DE2720079B2 - Method for improving the image quality in facsimile transmission - Google Patents

Description

The present invention relates to a method for improving the image quality in facsimile transmission according to the type mentioned in the preamble of claim 1.

In facsimile transmission technology, e.g. B. for office fax machines, weather map transmission or Transmission of other documents via lines or radio, the image quality of the recorded again suffers Image due to unconventionalities of the transmission channel and the limited availability standing bandwidth.

A common case is that only black and white information is transmitted, what in the case of documents, line drawings or the like. That is given by the submission by electro-optical The image signal obtained by scanning is a so-called 2-level signal, i. h, the signal only takes 2 states which signal black or white depending on the agreement.

There are short black and white sections within the template on, the scanner also delivers very small, short pulses, but these on in the course of the transmission falsified on the transmission channel and no longer with a recognizable amplitude on the recording side arrive, which is why their information is lost.

If a corresponding gain is made on the receiving side in order to raise these signals above the detection level, a number of errors occur. Echoes or other interference impulses falsify the image information, which is why this measure is not suitable is.

In today's transmission technology is often to save transmission bandwidth or to save To increase transmission speed, a method for bandwidth reduction is used in which

on the scanning side, the 2-level signal into a 3-level signal converted and transmitted as a 3-level signal. This method is for example in the US Patent 33 03 284 or in German Patent 17 62 644.

The 3-level signal becomes the 2-level signal obtained by changing the information, d. H. with each change of the 2-level signal, a flip of the signal, creating 3 levels. The signal values ± 1 and 0 occur, depending on the agreement, ± 1 for the status black and 0 for the status white of the original or vice versa can be assigned.

Due to the halving of the basic frequency achieved with the 3-level signal, the resultant can Signal can be transmitted either with half the bandwidth or twice as fast.

On the receiving side, the signal is regained into a 2-level signal and recorded But it has also been shown with this method that short black / white lines of a template that have already been reduced in their amplitude by the scanning diaphragm, or lines that are wide, but only have an amplitude below the threshold, are lost because the amplitudes these signals are not sufficient to cause a flip from the 2-level signal to the 3-level signal.

In DE-OS 19 25 978 there is also a system for reducing the amount required for signal transmission Transmission bandwidth described, also with the 2/3 level conversion described above works to save transmission bandwidth. In this system, the 2-level signal split up before further processing, the signal portion that is used for flipping, d. H. of the the flip criterion is derived, is given a minimum threshold level, which prevents that weak image signals that have been raised by the division lead to a flip of the

Convert a 2-level signal into a 3-level signal. Also will the division itself is reversed in this signal component, which is why short black / white lines are also used here a template will be lost because the amplitudes of these signals, which are below the, threshold, fall away.

The present invention is therefore based on the object of eliminating these disadvantages and the Improve the image quality of the transmission.

The invention achieves this through the features specified in the characterizing part of claim 1.

Advantageous further developments are specified in subclaims 2 to 6.

The present invention admittedly makes use of those known in transmission technology for contour division Averaging the two-fold differentiation of the image signal uses, but one crucial difference lies in the fact that this is not about the To sharpen the contours of the signal during transmission, but rather that the complete Information of the original is transmitted at the receiving location can be recognized and recorded cleanly.

In the known method of steepening the edges, as it is, for. B. is described in DE-PS 21 14 149, the information is very short during the transmission from the scanning to the recording side Black / white lines or gray lines - as described at the beginning - are lost, which is why those in this one The solution described in the patent is insufficient to achieve the aim of the present invention.

The invention is illustrated below with reference to FIGS. 1 -11 explained in more detail It shows

F i g. 1 a basic circuit diagram for the transmission of a facsimile signal,

F i g. 2 shows an example of the application of the method according to the invention in 3-level modulation,

F i g. 3 a circuit for the pulse splitting,

F i g. 4 is a schematic representation of the FIG. 3 occurring signals,

Fig.5 shows a circuit for converting a 2-level signal into a 3-level signal,

F i g. 6 curves for the well-known conversion of the 2-level to a 3-level signal,

F i g. 7 shows a schematic representation of signals as they occur in the transmission according to the invention appear,

Fig. 8 an example of the transmission by means of Amplitude modulation,

9 shows an example of the transmission by means of Frequency modulation,

F i g. 10 a circuit for demodulating the signal at the receiver,

F i g. 11 Schematic representations of the signals on the Demodulator and at the demodulator output.

