JPS6129078Y2 - - Google Patents

Description

[Detailed description of the invention] This invention receives an input signal that includes an analog fluctuation component and changes relatively steeply between two levels.
This invention relates to a circuit that detects the level change point.

For example, in a barcode reader, the barcode video signal output from the imaging device is divided into high and low levels corresponding to the black and white of the barcode, as shown in Figure 1.
The signal changes relatively sharply between levels, but
The two levels are not constant and change in an analog manner depending on various photographing conditions. For example, the white level and the black level increase or decrease in response to variations in light source illuminance, and in particular, the difference (amplitude) between the white level and the black level varies in response to variations in the photographing contrast of a barcode.
In this way, the video signal containing the analog fluctuation component is
In order to convert into a value, it is not possible to adopt a method of simply comparing it with a threshold value of a certain level, and some methods detect the level change points (rising, falling) of the video signal and directly or A method of indirect binarization has been devised.

A simple method for detecting the change point of a video signal as described above is to use a differentiating circuit that combines a resistor, a capacitor, or a coil, but this is extremely susceptible to noise and is prone to erroneous detection.

Further, a change point detection circuit shown in FIG. 2 is conventionally known. This inputs the video signal V _A applied to the input terminal IN directly to the comparator 1, and
Constant voltage V _O determined by resistor 3 and constant current source 4
A signal V _A −V _O obtained by subtracting (level shifting) is input to the comparator 2. In addition, the video signal V _A is delayed for a certain period of time by a delay circuit 5 (the gain of this delay circuit 5 is 1).
(there is no input/output level change), the delayed signal V′ _A is input directly to the comparator 2, and a constant voltage V _O determined by the resistor 6 and constant current source 4 is subtracted (level shifted). The signal V' _A -V _O is input to the comparator 1. Here, the high level voltage of the video signal V _A is expressed as V _AH and the low level voltage is expressed as V _AL . Then, before and after the video signal V _A falls from the high level V _AH to the low level V _AL , the two inputs of the comparator 1 change as shown in FIG. 3a, and the video signal V _A shown by the solid line and the dotted line The level of the delayed and level-shifted signal V′ _A −V _O shown in FIG _.
<V' _A -V _O , and the comparator 1 outputs a falling detection signal. Similarly, before and after the video signal V _A rises from the low level V _AL to the high level V _AH , the two inputs of the comparator 2 change as shown in FIG. 3b, and the level-shifted video signal V shown by the solid line changes.
Immediately after the levels of _A -V _O and the delayed signal V' _A shown by the dotted line rise, V _A -V _O >V' _A , and the comparator 2 outputs a rising detection signal.

As is clear from the above explanation, in the conventional change point detection circuit described above, the video signal V _A changes between the high level V _AH and the low level V _AL with a steep slope exceeding a certain value. , and the level difference between them V
When _AH - V _AL is V _AH -V _AL >V _O , the comparators 1 and 2 detect the change point. Here, the value of the level shift voltage V _O has a very significant meaning. In other words, when the shift voltage V _O is small, the detection sensitivity is high, and although it is appropriate when the light source illuminance of the shooting system is low and the image contrast is low, it is suitable when the light source illuminance is high and the image contrast is high. In this case, the sensitivity is higher than necessary, and there is a high possibility that unnecessary fluctuations (noise) in the video signal V _A caused by dirt on the bar code or the like will be erroneously detected as a level change. On the other hand, if the shift voltage V _O is large, it is strong against noise and is suitable when the contrast of the image is high, but when the contrast of the image becomes low, it becomes impossible to detect the level change point. As described above, the conventional circuit cannot always detect level change points with appropriate sensitivity over a wide range.

This idea was developed to eliminate the above-mentioned drawbacks of the conventional technology. For example, the detection sensitivity automatically changes according to changes in light source illuminance and image contrast, allowing accurate detection over a wide range with less noise influence. The object of the present invention is to provide a change point detection circuit that is operable.

Hereinafter, embodiments of this invention will be described in detail based on the drawings.

FIG. 4 shows a change point detection circuit according to this invention. As described above, the video signal V _A (input signal) that includes an analog fluctuation component and whose level changes relatively sharply between the high level V _AH and the low level _AL is applied to the input terminal IN. This video signal V _A is the comparator 1
While it is directly input to the ground potential, it is attenuated to 1/n (n is a positive number of 1 or more) by voltage dividing resistors R1 and R2 connected in series between the ground potentials, and these resistors R
1, R2 and a constant voltage Vm determined by the constant current source 4 is subtracted (level shifted), and the resulting signal 1/n·V _A -Vm is input to the comparator 2.

Further, the video signal V _A is delayed by a delay circuit 5 for a certain period of time. The gain of this delay circuit 5 is 1, and there is no change in input/output level. Here the delayed signal V′ _A
is input directly to comparator 2, while voltage dividing resistor R
3 and R4, and the constant voltage Vm determined by these resistors R3 and R4 and the constant current source 7 is subtracted, and the resulting signal is 1/n·V' _A − Vm is input to comparator 1. Note that R1=R3, R2=R4, and the constant current sources 7 and 7 are the same.

