NL8300686A - DEVICE FOR CONVERTING AN ANALOGUE VIDEO SIGNAL TO A TWO-VALUE SIGNAL. - Google Patents

Description

* v * PHN 10.601 1 N.V. Philips' Incandescent lamp factories in Eindhoven "Device for converting an analog video signal into a bivalent signal" A. Background of the invention A (1). Field of the invention

The invention relates to a device for converting an analog video signal into a bivalent signal, wherein this analog video signal is obtained by testing a document according to a determined line pattern. For example, in the manner as this happens in a facsimile scanning device, in a scanning device for postcodes or fingerprints and the like.

10 A (2). Description of the prior art

Sensors of the above type provide analog video signals, the instantaneous value of which is a measure of the average brightness of a given surface of the document. This surface is called picture element or Pel (Picture elsnent) and is very small. For documents with areas of different colors, the brightness of the Pels can vary widely, from Pel to Pel. It will be assumed that white areas have the maximum brightness and black areas have the minimum brightness. Gray and colored areas will then have a brightness between Lg and L ^.

This analog video signal can be stored directly in a suitable memory medium (for example a magnetic tape), or can be transferred via a transmission medium. In order to limit the required memory space during storage and to reduce the required transmission time during transmission, or to be able to compare patterns in a simple manner (for example for comparing fingerprints), it is now customary to convert the analog video signal into a bivalent signal. For this purpose, the analog video signal is applied to a first input of a comparator circuit, while a second input thereof is supplied with a reference signal supplied by a reference signal 8300 68 6 φ 10,601 2 τ generator. If the analog video signal is greater than the reference signal, the comparator circuit provides an output voltage E ^. If, on the other hand, the analog video signal is smaller than the reference signal, the comparator circuit supplies an output voltage Eg. It will be assumed that E ^ is greater than Eg.

Such a conversion is inevitably accompanied by a loss of information. In order to limit this loss, reference 1 has already suggested that the reference signal generator generate a reference signal equal to the mean value of the analog video signal. For this purpose, this video signal is supplied to this reference signal generator, which is more particularly provided with a positive and a negative peak detector or peak and valley detector, respectively. The peak detector provides a peak detection signal which is a measure of the highest signal level of the video-15 signal. The valley detector provides a valley detection signal which is a measure of the lowest signal level of the video signal. In an adder circuit, the mean signal whose current value is situated midway between the instantaneous values of the two detection signals is now determined. This mean signal now represents the reference 20 signal. In practice, this known relief device appears to have an important drawback. Namely, it appears that much information is lost if the document has gray or colored areas with brightnesses in the range from 0 to about 0.4 L ^, or in the range from about 0.6 to L ^. Examples include bank checks that can contain green areas with a brightness of about 0.6 L ^, while on these green areas red characters can appear with a brightness of about 0.7 L ^. It is assumed here that Lg = 0.

In order to overcome the drawbacks of the converter described in reference 1, it has been proposed in reference 2 to have the reference signal generator generate a reference signal, the instantaneous value of which is continuously equal to the average value of a number of, for example N, instantaneous values of the analog video signal. More specifically, for this purpose, the analog video signal is applied to a delay line tapped at N certain locations. The delayed version of the analog video signal occurring at these taps is added together and divided by a factor N. The resulting signal represents the 83 0 0 68 6 ΡΗΝ 10.601 3 Λ «reference signal. This known conversion device also appears to have a very unpleasant property in practice. Namely, irregularities in the document, such as a change in the fiber structure of the paper, contamination of the paper, or non-uniformly distributed ink, are found to be very strongly reflected in the output signal of the camper circuit.

B. Object and summary of the invention. The object of the invention is to indicate a converting device which does not have the drawbacks of the above-described known device.

According to the invention, the reference signal generator is provided for this purpose with: - a first auxiliary reference signal generator to which the analog video-15 signal is supplied and which provides a first auxiliary reference signal equal to the average value of a number of (N) instantaneous values of the analog video signal; a second auxiliary reference signal generator to which the analog video signal is applied and which provides a second auxiliary reference signal 20 which is equal to the mean value of the analog video signal; - means for multiplying the first auxiliary reference signal by a factor a and for multiplying the second auxiliary reference signal by a factor 1-a to generate a first and a second weighted auxiliary reference signal, respectively; - soy agents to which the weighted auxiliary reference signals are applied to generate a sum signal representing said reference signal.

It should be noted that the first auxiliary reference signal generator 30 can be used as the reference signal generator described in reference 2 and that the second auxiliary reference signal generator can be used the reference signal generator described in reference 1.

