JPH0439828B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION A. Background of the Invention A(1) Field of the Invention The present invention relates to a conversion circuit for converting an analog video signal to a two-level signal.
Analog video signals can be obtained by scanning a document according to a predetermined line pattern, as is done, for example, with a facsimile scanning device, a device that scans postal codes or fingerprints, and the like.

A(2) Description of the Prior Art The instantaneous value of an analog video signal produced by a scanning device of the type described above corresponds to the magnitude of the average brightness of a given segment of a document. Such segments are called pixels, or pels, and are extremely small. If your manuscript has various colored parts,
Pixel brightness varies significantly from pixel to pixel. If white areas exhibit maximum brightness L _W and dark areas exhibit minimum brightness L _B , then the brightness of gray and colored areas exhibits values between L _B and L _W.

Analog video signals can be stored directly on a suitable storage medium, such as magnetic tape, or transmitted via a transmission medium. However, this does not limit the storage capacity required to store analog video signals, limits the transmission time required for transmission, or makes it easier to compare patterns with each other (e.g., comparing fingerprints). It is currently customary to convert the analog video signal into a two-level signal in order to obtain the desired signal. For this purpose, an analog video signal is applied to a first input of the comparator circuit and a reference signal generated by a reference signal generator is applied to a second input of the comparator.

If the analog video signal is greater than the reference signal, the comparator circuit produces an output voltage _EW . On the other hand, if the analog video signal is smaller than the reference signal, the comparator circuit will generate an output voltage E _B.
Note that E _W is assumed to be larger than E _B.

Such a conversion method inevitably results in a loss of information. In order to limit such losses, Document 1 listed below proposes a method in which a reference signal generator is used to generate a reference signal equal to the intermediate value of the analog video signal. In this case, the video signal is fed to the reference signal generator, which includes positive and negative peak detectors. A positive peak detector generates a positive peak detection signal corresponding to the magnitude of the highest signal level of the video signal. A negative peak detector generates a negative peak detection signal corresponding to the magnitude of the lowest signal level of the video signal. Note that the intermediate signal is determined by an adder circuit, and the instantaneous value of this intermediate signal is located midway between the instantaneous values of the two peak detection signals. Such intermediate signal represents the reference signal. However, it has been confirmed that such conventional conversion circuits actually have the following drawbacks. That is, when a document has gray or colored areas with brightness ranging from 0 to about _0.4LW , or from about _0.6LW to _LW , conventional conversion circuits lose a large amount of information. This is the case, for example, in the case of a bank check that has green areas with a brightness of approximately _0.6LW and above these green areas red letters with a brightness of approximately _0.7LW . Note that it is assumed that L _B -0.

In order to eliminate the drawbacks of the conversion circuit shown in Document 1 as mentioned above, Document 2 listed later describes a method in which the instantaneous value corresponds to the average value of a large number of N instantaneous values of the analog video signal using a reference signal generator. A conversion circuit that generates a reference signal has been proposed. In this case, an analog video signal is applied to a delay line having taps at N predetermined positions. The delayed analog video signals occurring at these taps are added together and divided by factor N. The signal thus obtained represents the reference signal. Even with such conventional conversion circuits, irregularities in the original such as changes in the fiber structure of the paper, stains on the paper,
It has been determined that the non-uniformity of the ink coverage has a very obvious negative effect on the output signal of the comparator circuit.

B. Object and Summary of the Invention The object of the present invention is to provide a conversion circuit of the type described above, with suitable connections and arrangements so as to eliminate the drawbacks of the conventional circuits mentioned above.