F i g. 1 shows schematically a template 1 which is illuminated by a light source 2 and schematically by one scanning element shown, which consists of an optics 3, a scanning diaphragm 3a and an opto-electrical Converter 4 exists, is scanned. The generation of the relative movement between the original and the scanning element was not shown in the present case because it is not the subject of the invention, but more general State of the art.

The image signal reaches a transmission channel 7, which is a line or a radio transmission link, via a pulse shaper stage 5 and a modulation unit 6 can be. A demodulator 8 is provided at the receiving location, which transmits a video signal on line 9 which is recorded as a facsimile signal.

FIG. 2 shows an example of the application of the method in 3-level modulation. That from The signal coming from the scanner 4 arrives at a pulse shaper stage 5, which is linked to stage 5 in FIG. 1 matches, and from there it is transferred to a 2-level / 3-level converter 10 given, from which it arrives at a modulator 6. This modulator can, depending on Use case an amplitude or a frequency modulator. From the modulator 6, the signal arrives at the Transmission link 7.

On the receiving side, as in FIG. 1 a demodulator 8 is provided to which a 3-level / 2-level converter U is connected downstream, which supplies the facsimile signal on line 9

3 shows an example of the internal structure of the pulse shaper stage 5. In the present case, use is made of the two-fold differentiation of the image signal, which is known per se, the 2ma) differentiated signal being fed back to the image signal with the opposite sign.

From the scanner 4, the image signal passes via an RC element, which consists of the capacitor 12 and the operational amplifier 13 with the feedback resistor RK \ 3 , to a second ÄC element, which consists of the capacitor 14, the operational amplifier 15 and the feedback resistor RK \ s composed, via a resistor 16 to the negative input of an operational amplifier 17. The image signal is differentiated once at the first RC element and a second time at the operational amplifier 17, which is connected to the feedback resistor RKu the negative twice differentiated image signal and the image signal arriving via line 18 are added and added to the image signal shown in FIG. 5 2-level / 3-level converter 10 shown in more detail given.

F i g. 4 shows the curves a) and b), the curve a) representing the signal supplied by the scanner 4 and the curve ty representing the image signal at the output of the operational amplifier 17 generated with the aid of double differentiation.

Instead of double differentiation, the known method of scanning the surroundings can also be used, which also results in a division at edges that are parallel to the scanning direction, which leads to the same signal curve b) in which the weak signal trains are raised so far that they are unambiguous are recognized as lying above the threshold value black, which is representative of a change in the information in the original.

5 shows a circuit structure for a 2-level / 3-level converter with a downstream modulator 6, from which the modulated signal is sent to the transmission link 7. As a modulator As described above, both an FM and an AM modulator can be used. Such Modulators can be purchased and are therefore not shown in detail as a circuit

The 2-level signal emitted by the pulse shaper stage 5 arrives at the resistors 20a a comparator 20, which consists of a differential amplifier at the input and an output stage, which as Output signals only emits 2 states 0 V or +5 V, there is a decision threshold at the other input determining voltage of, for example, 4.5 V.

The voltage divider consisting of resistors 20a and 20Z »has the purpose of creating a switching hysteresis generate so that in the special case, when an input signal happens to be present, exactly the threshold signal of 4.5 V, no permanent due to noise effects undesired switching back and forth of the (comparator. The output of the! comparator is connected to a flip-flop 21, to whose outputs OR gates 22 and 30 are connected. the The second inputs of the OR gates 22 and 30 are connected to the output of the comparator 20. The Outputs of OR gates 22 and 30 form the switching inputs of analog switches 23 and 29 which are the positive and an inverted negative image signal

As a comparator, for. B. a module of the type LM 311 from National Semiconductors can be used.

The positive image signal is applied to the switch 23 via a resistor 24 and is either taken directly from the scanner 4 or from the pulse shaper stage S, or there is a constant positive potential at the switch 23 in the event that a black / white transmission is carried out The image signal fed in at terminal 25 is also applied to an inverter which consists of an operational amplifier 26 with its feedback _{resistor RK 26} and a resistor 27 at the input of the operational amplifier. The output of this operational amplifier is via a resistor 28 at the input of the analog switch 29. In the case of a pure black / white transmission, there is a negative constant potential of the same level as would be in positive polarity at the switch 23 in this case.

