GB1518177A - Method and device for producing binary signals from an original by means of a television camera - Google Patents

Abstract

1518177 Facsimile LICENTIA PATENT VERWALTUNGS GmbH 24 Oct 1975 [24 Oct 1974] 43792/75 Heading H4F When an original black and white document is scanned by a television camera the video signal contains undesired grey level amplitudes (for example due to uneven illumination or coloration of the document, differential spatial sensitivity of the camera tube and optics, and the finite size of the electron beam when scanning fine detail) which should be removed prior to transmission via a digital channel, e.g. in a high speed facsimile system using a telephone channel. A first variable threshold signal 4 is therefore produced by passing the signal 1 through a low-pass filter with a cut-off frequency of 2-3 times line frequency, and the amplitudes of signals 1 and 4 are compared to produce a corresponding two-level signal 5. During the blanking period 2 the filter is preferably rendered inoperable. If a substantial part of a line contains no image detail, signals I and 4 tend to become equal and the signal 5 may assume the incorrect value. A second threshold signal 7, of fixed waveform and an amplitude falling between these for black and white, is therefore produced in response to the blanking or sync signals and is compared with the first threshold signal 4 (6) to give a signal 8 of one of two levels depending on whether the line is white or black. Signals 5 and 8 form the inputs to an OR gate producing the final two level signal for transmission. The signal 7 may be at a fixed DC level, but, as shown, includes line and field frequency components so that it corresponds closely to the signal obtained by scanning a uniform grey line on the document.