Patent GB-1249873-A (GB)
Title: Discharge lamp modulation system
IPC: H04N1/40
Filed: 1968-10-14
Abstract: 1,249,873. Facsimile apparatus; light modulation. MATSUSHITA ELECTRIC INDUSTRIAL CO. Ltd. 14 Oct., 1968 [16 Oct., 1967], No. 48670/68. Heading H4F. To improve the quality of reproduction in a facsimile system a high-brightness discharge lamp e.g. a xenon lamp, is energised by (a) a sampled pulse signal Fig. 4(b) derived by sampling an input signal Fig. 4(a) at a frequency which is higher than the input signal frequency, or (b) a signal Fig. 4(b) derived by detecting the input signal Fig. 4(a) when amplitude modulated by a frequency higher than the input signal frequency, or (c) a pulse-width modulated signal Fig. 6(d) derived by pulse-width modulating input signal 6(a) with a frequency higher than the input signal frequency. The optical outputs of the discharge lamp for these methods are shown in Figs. 4(d), 4(e) and 6(e) respectively. In each case a discharge sustaining current is supplied to the lamp, and the width of each pulse applied thereto is shorter than the duration of the transient rising portion of light emission from the lamp. In the facsimile system described an original picture on a rotating drum is scanned and the modulated output beam of the discharge lamp is scanned across a recording sheet by means of an oscillating mirror, Fig. 1 (not shown). In the pulse-width modulation method a xenon lamp 17, Fig. 5, is connected to a highvoltage generator 14 supplied by a D.C. power supply 13, and to a variable resistor 20 via a resistor 18. A switch 19 is connected in parallel with variable resistor 20 and mechanically through relay 16 with a starting switch 15 for the generator 14. A transistor 21 is connected in parallel with resistor 20 and an input video signal Fig. 6(a) is applied at input 22, added to a sawtooth or triangular waveform Fig. 6(b) from generator 27 to produce the waveform of Fig. 6(c), the D.C. level E of which is adjustable by means of resistor 25, sliced at levels El and E 2 , and shaped to produce a pulse-width modulated signal having a waveform as shown in Fig. 6(d) which is applied to the base of transistor 21. In operation when starting switch 15 is closed, a high voltage is applied to lamp 17 and relay 16 is energized thus closing switch 19. With transistor 21 cut off i.e. effectively not present the current for lamp 17 is solely limited by resistor 18 so that a large starting current is supplied to lamp 17 to initiate an arc discharge. In response to this arc discharge relay 16 is deenergized and switch 19 opens causing resistors 18 and 20 to limit the lamp's current. Thus a bright luminescence occurs when resistor 20 is short-circuited while a dark luminescence occurs when it is not short-circuited, so that the switching operation of transistor 21 in response the signal waveform of Fig. 6(d) produces a bright or dark luminescence depending on the width of pulses in the signal and the lamp delivers an optical output as shown in Fig. 6(e). By energizing the discharge lamp by a train of slender pulses in the no-signal period p-q, the luminous discharge is stabilized. A power unit for three discharge lamps which may be utilized in a colour facsimile system is described with reference to Fig. 8 (not shown).