Patent GB-1467877-A (GB)
Title: Demodulation system for digital information
IPC: G06K7/016
Filed: 1974-06-19
Abstract: 1467877 Reading recorded data TOKYO MAGNETIC PRINTING CO Ltd 19 June 1974 [27 Nov 1973] 27163/74 Headings G4C and G4M Digital data on a recording medium, e.g. a magnetic card or optical bar coded medium is demodulated by (1) generating an electrical signal a (Fig. 5) representative of the data in a bit frame, (2) generating a signal e representing the duration of the preceding bit frame (that is representing the speed of the medium), (3) generating for each bit frame a clock pulse f the duration of which is dependent on the value of the signal e and (4) using the clock pulse f to demodulate the data to provide an output signal g. In the embodiment of Fig. 6, slightly displaced signals a, b, c and a<SP>1</SP>, b<SP>1</SP>, c<SP>1</SP> (see Fig. 5) are derived for each transition of a frequency modulated recording. One of the signals, b<SP>1</SP>, is used to generate a sawtooth signal d which is sampled in an analogue memory M at the signal a<SP>1</SP> to derive the signal e representing the duration of the preceding frame. This is compared with the signal d in a comparator 17 which at relative predetermined values, e.g. 4 : 3 provides a reset pulse to bi-stable 13, set by signal c<SP>1</SP>. The output f of the bi-stable 13 is used to clock gates 18a, 18b receiving respectively signals b, c and setting and resetting respectively a bi-stable 14, the output from which is the demodulated data signal g. In a modification (Figs. 7 and 8, not shown) the output of the analogue memory drives a voltage controlled oscillator (28), the frequency signal from which is such that each bit frame has a constant predetermined number of pulses (since the amplitude e is inversely proportional to the frame duration and consequently the frequency is also inversely proportional to the duration). A counter (29) provides a clock signal at this predetermined count to control the gates. In a further embodiment (Fig. 10, not shown) the voltage controlled oscillator and the counter are replaced by a monostable multivibrator (40) providing a pulse width proportional to the preceding frame. In a fourth embodiment the pulses from an oscillator 44 (Fig. 12) are counted in a counter 43, the count at the signal a being stored in a latch 42 providing via a digital/  analogue converter an analogue signal e to control a monostable vibrator 40. In a fifth embodiment (Fig. 13, not shown) an oscillator (51) drives via a divider (52), providing three pulses for every four input pulses, a counter (54). The counter (54) is set to zero at each pulse signal d<SP>1</SP> and its count at pulse signal a<SP>1</SP> is transferred to a second counter (53) which is then decremented by the oscillator, a "borrow" pulse being generated as the clock pulse when the count goes below zero. In a sixth embodiment (Fig. 15, not shown) a first divider (57) provides one quarter of the input pulses from an oscillator (51) to a counter (54) reset by pulse b<SP>1</SP>. A second divider (56) also connected to the oscillator provides one third the number of input pulses to decrement a counter (53) set to the count in the first counter (54) at the pulse signal a<SP>1</SP>, the "borrow" pulse from the counter again providing the clock signal. An alarm to indicate a too fast or too slow movement of the medium may be generated by checking the contents of the counters. The system may also be applied to phaseencoded signals.