GB2135852A - Arrangement for converting an analog video signal into a two-level signal - Google Patents

Abstract

An analog video signal x(t), which may be produced by line-wise scanning of a document (for example for facsimile transmission), is applied together with a reference signal r(t) to a comparator circuit of an arrangement for converting the analog video signal into a two-level signal. In order to prevent the output signal of this comparator circuit from being effected by irregularities of the document such as irregularities in the fibre structure or contamination of the paper, or non-uniformly distributed ink and in order to limit the loss of information when the document has grey or coloured areas, the reference signal r(t) is formed by the sum of weighted versions of two or more auxiliary reference signals. A first auxiliary reference signal r1(t) is equal to the average value of a number (N) of instantaneous values of the analog video signal. A second auxiliary reference signal r2(t) is equal to the intermediate value of the analog video signal. <IMAGE>

Description

SPECIFICATION Arrangement for converting an analog video signal into a two-level signal.

A. Background of the invention A (1). Field of the invention The invention- relates to an arrangement for converting an analog video signal into a two-level signal. The analog video signal can be obtained by scanning a document in accordance with a predetermined line pattern, for example in the manner effected in a facsimile scanning arrangement, in an arrangement for scanning postal codes, or finger prints and such like.

A (2). Description ofthepriorart.

Scanning arrangements of the above-mentioned type produce analog video signals whose instantaneous value is a measure of the average luminance of a given segment of the document. This segment is called a picture element or pel and is very small. For documents having differently coloured areas the luminances of the pels may vary considerably from pel to pel. Let it be assumed that white areas have the maximum luminance Lw and black areas the minimum luminance LB- Grey and coloured areas will then have a luminance between La and Lw.

The analog video signal can be directly stored in a suitable storage medium, for example a magnetic tape, or it can be transmitted via a transmission medium. In order to limit the storage capacity required for storage and the transmission time required during transmission, or to render it possible to compare patterns with each other in a simple way (for example for comparing finger prints), it is at present customary to convert the analog video signal into a two-level signal. For that purpose the analog video signal is applied to a first input of a comparator circuit, while a reference signal produced by a reference signal generator is applied to its second input.

When the analog video signal is larger than the reference signal, the comparator circuit produces an output voltage Ew. When the analog video signal is smaller than the reference signal, the comparator circuit produces an output voltage Ea. It is assumed that Ew exceeds E8.

Such a conversion is unavoidably accompanied by loss of information. In order to limit this loss, Reference 1 proposes that the reference signal generator generates a reference signal which is equal to an intermediate value of the analog video signal. To that end, the video signal is applied to this reference signal generator which comprises a positive and negative peak detector. The positive peak detector produces a positive peak-detection signal which is a measure of the highest signal level of the video signal. The negative peak detector produces a negative peak-detection signal which is a measure of the lowest signal level of the video signal. In an adder circuit an intermediate signal is now determined whose instantaneous value is located halfway between the instantaneous values of the two peak-detection signals. This intermediate signal represents the reference signal.In practice it has been found that this prior art conversion arrangement has one disadvantage, in that much information is lost when the document has grey or coloured areas having luminances located in the range from 0 to approximately 0.4 Lw or in the range from approximately 0.6 Lw to Lw. This is, for example, the case with bank cheques which may have green areas having a luminance of approximately 0.6 Lw, while there may be red characters having a luminance of approximately 0.7 Lw on these green areas. It is assumed that L8 = 0.

In order to obviate the disadvantages of the conversion arrangement shown in Reference 1, Reference 2 proposes that the reference signal generator generates a reference signal whose instantaneous value is equal to the average value of a number N of instantaneous values of the analog video signal. The analog video signal is applied for that purpose to a delay line having taps in N predetermined positions. The delayed analog video signal versions occurring at these taps are added together and divided by a factor N. The resulting signal represents the reference signal. This prior art conversion arrangement has also been found to have a disadvantage in that irregularities in documents, such as changes in the fibre structure of the paper, contamination of the paper or non-uniformly distributed ink, have a very pronounced affect on the output signal of the comparator circuit.

B. Object and summary of the invention.

The invention has for its object to provide a converter arrangement which does not have the disadvantages of the above-described prior art arrangement.