In the above configuration, before and after the video signal V _A falls from the high level V _AH to the low level V _AL , the two inputs of the comparator 1 change as shown in FIG. 5a, and the video signal V _A shown by the solid line changes. , the level relationship with the delayed, attenuated and shifted signal 1/nV' _A -Vm shown by the dotted line becomes V _A <1/n V' _A -Vm immediately after the fall of the signal V _A , and the comparator A falling detection signal is output from 1.

Similarly, before and after the video signal V _A rises from the low level V _AL to the high level V _AH , the comparator 2
The input changes as shown in FIG. 5, and the level relationship between the signal 1/nV _A -Vm, which is the attenuated and level-shifted signal V _A shown by the solid line, and the delayed signal V' _A is the video signal V Immediately after _A rises, V' _A <1/n·V _A -Vm, and the comparator 2 outputs a rising detection signal.

As is clear from FIGS. 5a and 5b,
When the video signal V _A changes between the high level V _AH and the low level V _AL , the slope is sufficiently steep, and 1/nV _AH Vm>V _AL , that is, V _AH −V _AL >(1− 1/n) V _AH +Vm, the point of change is correctly detected. The value of (1-1/n)V _AH , which determines the detection sensitivity here, changes in proportion to the high level voltage V _AH of the video signal V _A , so the detection sensitivity depends on the video signal depending on the light source illuminance and video contrast. It changes following the fluctuation of V _A.

In other words, when the light source illuminance is low and the image contrast is low, V _AH is low, and both V _AH and V _AL are also small. In this case, the value of (1-1/n) V _AH + Vm is small, and therefore the detection sensitivity is automatically becomes high, and changes in the level of the video signal V _A can be detected with high sensitivity. Furthermore, when the light source illuminance is high and the image contrast is high, V _AH becomes high and V _AH - V _AL also becomes large. In this case, the value of (1-1/n) V _AH + Vm becomes large, and therefore the detection The sensitivity is automatically lowered, making it possible to perform highly reliable detection operations without being affected by unnecessary fluctuations due to noise or the like in the video signal _VA .

Next, other embodiments of this invention will be described. In the above embodiment, in addition to attenuating the signal to 1/n, a constant current source is used to shift the level, but this prevents the sensitivity from increasing more than necessary when V _AH approaches zero. This is to prevent this, and in some cases, it may not be necessary to perform this level shift. Further, in the embodiment described above, one ends of the resistors R2 and R4 are connected to the ground potential, but an appropriate constant voltage may be applied to this point if necessary. Furthermore, in the above embodiment, either the video signal V _A or the delayed signal V′ _A is attenuated at a predetermined ratio and the level is compared with the other, but either one is amplified at a predetermined ratio, The same is true when comparing with the other one. Furthermore, although the above description uses as an example a barcode video signal in a barcode reader, it is of course possible to handle various other analog input signals. Further, if necessary, it may be possible to detect either the rising edge or the falling edge.

As explained in detail above, the change point detection circuit according to the invention includes: an input signal delay circuit that delays an input signal by a predetermined time; a first attenuation circuit that attenuates the input signal before the delay at a predetermined ratio; a second attenuation circuit that attenuates the delayed input signal at a predetermined ratio; and a second attenuation circuit that attenuates the delayed input signal at a predetermined ratio;
This is because it includes a first discrimination circuit that discriminates the level of the non-delayed input signal, and a second discrimination circuit that discriminates the level of the delayed non-attenuated input signal based on the attenuated non-delayed input signal. , the detection sensitivity changes automatically to follow extremely low frequency fluctuations in the input signal, and it always reliably detects changing points, that is, both rising and falling points, with appropriate sensitivity over a wide range. be able to.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing an example of the waveform of an input signal that is the subject of this invention, Fig. 2 is a circuit diagram of a conventional change point detection circuit, and Figs. 3a and 3b are waveform diagrams showing the operation of the same conventional circuit. , FIG. 4 is a circuit diagram of an embodiment of the change point detection circuit of this invention, FIG.
Figure b is a waveform diagram showing the operation of the circuit according to the present invention. 1, 2... Comparator, 5... Delay circuit, 4, 7...
...constant current source, R1 to R4...resistance.

Claims (1)

[Claims for Utility Model Registration] An input signal delay circuit that delays an input signal by a predetermined time; and a first circuit that attenuates the input signal before the delay by a predetermined ratio.
attenuation circuit, and a second attenuation circuit that attenuates the delayed input signal at a predetermined ratio.
a first discrimination circuit that discriminates the level of the non-delayed input signal using the delayed and attenuated input signal as a reference; and a first discrimination circuit that discriminates the level of the non-delayed input signal using the delayed and attenuated input signal as a reference; An analog signal change point detection device, comprising: a second discrimination circuit for level discrimination; and a second discrimination circuit for level discrimination.