The switches 23 and 29 can be designed as simple analog switches, e.g. B. as transistors or as a digitally controllable MOS switch, for the z. B. the type IH 5011 module from Intersil can be used.

The operation of this circuit is described in more detail below. The comparator 20, which consists of the operational amplifier 20, may at its minus input z. B. have a voltage of 43 volts. If the input voltage of the comparator 20 at the plus input, which is formed from the output signal of the pulse shaper stage 5, the output voltage Ua of the comparator 20 and the voltage divider from the resistors 20a and 20i>, is less than 4.5 volts, ie less than the black level , then the following applies.

Let the flip-flop 21 not be set; h, at its Q output the value zero and at its Q output the value 1. Since the output of the comparator is also zero, the following applies to the OR gates 22 and 30:

55

Gate 22 has the combination 0/0, Gate 30 the combination 1/0. That is, the switch 23 blocks and the switch 29

is permeable. As a result, the negative

Signal at switch 29 is passed.

60

If the voltage at the plus input of the comparator 20 exceeds the value of 4.5 volts, the voltage increases Output voltage of the comparator t / * to +5 V, and the input of the flip-flop becomes positive, his Outputs remain unchanged, however, as it is only switched by a negative edge. At the entrances the OR gates 22 and 30 but changes occur:

Gate 22 has the combination 0/1,
Gate 30 the combination 1/1.
That is, both gates switch through and so do switches 23 and 29, which has the effect that an equally large positive and negative signal are present at the addition point 31, which leads to a total signal of zero.

If the input voltage at the positive input of the comparator 20 falls below the threshold of 4.5 volts again, the output of the comparator becomes zero, i.e. h “a negative edge occurs, which flip-flop 21 switches. This leads to a combination 1/0 at the output of the flip-flop and to the ports 22 and 30 the following combination;

Gate 22 has the combination 1/0, Gate 30 the combination 0/0,

i.e. switch 23 lets through, i.e. at the exit a positive signal appears.

If the input signal at the operational amplifier 20 again exceeds the threshold value, that remains the case Flip-flop 21 unchanged but other signal combinations occur at OR gates 22 and 30:

Gate 22 has the combination 1/1, Gate 30 the combination 0/1,

i.e. both switches 23 and 29 open, i.e. you

The sum signal is again zero.

The processes just described can easily be seen in the following FIGS. 6a and 6b as well as 7a and 7b check.

In FIG. 5, another operational amplifier 32 is connected downstream of the switches 23 and 29, the feedback resistor RK 23 and the resistors 24 and 28 are dimensioned so that this stage works as a simple adder Signal reaches the transmission line

In order to express more clearly where the difference between the present invention and the known methods of 3-level modulation lies, let us consider a simple case in which only black / white transmission takes place in a comparison of FIGS. Figures 6 and 7 show F i g. 6 shows the transmission without pulse splitting for the flip and FIG. 7 the transmission with pulse splitting for the flip.

Case a) in FIG. FIG. 6 shows a signal supplied by the scanner, which is approximately the same as signal a in FIG. 4 corresponds to It has the signal sequence WS, SW, WS, SW, WS, SW, WS, SW, WS, where WS means white and SW black.

In Fig. 6b shows the signal sequence of the 3-level signal, WS, SW, WS, SW, WS . As can be easily seen, the information about the WS and SW values is in the middle area of the curve according to FIG. 6a has been lost because their amplitudes were insufficient to exceed the threshold value for black, which has resulted in all values being transmitted as white.

Fig.7a shows the same signal train of Fig.6a, but with pulse splitting. It is easy to see here that the mean black values of the curve also decrease Exceed the threshold value for black, whereby these values as Black / white jumps built into the 3-level signal and be transferred.

However, the present invention is not limited

to the pure transmission of black / white pulses, as shown in FIG. 7 specified, but it can be done with this Process also semitones are transmitted. When transmitting using amplitude modulation, shown that the input signal for the comparator 20 of FIG. 5 advantageously the established Image signal and as an input signal for terminal 25, d. H. for the switches 23 and 29 that is not set up Image signal selects what is shown in Figs. 8a, 8b and 8c is shown. ι ο

In the case of frequency modulation, it is preferable to use the established image signal in both cases, so that the terminal 25 can be connected to the input of the comparator 20. The corresponding Curves are shown in Figures 8a and b and 9 shown.