According to the invention an arrangement for converting an analog video signal into a two-level signal, comprising: a) a converter input for receiving the analog video signal; b) a converter output at which the two-level signal is required; c) a reference signal generator coupled to the converter input and thus receiving the analog signal and producing a reference signal; d) a comparator circuit coupled to the converter input and to the said reference signal generator, and also to the converter output, for supplying the two-level signal to the inverter output, is characterized in that the reference signal generator comprises:: - a first auxiliary reference signal generator coupled to the converter input and to which the analog video signal is applied, which generator produces a first auxiliary reference signal which is equal to the average value of a number (N) of instantaneous values of the analog video signal; - a second auxiliary reference signal generator coupled to the converter input and to which the analog video signal is applied, which generator produces a second auxiliary reference signal which is equal to the intermediate 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 and - summing means to which the weighted auxiliary reference signals are applied for generating a sum signal which represents the said reference signal.

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

C. References.

1. Electric Pulse Wave Clipping Circuitry; D.D. Baumann; United States Patent Application No 3,566,281 (granted February 23rd 1971).

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.

D. Short description of the Figures.

Figure 1 shows schematically an embodiment of a converter arrangement according to the invention; Figure 2 shows in greater detail an embodiment of a converter arrangement according to the invention; and Figure 3 and Figure 4 show some time diagrams to explain the operation of the converter arrangement shown in Figure 2.

E. Description ofthe embodiments given by way ofexample.

Figure 1 shows schematically a converter arrangement having a structure in accordance with the invention. It has a converter input 1 to which the analog video signal x(t) is applied, which signal is to be converted into a two-level signal y(t) which is present at a converter output 2. this converter output 2 forms the output of a comparator circuit 3 which has two inputs 3(1) and 3(2). The input 3(1) is connected to the input 1 of the converter arrangement via a delay device 4. If the time delay produced by the delay device 4 is assumed to be equal to T,, then this comparator input 3(1) receives the signal x(t-To). The other comparator input 3(2) receives a reference signal r(t). In response to these input signals, y(t) assumes a value Ew when x(t-To) is larger than r(t).When x(T-T,) is less than r(t), y(t) assumes a value E5 which is lowerthan Ew.

The reference signal r(t) is produced by a reference signal generator 5 which comprises a first and a second auxiliary reference signal generator 51 and 52, respectively producing a first and a second auxiliary reference signal r, (t) and r2(t). The generators 51 and 52 are both connected to the converter input 1 and thus each receives the analog video signal x(t). In a multiplying circuit 53, the auxiliary reference signal r1(t) is multiplied by a constant factor a and the auxiliary reference signal r2(t) is multiplied in a multiplying circuit 54 by a constant factor 1 -a. The quantity a is larger than zero and less than 1. A practical value was found to be a = 0.9.The weighted auxiliary reference signals ar(t) and (1 -a)r2(t) produced by the multiplying circuits 53 and 54 are added together in an adder arrangement 55. This results in the sum signal a r,(t) + (1-a) r2(t) which represents the said reference signal r(t).

The first auxiliary reference signal r1(t) is equal to the average value of N instantaneous values of the analog video signal x(t) and can be obtained by constructing the first auxiliary reference signal generator 51 in the manner described in Reference 2.

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

For the sake of completeness, Figure 2 shows a practical embodiment of the converter arrangement. 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 are each provided by a cascade arrangement of three delay sections 4(.) and 6(.), respectively, each having a time delay Two/3 The outputs of all the delay sections 4(.) and all the delay sections 6(.) are connected to a common lead 7 via resistors which each have a resistance value R. The common lead 7 is connected to the input of an operational amplifier 8. This amplifier comprises a feedback loop which includes a resistor having the resistance value R1. The output of this operational amplifier 8 is connected to the comparator input 3(2) via an inverting amplifier 9.

The above described portion of the circuit shown in Figure 2 represents the first auxiliary reference signal generator 51,which is shown schematically in Figure 1. If it is chosen therein that N = 6 and R1 = R/6, then the inverting amplifier 9 produces the first auxiliary reference signal r(t) which is defined by the expression::

r(t) - R x(t-T,/3) + x(t-2T,/3) + x(t-3T,/3) + x(t-4To/3) 1 6 R R R R + + x(tR TJ3 + x(t-6T,/3 R R 1 x(t-T,/B) + x(t-2T,/3) + ..... + x(t-6T,/3 If the particular video signal x(t) shown in Figure 3 is applied to this first auxiliary reference signal generator, then the first auxiliary reference signal r1(t) has the shape which is also shown in Figure 3. The desired weighting of this first auxiliary reference signal r(t) with the factor a can be realised by making R1 equal to aR/N.

