Patent GB-1059238-A (GB)
Title: Facsimile communication system
IPC: H04N1/403
Filed: 1964-11-30
Abstract: 1,059,238. Copying by scanning. RANK XEROX Ltd. Nov. 30, 1964 [Dec. 11, 1963], No. 48602/64. Heading H4F. An automatic level control in a facsimile reproduction system incorporating a flying spot scanner CRT operating in conjunction with a photo-multiplier, Fig. 1 (not shown), functions such that (a) the background level of the video signal for any particular scanned document is maintained relatively constant, the background levels of the image video signals for changes resulting from different document backgrounds being instantaneously adjusted, (b) the extraneous noise signals generated by the accompanying photo-multiplier and document paper texture are eliminated by producing a noise clipping circuit operative at a level derived and measured from an established background level for the transmission system, and (c) the signal pulses that are indicative of high resolution pictorial information are distinguished from the background noise pulses. In the automatic level control, Fig. 2, the video input signal from the photo-multiplier is fed into a video amplifier 25, the amplified signal being then conducted into two major branches of the control by way of conductors 26 and 27. In the first branch, the composite signal is fed to a first background level reader 28 wherein the signal is measured to determine the voltage level of that portion of the waveform which represents background noise and then to a second background level reader 29 wherein a further refinement relative to the background noise of the measurement of the first reader is made. Circuits 28 and 29 are operative to determine whether background level changes are intelligence signals, noise, or a change in background, circuit 29 having a slow response to changes of background as may occur from document to document and circuit 28 having a fast response to fast changes in the background level such as may occur on a single document. The resultant signal is fed to a CRT brightness control amplifier 30 to vary the brightness of the light spot in the CRT (4), Fig. 1 (not shown), to compensate for variations of the document background and thus to maintain the background voltage at a constant level. In the other branch the output of the amplifier 25 is fed to a noise clipping level gate circuit 31 operative in conjunction with the background level amplifier 32 to modify the video signal to eliminate the background noise from the composite signal. The signal from circuit 31, which is indicative of the intelligence information, is conducted to a filter and video amplifier 33 where there is separated and amplified the alternating current component of the video signal. This component may comprise the minute signal produced by scanning images that approach the limit of resolution of the scanning system. The amplified video signal is fed into a video trigger circuit 34 where the video signal is converted from analogue to binary, thus enhancing the high resolution pictorial information. In burst gate 35 the binary signal is gated with a blanking pedestal and synchronizing burst circuits to produce a composite video signal suitable for transmission. The video signal output of amplifier 25 is also conducted to black to white delay circuit 36 which compares the composite video signal input signal with the signal from the video trigger circuit 34. The signals from the delay circuit 36 and trigger circuit 34 are used as the control signals for the background level readers 28 and 29 to determine which part of the video signal is to be considered background. The video signal waveform entering circuit 28, i.e. at point A, Fig. 5a (not shown), includes background noise (44), character information (45), solid material of intelligence (46) and some high resolution material (47), the signal being able to vary between ground potential and the control's bias voltage, e.g. -12 volts. The signal further includes voltages (48) illustrating general background and voltages (49) which are shifted slightly toward ground potential as indicative of two dark areas of document background occurring for example when the electron beam scans coloured areas of the document. In the detailed embodiment described, Fig. 3 (not shown), the brightness of the CRT is controlled by means of amplifier 30 fed with signals from readers 28 and 29, within the range -9À5 volts to -10À5 volts, e.g. within voltages 48 and 49, so that the effect of a darker background which caused the background level to go positive in the first instance is reduced. However, where the resultant background level extends out of the -9À5 volt to -10À5 volt range, citcuitry is provided to determine whether the voltage level changes are intelligence or a change in background, and where it is determined to be a change in background condition, the circuit level is adjusted to eliminate this disturbance from the video signal by means of control signals from circuits 36 and 34 which are fed to the readers 28 and 29, see later. The noise is removed by clipping circuit 31 at a voltage more positive than background level (72) and as the background level is not constant the level at which noise clipping is accomplished (72d), is a voltage which is more positive by a percentage than the actual background level. The noise clipping level, e.g. (72e), is determined in background level amplifier 32 and combined with the video signal at point C to produce at point F a voltage having the waveform (96), Fig. 5b (not shown). The video signal at the output of trigger circuit 34, Fig. 5d (not shown), does not include provisions for the changes in document background from lighter to darker areas, e.g. from portions 48 to 49. In operation, the black to white circuit 36 by comparing the video signal with the background level voltage, modifies the video trigger circuit 34 output and is used as the control input to the first background level reader 28. In this way, the noise clipping level of the first background level reader 28 distinguishes between noise and high resolution signals. Further the output trigger circuit 34 determines when the scanned document is black or dark and not the result of a change in background, i.e. at points (73), Fig. 5a (not shown), the trigger circuit determines whether the rise is indicative of a background change or the introduction of character intelligence. The receiving and recording section of the facsimile reproduction system, Fig. 4 (not shown), incorporates a flying spot scanner CRT and a xerographic drum having a photoconductive surface which retains an electrostatic change, a powder image being transferred to a continuously moving web.