DE2039935A1 - Method for the transmission of facsimile signals with halftone properties - Google Patents

Description

Patent attorneys Dipl.-Ing. F. Weickmann,

Dipl.-Ing. H. Weickmann, D1PL.-PHYS. Dr. K. Fincke Dipl.-Ing. F. A. Weickmann, Dipl.-Chem. B. Huber

8 MUNICH 86, DEN

PO Box 860 820

MÖHLSTRASSE 22, NUMBER 48 39 21/22

XEROX CORPORATION, Rochester, F.Y "14605, T.St.A.

Method for the transmission of facsimile signals with

Halftone properties

Many known facsimile transmission methods work with two signal levels, the one on a document or the like. contained and evaluated information before it is transmitted converted into an analog or digital two-level signal will. Such a conversion into a two-level signal is carried out for data encoding prior to transmission or to reduce the bandwidth required, for example, for the transmission of a written page. Contains the document all black and white information, such as typewritten or a design drawing by f If so, the two-level transmission is economical and rational. On the other hand, they contain photographs or other original images Gray level values, which in facsimile transmission are either as Black or white values can be reproduced. This effect can lead to a loss of background information and / or at. a distant scribe can produce an entirely black or white signal, which makes up a large part of the structure and the Tint of the original document is lost.

The object of the invention is to provide a transmission

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to create the method described above Avoids disadvantages and enables a transmission of halftone properties in two-level facsimile signals.

Such a transmission method is designed according to the invention to achieve this object in such a way that the video signals are composed with a triangular or sawtooth signal of a predetermined frequency and that the composite Signal is cut off at a predetermined Sohwellwert, whereby increased signal transitions in the range of the threshold value be generated.

The analog data signal generated by the photodetector of a facsimile abuser can be superimposed with a triangular signal, the composite signal then generates increased two-level pulses when the analog signal is in a range close to the threshold value. This means that the signal amplitudes around the threshold value a number signaled · le instead of a sharp transition from white to black or from black to white information generating ₀ If this information is written to a remote recorder, as seen at its FaksiimLekopie a halftone effects

^ The invention is described below in terms of its further The advantages and characteristics described using the figures show:

Figol is a block diagram of a facsimile transmitter operating according to the invention,

Fig. 2 is an illustration of normal facsimile transmission with a trigger or threshold level and

Figo3 shows the signal curves for understanding of the method according to the invention.

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In Figo1 there is a block diagram of one according to the invention working facsimile sender. A facsimile abbot The most well-known type generates a beam of light that passes through a Document is reflected and modulated according to its information o The reflected light is incident a photodetector 10, which converts the modulation information into electrical signals. The photodetector 10 may be of a known arrangement, for example a phototransistor or a photomultiplier tube The signals generated by it are fed to a video amplifier 12 of known type, which they for the subsequent Circuits reinforced. There are also not shown timing circuits, quantization or limiter circuits intended for amplitude limitation, temporal quantization or digitization of the facsimile signals, they can be included in amplifier 12

Up to this point, the arrangement shown in FIG. D is known. FIG. 2 shows the signal processing in such a known arrangement. FIG _o 2a represents the bi-level video signal generated with the information contained in the video amplifier 12 circuits _o From the video amplifier 12, this signal is supplied to the Schmitt-trigger supplied to 16, the signals of in Fig type shown "2b of the parts of the signals in Fig _o 2a generates, the rise above the trigger level These signals are then fed to the modulator 18, which can be designed in a known manner for transmission via a line, a telephone channel or radio channels, microwaves, etc. The modulation methods that can be used are amplitude modulation, frequency modulation, pulse modulation, etc. _o Fig · 2c shows a video signal of a continuously toned image at the point where it intersects the trigger level with a signal transition from white to gray to black. If the white signal approaches the trigger level, it becomes the Schmitt trigger

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is not driven, and its output signal appears at the low level shown in Figure 2d. If the signal intersects the trigger level, the Schmitt trigger 16 generates the _{upper signal level shown in Mg 0} 2d Independent of the form of the image signal below the trigger level, the output signal of the Schmitt trigger has the lower level, while every deflection above the trigger level has the Generation of the upper signal level shown in Pig «2d effected In this way a continuous tone signal of the kind shown in Fig. 2c is transmitted as a two-level signal, and the facsimile copy produced thereby contains only black and white characters without shades of gray A detailed description of this known type of facsimile transmission can be found in U.S. Patent 3 H9,201.

In Fig.3b is a generator with the one shown in Fig * 1 14- generated triangle or sawtooth signal shown · Has the signal of the video reception more 12 In the case of a continuous toned image, for example that shown in Fig. 3a Curve, the superposition of the signals shown in FIGS. 3a and 3b results in the signal shown in FIG is a combination of the continuous tone video signal and the triangle or sawtooth signal. It will the Schmitt trigger 16 shown in FIG. 1. The clipper level shown in FIG. 30 is the trigger level of the Schmitt triggers. FIG. 3d shows the output signal of the Schmitt trigger 16 when activated with the signal shown in FIG. The part of the signal containing the white information has a corresponding level, given the amplitude of the added Three-way signal · still below the trigger level of the Schmitt-Triggers is “However, the white signal increases to the black signal via the gray tones, so »classify periodic signal duration of the Dreieoksignale through the Clipper level with the frequency of the triangle signal. Approaching

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If the _{signal shown in PIg 0} 3a is the black level, a larger and larger part of the triangular ocular appears above the oil lip level, until the black level is reached, when the entire signal is above the clipper level and only contains black information to recognize that between the signal state white and the signal state black the upper peaks for a small percentage of the black signal and then the lower peaks for the larger percentage of the black signal cause the generation of signals with variable width by the Schmitt trigger · With the writer the reception · »^

station, all signals are usually either black or white, with no gray tones · However, since the signal transitioning from white to black is in If a small black signal is split, the viewer sees an irregular, split black signal that shows a Creates a halftone effect or a shade of gray.

With the facsimile scanner, a normal black and white Sohrift piece is used sampled without shades of gray, the generator for the three-way signal has no effect on the output signal, there between the black and white signals of the potentiometer there are sharp transitions · However, a document with a continuously toned image will appear- ™ probes, there are no sharp signal transitions, but different shades of gray. The method according to the invention then causes a conversion of the continuously tinted image into a signal curve with constant prequena variable duty cycle »so that on the written facsimile copy a halftone effect is achieved · ■

The method described above for implementing a continuous and variable video signals into one waveform with variable duty cycle and constant prequena

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creates a halftone effect in facsimile transmission of documents. A triangle or Called a sawtooth signal, but any other signal can also be used which generates transitions in the range of the Olipper level of the video signal. The invention can therefore be implemented in the most varied of ways by Faohmann without deviating from their basic idea · Furthermore, changes and further developments can be made, which as a whole are encompassed by the basic idea of the invention

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Claims (3)

Claims 1 * Method of reproducing continuously toned images with a two-level facsimile signal with random Signal transitions, characterized in that a continuously changing part of the video signal with a Triangular or sawtooth signal of predetermined frequency is superimposed, whereby the signal transitions in the range a threshold value are increased and a signal of constant frequency with a variable duty cycle is created, that has semitone properties. | 2. The method according to claim 1, characterized in that the overlay signal is clipped at the threshold value will. 3. The method according to claim 2, characterized in that for the periods in which the overlay signal exceeds the threshold value, signals with a constant peak value are generated, the generation of which is carried out by the Triangle or sawtooth signal is concentrated on the area where the beat signal is close to the Threshold value lies. ORIGINAL INSPECTED 10 9 8 1 «/ 1 3 R 9 first page Copy