Patent GB-1138609-A (GB)
Title: Improvements relating to the handling of digital information signals
IPC: H04L25/49
Filed: 1966-06-13
Abstract: 1,138,609. Self-clocking. RADIO CORPORATION OF AMERICA. 13 June, 1966 [29 June, 1965 (3)], No. 26260/66. Heading G4C. In self-clocking signals a transition in the middle of a bit cell is recognized as representing one binary value and a transition between adjacent bit cells is recognized as representing the other binary value. Bit signals in NRZ form are shifted from a shift register by clock signals, and converted by means responsive to the clock signals, to a form in which each 1 is represented by a transition in the middle of a bit cell and a transition occurs on the boundary between two adjacent bit cells both representing 0, these being the only transitions. In this form, the signals are recorded on magnetic tape. On read-out, the clock signals are extracted and used to shift the data signals, since reconverted into NRZ form, into a shift register. Conversion from NRZ prior to recording.-In Fig. 2 one clock pulse a occurs in the second half of each bit time of the delayed data at b, a pulse d from AND G1 reversing recording flipflop TF for each 1 bit via OR G2. Due to halfbit delay D2 and inverter I1, each 0 bit which is immediately followed by a 0 bit causes AND G4 to reverse flip-flop TF. Fig. 7 (not shown) shows a modification in which by means of two clock pulse trains derived from a single train and having pulses at the middle (c) and end (d) of each bit time respectively, gating the NRZ data through two flip-flops (F1, F2) in series, the data is provided in true form with half-bit delay to reverse a recording flip-flop for each 1 bit, and in inverse form with one-bit delay to enable a gate (G5) to reverse the recording flipflop for any 0 bit provided the next bit is also 0 (undelayed inverted input to the gate). Conversion to NRZ after read-out.-In Fig. 4, each transition in the read data at 25 produces a pulse at b to synchronize bit-time oscillator 36 via the network comprising ANDs G5, G6, halfbit delay D3, and OR G7, the feedback paths 40, 42 (the latter via half-bit delay D4) to ANDs G5, G6 causing a sync pulse to be applied for each transition at a bit cell boundary and centre respectively. Three-quarter-bit delays D5, D6 produce at f, e a pulse in the first and second halves respectively of each bit time. The " first half " pulses at f gate the data input a in true and inverse form via half-bit delays D7, D8 for comparison with the undelayed data a in ANDs G10 to G13 enabled by the " second half " pulses at e. Flip-flop F1, providing the NRZ output, is set if gates G10 to G13 indicate the signal level is different in the two halves of the bit time and reset if they indicate it is the same. A drop-out error signal is produced at 44 if neither a set nor reset signal occurs during a bit time. The recorded message starts with a sequence of all is (or all 0s) to initially synchronize the oscillator 36. Fig. 11 (not shown) shows a modification in which a half-bit-time oscillator is used, followed by a flip-flop for frequency halving, and another flip-flop performs the delaying function of delays D7, D8.