The present invention provides a conversion circuit for converting an analog video signal into a 2-level signal, comprising: a a converter input terminal for receiving an analog video signal; b a converter output terminal for which a 2-level signal is required; c a reference signal generator coupled to said transducer input terminal for receiving an analog signal and generating a reference signal; d coupled to said transducer input terminal and said reference signal generator and said transducer output terminal; a comparator circuit also coupled to the converter output terminal for providing a two-level signal to the converter output terminal; a converter circuit comprising: a first auxiliary reference signal generator supplied with an analog video signal and generating a first auxiliary reference signal equal to the average value of a number (N) of instantaneous values of the analog video signal; f coupled to the converter input terminal; a second auxiliary reference signal generator supplied with the analog video signal and generating a second auxiliary reference signal equal to an intermediate value of the analog video signal; g multiplying the first auxiliary reference signal by a factor a; , and multiplying the second auxiliary reference signal by a factor 1-a to obtain first and second auxiliary reference signals, respectively.
a multiplier for generating a weighted auxiliary reference signal; h) an addition means to which the weighted auxiliary reference signal is supplied and generates an addition signal representing the reference signal; It is in.

The first auxiliary reference signal generator is described in Document 2.
The reference signal generator described in Document 1 can be used, and the reference signal generator described in Document 1 can be used as the second auxiliary reference signal generator.

C Reference 1 Electric Pulse Wave Clipping Circuitry;
DDBaumann; U.S. Patent Application No. 3,566,281 (granted February 23, 1971) 2 Fingerprint Image Enhancement
System; YMTing, W. Wilson; IBM
Technical Disclosure Bulletin, Vol.16, No.
8, January 1974, pp. 2684-2685 D The explanation of the attached drawings is described in 4.

E Description of Examples The present invention will be explained below with reference to the drawings.

FIG. 1 is a block diagram showing an example of a conversion circuit according to the present invention. Input terminal 1 of this conversion circuit
is supplied with an analog video signal x(t), converts this signal into a 2-level signal y(t), and outputs it as an output signal 2. The output terminal 2 of such a conversion circuit is
It forms the output terminal of a comparator circuit 3 having two input terminals 3(1) and 3(2). One input terminal 3(1) of the comparator circuit 3 is connected to the input terminal 1 of the conversion circuit via a delay device 4. If the delay time delayed by the delay device 4 is _T0 , the input terminal 3(1) of the comparator circuit 3 receives the signal x(t- _T0 ). The other input terminal 3(2) of the comparison circuit 3 receives the reference signal r(t). The output signal y(t) responds to the input signals of these comparison circuits, and when x(t-T ₀ ) is larger than r(t), the value of the output signal y(t) becomes E _W and x(t
-T ₀ ) is smaller than r(t), then the value of y(t) is E _B which is lower than E _W .

The reference signal r(t) is generated by a reference signal generator 5. This reference signal generator 5 includes first and second
first and second auxiliary reference signal generators 51 and 5 generating auxiliary reference signals r ₁ (t) and r ₂ (t), respectively;
It has 2. These auxiliary reference signal generators 5
Input terminals 1 and 52 are both connected to input terminal 1 of the conversion circuit, so that they receive the analog video signal x(t). The auxiliary reference signal r ₁ (t) is
A multiplier circuit 53 multiplies the signal by a constant factor a , and a multiplier circuit 54 multiplies the auxiliary reference signal r ₂ (t) by a constant factor 1- a . The value of the multiplication amount a is greater than 0 and smaller than 1.
In reality, it was confirmed that a=0.9. Weighted auxiliary reference signals ar ₁ (t) and (1-a)r ₂ (t) formed by multiplier circuits 53 and 54 are added together in addition circuit 55 . The added signal ar ₁ (t) + (1-a) r ₂ (t) obtained in this way is the reference signal r(t)
represents.

The first auxiliary reference signal r ₁ (t) is equal to the average value of N instantaneous values of the analog video signal x(t), and such signal r ₁ (t) is generated by the first auxiliary reference signal generator 51 can be obtained by configuring as described in Document 2 above.

The second auxiliary reference signal r ₂ (t) is equal to the intermediate value of the analog video signal x(t), and such signal r ₂ (t) is
This can be obtained by configuring the second auxiliary reference signal generator 52 as described in the above-mentioned document.