The analog video signal x(t) is not only applied to the above-described first auxiliary reference signal generator but also to the second auxiliary reference signal generator 52, which comprises a positive peak detector 56 and a negative peak detector 57. As shown in Figure 2 for the sake of completeness, these detectors 56 and 57 may be formed by a diode D, a resistor having the resistance value R2, a capacitor having capacitance C and a buffer amplifier B. In response to the analog video signal x(t), the positive peak detector 56 produces the positive peak signal p(t) and the negative peak detector 57 produces the negative peak signal d(t) both these signals also being shown in Figure 3. These signals are applied to a potentiometer circuit 58 which supplies from its output the second auxiliary reference signal r2(t), also shown in Figure 3.

Let it be assumed that the potentiometer circuit 58 is adjusted such that r2(t) = 1/2 p(t) + d(t). To obtain the desired weighting of this second auxiliary reference signal, it is applied to the input of the operational amplifier 8 via a resistor having a resistance value R3. The resistance value R3 is chosen equal to - a R R3 = (1 -a) N and the resistance value of potentiometer 58 is chosen much smaller than R3.

The reference signal r(t) which is equal to the sum ar1(t) + (1-a)r2(t) and whose shape for a = 0.9 is shown in Figure 3 now occurs at the output of the inverting amplifier 9.

As can be seen from Figure 3, the converter arrangement of the construction in accordance with the invention makes advantageous use of the advantages of the prior art converter arrangement described in Reference 2 when the luminance of the pels is located in the range extending from approximately 0.05 Low to approximately 0.95 Lw as in that case r(t) is substantially equal to r1(t). In extremely white areas, i.e. those areas of the document which do not contain information, small luminance changes do not now show in the two-level signal y(t). This is because of for those areas the reference signal r(t) is somewhat smaller than r1(t). Also now small luminance changes in extremely black areas of the document do not show in the two level signal y(t) because for such areas the reference signal r(t) is now somewhat larger than r1(t).

In practice it has been found to be advantageous to accentuate, as is also customary with photocopying equipment, either the black information or the white information for some documents. This is, for example, the case when the original document is of a poor qualtiy. This so-called light/dark control can be realized for the converter arrangement 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 which level can be intensified.

b) By changing the resistance value R3the proportion of r2(t) in r(t) can be increased or reduced.

c) By applying an additional off-set voltage to the operational amplifier 8 the reference signal level can be increased or reduced.

As shown in Figure 2 the off-set voltage mentioned in c) can be derived by means of a potentiometer circuit 59 from two constant voltage sources which produce voltages +VO and -V0, respectively. The voltage occurring at the output of this potentiometer 59 is applied to the input of the operational amplifier 8 via a resistor having a resistance value R4.

It should be noted that in the embodiment shown in Figure 2 the second auxiliary reference signal generator 52 is connected to the converter input 1. It may, however, alternatively be connected to the output of one of the delay sections 4(.) or 6(.).

It should also be noted, that it is not necessary for the delay sections 4(.) and 6(.) to have all the same time delay.

It is possible that the video signal is not a continuous analog signal but a time-discrete analog signal which is consequently formed by a sequence of samples of the continuous analog video signal, these samples being produced at a predetermined sampling rate f,. This is, for example, the case when the document is scanned by a series of in-line CCD image sensors. In that case the time delay To/3 of the delay sections 4(.) and 6(.) will preferably be chosen equal to 1/fas.

Claims (2)

1. An arrangement for converting an analog video signal into a two-level signal, comprising: a) a converter input for receiving the analog video signal; b) a converter output at which the two-level signal is required; c) a reference signal generator coupled to the converter input and thus receiving the analog signal and producing a reference signal; d) a comparator circuit coupled to the converter input and to the said reference signal generator, and also to the converter output, for supplying the two-level signal to the converter output, characterized in that the reference signal generator comprises:: e) a first auxiliary reference signal generator coupled to the converter input and to which the analog video signal is applied, which generator produces a first auxiliary reference signal which is equal to 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 to which the analog video signal is applied, which generator produces a second auxiliary reference signal which is equal to the intermediate value of the analog 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, respective, and h) summing means to which the weighted auxiliary reference signals are applied for generating a sum signal which presents the said reference signal.

2. An arrangement for converting an analog video signal into a two-level signal, substantially as hereinbefore described with reference to the accompanying drawings.