out by the circuit of Fig. 8, clock signals being produced from the data signals by a circuit 57 shown in detail in Fig. 6. The read-out waveform is applied to a terminal 31 connected directly and via full and half cell period delays 36, 34, Fig. 6, to an OR gate 33 to produce an output with a pulse for every cell boundary and mid-point which is applied to an AND gate 39 together with a gating waveform derived by feeding back the output of a flip-flop 41 so that the final output at 56 consists of clock pulses at cell boundaries only. Assuming that the digits shown at the top of Fig. 9 are read out, the corresponding signal is applied to the input terminal 31, Fig. 8, clock pulses produced by the clock recovery circuit 57 being delayed at 58 and causing a generator 60 to produce a pulse train P4, Fig. 9 effective to gate mid-cell signals only at an AND gate 74, the output of this AND gate being further gated in an AND gate 76 with a signal derived from the input signal at 31,the arrangement being such that the output of the AND gate 76 passes mid-cell signals only when they are not preceded by a cell boundary pulse, i.e. the output represents ternary "2". To recover "0"s and "1"s, the pulse train P4 is inverted at 64 the inverted train being applied together with the input signal from terminal 31 to an AND gate 66 to provide an output consisting of cell boundary signals, this output, together with an output from an AND gate 85 consisting of all mid-cell signals which are preceded by a boundary pulse, being applied to an OR gate 69 whose output will represent "0"s and "1"s in the signal at 32. A circuit for recording and reproducing the two bit binary code, right hand of Fig. 1, is described with reference to Figs. 10 and 11 (not shown). A conventional tape drive arrangement is described with reference to Fig. 12 (not shown).