For completeness, a detailed block diagram of the conversion circuit of the present invention is shown in FIG. Input terminal 1 of conversion circuit
is connected to the input terminal 3(1) of the comparator 3 via the delay line device 4,
It is connected to the input terminal of the second delay device 6. Each of these delay devices 4 and 6 consists of three delay stages 4(.)
and 6 (.) are connected in cascade to form. In addition,
The delay time of each of these delay stages is assumed to be T _0/3 . The output terminals of each delay stage 4(.) and 6(.) are connected to the common lead wire 7 through a resistor having a resistance value of R. This common lead wire 7 is connected to an input terminal of an operational amplifier 8. The operational amplifier 8 has a resistance value of R ₁
It has a feedback loop containing a resistor. The output terminal of the operational amplifier 8 is connected to the comparator input terminal 3(2) via an inverting amplifier 9.

The above-described portions of the circuit shown in FIG. 2 represent the first auxiliary reference signal generator 51 shown in FIG. Here, if N=6 and R ₁ =R/6, the first auxiliary reference signal r ₁ (t) generated by the inverting amplifier 9 is expressed as follows. That is, r ₁ (t)=R/6[x(t-T _0/3 )/R+x(t-2T _0
/3 )/R+x(t-3T _0/3 )/R+x(t-4T _0/3 )/R+x(t-5T ₀ /3/R+x
(t-6T _0/3 /R) = 1/6 [x (t-T _0/3 +x (t-2T _0/3 ) +...
+x(t-6T _0/3 )] The specific video signal x(t) shown in FIG.
When supplied to the auxiliary reference signal generator, the signal shape of the first auxiliary reference signal r ₁ (t) is as shown in this FIG. 3 as r ₁ (t). The desired weighting of this first auxiliary reference signal r ₁ (t) by factor a is such that R ₁ is aR/N
This can be achieved by making it equal to .

The analog video signal x(t) is not only fed to the first auxiliary reference signal generator described above, but also to a second auxiliary reference signal generator 52 comprising a positive peak detector 56 and a negative peak detector 57. will also be supplied. As shown in FIG. 2, both the detectors 56 and 57 have a diode D and a resistance R.
, a capacitor with a capacitance of C, and a buffer amplifier B. In response to the analog video signal x(t), a positive peak detector 5
6 generates a positive peak signal p(t), and a negative peak detector 57 generates a negative peak signal d(t),
Both of these peak value signals are also shown in FIG.
The positive and negative peak signals are supplied to a potentiometer circuit 58 from which a third
A second auxiliary reference signal as shown in the figure is supplied. Note that the potentiometer circuit 58
shall be adjusted so that r ₂ (t)=1/2 [p(t)+d(t)]. In order to weight this second auxiliary reference signal as desired, this signal is
It is supplied to the input terminal of the operational amplifier 8 via a resistor _R3 . The value of the resistance value _R3 is selected so that _R3 =(a/1-a)R/N, and the resistance value of the potentiometer 58 is selected to be much lower than _R3 ! .

The reference signal r(t) is the addition value, i.e. ar ₁ (t)+(1-a)
r ₂ (t), and the signal shape of this signal when a=0.9 is shown in FIG. 3, and the reference signal r(t)
is generated at the output terminal of the inverting amplifier 9.

As is clear from FIG. 3, according to the conversion circuit configured according to the present invention, the luminance of the pixel varies from approximately 0.05L _W to
within the range up to 0.95L _W , where r(t) is r ₁ (t)
When approximately equal to , the advantages of the conventional conversion circuit described in document 2 can be advantageously used. In extreme white areas, ie, locations in the document that do not contain information, there is almost no change in brightness in the 2-level signal y(t). This is because the reference signal r(t) is somewhat smaller than r ₁ (t) at such a location. Furthermore, even in extremely dark areas of the document, there is almost no change in brightness in the 2-level signal y(t). The reason is that the reference signal r(t) is somewhat larger than r ₁ (t) at such a location.