35 C. References T. Electric Pulse Wave Clipping Circuitry; D.D, Baumann;

U.S. Patent No. 3,566,281 (issued February 23, 1971) B 3 0 0 ύ 8 δ EHN 10,601 4 2. Fingerprint Image Enhancement System; Y.M. Ting, W. Wilson; I.B.M. Technical Disclosure Bulletin, Vol. 16, no. 8, January 1974, pages 2684-2685.

5 D. Brief description of the figures

Figure 1 schematically shows a conversion device designed in accordance with the invention;

Figure 2 shows a more detailed converting device designed according to the invention;

Figure 3 and Figure 4 show some time diagrams for explaining the operation of the converter shown in Figure 2.

E. Description of the embodiments

Figure 1 schematically shows a conversion device with a construction according to the invention. It is provided with a converter input 1 to which the analog video signal x (t) is applied and which must be converted into a bivalent signal y (t) which is desired at a converter output 2. This converter output 2 forms the output of a comparator circuit 3 which has two inputs 3 (.). The input 3 (1) is connected via a delay device 4 to the input 1 of the converter. If the delay time of the delay device 4 is set equal to T, then this comparator input 3 (1) receives the signal x (t-T). The other comparator input 3 (2) receives a reference signal r (t). Due to these two input signals, y (t) assumes a value E ^ when x (t-TQ) is greater than r (t). On the other hand, if x (t-TQ) is less than 30 r (t), y (t) assumes a value Eg that is less than E ^.

The reference signal r (t) is provided by a reference signal generator 5. This is provided with a first and a second auxiliary reference signal generator 51 and 52, respectively, which supply a first and a second auxiliary reference signal r ^ (t) 35 r2 ^. They are each connected to the converter input 1 and thus each receive the analog video signal x (t). The auxiliary reference signal (t) is multiplied in a multiplier circuit 53 by a constant factor a and the auxiliary reference signal 83 0 0 68 6

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EHN 10,601 r2 (t) is multiplied in a multiplier circuit 54 by the constant factor 1-a. The quantity a is greater than zero and less than 1. A practical value appears to be a = 0.9. The weighted auxiliary reference signals ar ^ (t) and (1-a) r2 (t) supplied by the multiplier circuits 53 and 54 are added together in an adder 55. This makes the scan signal ar ^ (t) + (1- a) r2 (t) obtained which represents said reference signal r (t).

The first auxiliary reference signal r ^ (t) is equal to the mean value of N instantaneous values of the analog video signal and can be obtained by building the first auxiliary references signal regenerator 51 in the manner described in reference 2 .

The second auxiliary reference signal r2 (t) is equal to the mean value of the analog video signal x (t) and can be obtained by building the second auxiliary reference signal generator 52 in the manner described in detail in reference 1.

For the sake of completeness, a practical embodiment of the converter is shown in Figure 2. In this case, the converter input 1 is connected via the delay device 4 to the comparator input 3 (1) and to the input of a second delay device 6.

These delay devices 4 and 6 each consist of a cascade circuit of three delay sections 4 (.) And 6 (.), Each having a delay time T ^ 3. The outputs of both the delay sections 4 (.) And of the delay sections 6 (.) Are connected to a common line 7 via resistors each having the resistance value R. The common line 7 is connected to the input of an operational amplifier 8. It is provided with a feedback loop which includes a resistor having the resistance value. The output of this operational amplifier 8 is connected via an inverting amplifier 9 to the comparator input 3 (2).

The above-described part of the circuit shown in FIG. 2 represents the first auxiliary references signal generator 51 schematically shown in FIG. 1 if N = 6 and R 1 = R / 6 are selected therein. In that case, the inverting amplifier supplies the first auxiliary reference signal r ^ (t) that satisfies the expression: 830 0 88 6 o PHN 10.601 6 _ _ r fx (t-V3) x (t-2To / 3) x (t- 3V3) x (t-4V3) r1 (t) = 6 [. R + R + R + R + x (t-5To / 3) ix (t-STo / 3) '+ g + g 5 = 1 + Xtt-ZT ^) + ..... + x (t-6T0 / 3)

More in particular, if the video signal x (t) shown in Fig. 3 is supplied to this first auxiliary reference signal generator, the first hmp frequency signal r ^ (t) has the form also shown in Fig. 3. The desired weighting of this first auxiliary reference signal r ^ (t) with the factor a can be realized by equaling aR / N.