F i g. 8a shows the signal supplied by the scanner 4, FIG. 8b the output from the pulse shaper stage 5 Signal and Fig.8c the signal at the output of the 2-level / 3-level converter of Fig.5. The intersections of the established image signal of FIG. 8b with the threshold representing the black level the fold-over points for the 2-level / 3-level converter, which are drawn as vertical lines in FIGS. 8a to 8c. The curve of FIG. 8c, which supplies the signal supplied by the converter, results from the operation of the switches 23 and 29 in Dependence on the control of the comparator and the downstream OR gates of Fig. 5. To the Places at which the 3-level signal has the value zero assumes, both switches 23 and 29 are open, so that there is a sum signal of zero. In the positive Areas If the switch 23 is open and in the negative areas the switch 29. The F i g. 8c represents the case that the analog switches 23 and 29 the non-erected image signal of Fig. 8a, be it in positive or negative polarity.

The F i g. 9 shows an output signal of the 2-level / 3-level converter, in which the established signal of the ίο Fig.8b is given to the analog switches 23 and 29 becomes what z. B. when using frequency modulation is advantageous.

F i g. 10 shows the receiver circuit consisting of a demodulator 8 and a 3-level / 2-level converter 11. For the demodulator, a commercially available Circuit come to use, as z. B. in the textbook "Electrical communications engineering" volumes I and II, D. Ing. Heinrich Schröder, Verlag für Radio-Foto-Kinotechnik, Berlin-Borsigwalde, described is.

The 3-level / 2-level converter 11 consists of a full-wave rectifier, which consists of two parts, namely an operational amplifier 40, in whose feedback branch the two diodes 41 and 42 are connected anti-parallel to the feedback resistor RK40, as well as one simple adding circuit, consisting of an operational amplifier 43 with its feedback resistor RKu- The resistors of the operational amplifier _{RfU 0} and the series resistors 44, 45 and 46 are dimensioned so that half of the non-rectified input signal is added to the full amplitude of the rectified signal, resulting in the 3-level signal forms a 2-level signal. The mode of operation of this circuit is shown in FIG. 11 shown in more detail. The curve I is the non-rectified signal, the curve II is the rectified signal of the amplitude and the curve III is the inverting sum signal that is present at the output of the operational amplifier 43. As can be easily seen, this signal is a 2-level signal. In Fig. 11 a pure sinusoidal oscillation was used as a basis for the sake of better understanding. FIG. 12 shows the same relationships for a signal as it is transmitted over the line in the present case. Fig. 12a shows the transmitted and received over the line 3-level signal z. B. originates from the transmitted signal of FIG. 9 and has become a bit sluggish due to the band-limited line. F i g. 12b shows the 2-level signal which represents the states black and white after the rectification.

In addition 6 sheets of drawings

Claims (6)

Patent claims: 1. Process for improving the image quality the transmission of facsimile signals which are derived from a kids signal, which is essentially a signal of one polarity with a minimum amplitude for an information state (white or black) and a maximum amplitude for one other information state (black or white), whereby the information changes of this signal, flip-over pulses for the formation of a signal of alternating polarity are obtained, whose opposite polarities represent a state of information (black or white) and its zero position represents the second information state (white or black) and the signal alternating polarity is formed in that the operations successively with each flip pulse Switching on a signal of one polarity, changing to the zero position, switching on a signal of one different polarity, change to zero position and where the formation of the flip-over pulses is based on an image signal that was previously subjected to an amplitude division, characterized in that the Flip impulses directly by comparing the displayed image signal with a threshold value be won. 2. The method according to claim 1, characterized in that the formation of the signal is alternating Polarity by means of the flip-over pulses by switching to 2 constant voltages in opposite directions Polarity is gained. 3. The method according to claim 1, characterized in that the formation of the signal is alternating Polarity by means of the flip-over pulses by switching to the image signal and au? one from the image signal signal obtained by inversion is obtained. 4. The method according to claim I, characterized in that the formation of the signal is alternating Polarity by means of the flip-over pulses by switching to the split image signal and to on is obtained from the erected image signal by inversion. 5. The method according to any one of claims 1 -4, characterized in that the image signal twice is differentiated, is added to the original image signal with opposite polarity and to Formation of the flip-over pulses is compared with a threshold value. 6. The method according to any one of claims 1-4, characterized in that the image signal is a Environment signal is superimposed, and that the sum signal to form the flip-over pulses with a Threshold value is compared.