In practice, it has been found advantageous to emphasize either black or white information in some documents, as is commonly done in reproduction machines. This is true, for example, when the quality of the manuscript is poor. Such so-called bright/dark control can be performed in the conversion circuit shown in FIG. 2 by any of the methods listed below.

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

b By changing the value of the resistance value R ₃ , the magnitude of the second auxiliary reference signal r ₂ (t) in the reference signal r(t) can be increased or decreased.

c Additional offset (compensation) for operational amplifier 8
By supplying a voltage, the reference signal level can be increased or decreased.

The off-set voltage mentioned in section c is determined by potentiometer 59 from two constant voltage sources generating voltages +V ₀ and -V ₀ respectively, as shown in FIG.
It can be taken out via. The voltage generated at the output terminal of this potentiometer 59 has a resistance value of
It is supplied to the input terminal of the operational amplifier 8 via a resistor _R4 .

In the example shown in FIG. 2, the second auxiliary reference signal generator 52 is connected to the input terminal 1 of the conversion circuit, but the second auxiliary reference signal generator 52 is connected to the delay stage 4 (.)
Alternatively, it can also be connected to one of the output terminals of 6(.).

It is also clear that the delay times of delay stages 4(.) and 6(.) do not necessarily have to be the same.

The video signal is not necessarily a continuous analog signal, but may also be a time-varying analog signal formed by a series of samples of a continuous analog video signal, such samples being sampled at a predetermined sampling rate f _s
Generate at. Note that such a case is, for example, a case where a document is scanned by a series of in-line CCD image sensors. In such a case, the delay times of delay stages 4(.) and 6(.) are preferably selected to correspond to 1/f _s .

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an example of the conversion circuit according to the present invention; FIG. 2 is a detailed block diagram thereof; FIGS. 3 and 4 are signal waveform diagrams for explaining the operation of the conversion circuit shown in FIG. 2. It is. 1...Converter (circuit) input terminal, 2...Converter (circuit) output terminal, 3...Comparison circuit, 4, 6...
Delay device, 4(1), 4(2), 4(3)...Delay stage, 5...
Reference signal generator, 6(1), 6(2), 6(3)...delay stage,
7... Common lead wire, 8... Operational amplifier, 9...
Inverting amplifier, 51...first auxiliary reference signal generator,
52...Second auxiliary reference signal generator, 53, 54...
... Multiplier circuit, 55 ... Addition circuit, 56 ... Positive peak detector, 57 ... Negative peak detector, 58 ... Potentiometer circuit, 59 ... Potentiometer, D ... Diode, C ... Capacitor, B …
...Buffer amplifier.

Claims (1)

[Claims] 1. A conversion circuit for converting an analog video signal into a 2-level signal, comprising: a) a converter input terminal for receiving the analog video signal; b) a converter requiring a 2-level signal; an output terminal; c a reference signal generator coupled to the transducer input terminal for receiving an analog signal and generating a reference signal; d coupled to the transducer input terminal and the reference signal generator; a comparator circuit also coupled to a transducer output terminal for providing a two-level signal to the transducer output terminal; in a transducer circuit, the reference signal generator: e coupled to the transducer input terminal; a first auxiliary reference signal generator to which the analog video signal is applied and which generates a first auxiliary reference signal equal to the average value of a number (N) of instantaneous values of the analog video signal; f the converter; a second auxiliary reference signal generator coupled to an input terminal and supplied with the analog video signal and generating a second auxiliary reference signal equal to an intermediate value of the analog video signal; g. a , and the second auxiliary reference signal is multiplied by a factor 1-a to obtain the first and second auxiliary reference signals, respectively.
a multiplier for generating a weighted auxiliary reference signal; h) an addition means to which the weighted auxiliary reference signal is supplied and generates an addition signal representing the reference signal; .