In addition to the above-described first auxiliary reference signal generator, the analog video signal x (t) is also supplied to the second auxiliary reference signal generator 52 which includes a peak detector 56 and a valley detector 57. As shown for the sake of completeness, these detectors 56 and 57 can be constructed with a diode D, a resistor with resistor value R2, a capacitor with capacitance C and a buffer amplifier B. In accordance with the analog video signal x (t) shown in Fig. 3. The peak detector 56 supplies the peak signal p (t), also shown in FIG. 3, and the valley detector 57 supplies the peak signal d (t). These signals are applied to a potentiometer circuit 58 which supplies at its output the second auxiliary reference signal r2 (t), which is also shown in Fig. 3. It is assumed here that the potentiometer circuit 58 is set such that r3 (t) = j jp (t) + d (t) ~ j. In order to obtain the desired weighting of this second auxiliary reference signal, it is applied via a 3 ° resistor with resistance value R3 to the input of the operational amplifier 8. The resistance value R ^ is thereby chosen equal to

R3 a-a5 N

35 and the resistance value of potentiometer 58 is taken much smaller than R3

At the output of the inverting amplifier 9 the reference signal r (t), which is equal to the sum 8300686 PHN 10.601 7 aïj (t) + (1-a) r2 (t), whose form for a = 0 9 is shown in FIG. 3. In response to both signals x (t) and r (t) shown in FIG. 3, the comparator circuit now supplies the divalent signal y (t) shown in FIG.

As can be read from Fig. 3, the converting device built up according to the invention takes advantage of the advantages of the known ref. 2, when the brightness of the Pels is in the range ranging from about 0.05 to about 0.95 L. hammers in that case r {t) is approximately 10 equal to (t). In extremely white areas, those are areas of the document where no information is present, small brightness changes do not express the bivalent signal y (t). This is due to the fact that for those regions, the reference signal r (t) is somewhat smaller than r ^ (t). Also, 15 small brightness variations in extremely black areas of the document are now not reflected in the bivalent signal y (t) because for such areas the reference signal r (t) is now slightly greater than r ^ (t).

In practice it has been found to be advantageous to accentuate either the black information or the white information for some documents, as is customary with photocopiers. This is the case, for example, when the original document is of poor quality. This so-called light / dark control can be realized in the converting device shown in figure 2 in one of the following ways.

a) By changing the position of the potentiometer 58, either the black level or the white level can be favored.

b) By changing the resistance value, the proportion of r2 (t) in r (t) can be increased or decreased.

C) By supplying an additional off-set voltage to the operational amplifier 8, the reference signal can be raised or lowered.

The offset voltage referred to under c) can be derived, as indicated in figure 2, by means of a potentiometer circuit 59 35 from two constant voltage sources, resp. supply a voltage + VQ and - VQ. The voltage occurring at the output of this potentiometer circuit 59 is applied through a resistor with resistance value R ^ to the input of the operational amplifier 8300686 τ ΕΉΝ 10.601 8 stronger 8.

It should be noted that in the exemplary embodiment shown in Figure 2, the second auxiliary reference signal generator 52 is connected to the converter input 1. However, it can also be connected to the output of one of the delay sections 4 (.) Or 6 (0.

It should also be noted that the delay sections 4 (.) And 6 (.) Do not all have to have the same delay time.

It is possible that the video signal is not analog, but is time-discrete and is thus formed by a series of samples of the analog video signal, these samples occurring at a certain sampling frequency f. This is e.g. the case as the document

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is scanned by a series of aligned CCD image sensors.

In that case, the delay time 1 ^ 3 of the delay sections 15 4 (.) And 6 (.) Will preferably be chosen equal to 1 / f.

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20 25 30 35 83 0 0 68 6

Claims (1)

An apparatus for extracting an analog video signal into a bivalent signal, comprising: a) a converter input for receiving the analog video signal; b) a decoder output to which the bivalent signal is desired; gc) a reference signal generator coupled to the converter input and thus receiving the analog video signal and providing a reference signal, d) a comparator circuit coupled to the converter input and to said reference signal generator, as well as to the converter output, supplying the bivalent signal to said setter output, characterized in that the reference signal generator comprises: e) a first auxiliary reference signal generator coupled to the converter input and thus supplying the analog video signal equal to the first auxiliary reference signal is the average value of a number of (N) instantaneous values of the analog video signal, f) a second auxiliary reference signal generator coupled to the converter input and thus supplied with the analog video signal and providing a second auxiliary reference signal equal to the secondary value of the an aloge video signal, g) means for multiplying the first auxiliary reference signal by a factor a and for multiplying the second auxiliary reference signal by a factor 1-a, to generate a first and a second weighted auxiliary reference signal respectively, h) scramblers to which the weighted auxiliary reference signals are applied to generate a scm signal representing said reference signal. 30 35 8300686