JP2011503712A - Signal processing for light detection - Google Patents

Abstract

In optical code sensors of the type that produce a signal resulting from their transitions from rays encoded with digital data, a variable bandwidth filter is placed before processing to extract the data. The optical code distance is detected and the filter bandwidth is increased in relation to the bar code distance. The input signal of the filter is input to a shift register, and the output at each stage of the shift register is multiplied by a predetermined coefficient calculated to be a specific waveform, preferably a Gaussian waveform, using all the coefficient sets. . The outputs multiplied by each coefficient of the shift register are added together to produce a filtered output signal that is input to the processing unit.
[Selection] Figure 2

Description

  The present invention relates to signal processing, and more particularly to a method and filter for processing a ray encoded with data to improve the signal to noise ratio (S / N) of the signal that is extracted after processing.

  Data communication using light as a transmission medium has received wide support. One application in which light-encoded data has been used for some time is an optical code scanning device such as a barcode scanning device (scanner).

  Today, barcode scanners typically use laser light to illuminate remote barcodes. FIG. 1 is a block diagram for explaining the operation of a conventional barcode scanning apparatus. The laser beam L reflected from the barcode is modulated by the dark portion and the bright portion of the barcode, and is detected by the photodiode PD that generates a current corresponding to the modulated light. The current signal is input to the preamplifier 10 and converted into a voltage signal. The voltage signal is supplied to the differentiation circuit 12, and a pulse corresponding to each transition (transition) in the output of the preamplifier is generated. The automatic gain control (AGC) circuit 14 keeps the amplitude of the signal substantially constant regardless of the fluctuation of the received signal. Next, the gain-controlled signal is input to a low-pass filter (LPF) 16 to remove external high-frequency fluctuations, and then input to an analog-digital converter (ADC) 18. The resulting digital signal is input to a processor 20 that decodes the barcode from the signal.

  In this barcode decoding, the frequency spectrum of the signal supplied to the filter changes according to the distance from the scanning device to the barcode, which causes a problem of limiting the effectiveness of the filter. In the case where the processing device cannot accurately recognize the barcode, conventionally, a kind of search processing for adjusting the bandwidth of the filter stepwise has been performed in order to improve the recognition of the processing device. However, this is a time-consuming and complicated process if not impossible, making it difficult to read barcodes efficiently and accurately.

  One feature of the present invention is to replace the low pass filter with a variable bandwidth filter in an optical code sensor of the type described above. The optical code distance is sensed and the filter bandwidth is increased as a function of the bar code distance. The input signal of the filter is fed to a shift register and the output of each stage of that register may be multiplied by a predetermined coefficient calculated to be a specific waveform, preferably a Gaussian waveform, using all the coefficient sets. . The outputs of each stage of the shift register multiplied by the coefficients are added together to produce a filtered output signal that feeds the processing unit.

  In order to adjust the bandwidth, the clock ratio of the register may be adjusted. In particular, the sensed distance to the barcode is used to control the frequency of the voltage controlled oscillator (VCO) that generates the clock for the shift register.

  The foregoing brief description of the invention, as well as further objects, features, and advantages thereof will be more fully understood by a detailed description of the preferred embodiment of the invention with reference to the accompanying drawings.

It is a block diagram for demonstrating the effect | action of the conventional barcode scanning apparatus. It is a block diagram for demonstrating the effect | action of the Example of the barcode scanning device by this invention. It is a figure which shows the Gaussian waveform of the filter which has a different standard deviation (bandwidth). It is a block diagram which shows the suitable Example of a digital filter. It is a wave form diagram useful for understanding the operation of a digital filter.

  FIG. 2 is a block diagram for explaining the operation of the embodiment of the barcode scanning apparatus according to the present invention. The laser beam L reflected from the barcode is modulated by the dark portion and the bright portion of the barcode, and is detected by the photodiode PD that generates a current corresponding to the modulated light. The current signal is input to the preamplifier and converted into a voltage signal. The voltage signal is supplied to the differentiation circuit 12, and a pulse corresponding to each transition (transition) in the output of the preamplifier is generated. The automatic gain control (AGC) circuit 14 keeps the signal amplitude substantially constant regardless of fluctuations in the received signal. Thereafter, the gain-controlled signal is input to an analog-to-digital converter (ADC) 18. The resulting digital signal is input through the digital filter 30 and the digitizer 22 to the processing device 20 that decodes the barcode from the signal.

  Generally speaking, the digital filter 30 is a variable filter whose bandwidth can be controlled. The distance detector 24 detects the distance to the barcode, and the bandwidth of the digital filter 30 is controlled in direct relation to the distance.

  The pulse received by the digital filter 30 has a substantially Gaussian waveform. Those skilled in the art of communication theory will appreciate that the best S / N response is obtained with a matched filter. That is, matching the frequency spectrum with the time domain characteristics of the input signal. Thus, in the preferred embodiment, the filter 30 was designed to have a Gaussian time domain response.

FIG. 3 is a diagram showing Gaussian waveforms of filters having different standard deviations (bandwidths). Points K ₀ , K ₁ , K ₂ , and K ₃ are marked on one waveform. These indicate coefficient values used for drawing a Gaussian curve or a normal curve described later.

FIG. 4 is a schematic block diagram showing a preferred embodiment of the filter 30. A signal from the ADC 18 is supplied as an input to the shift register 32, and an output is taken out from each stage (stage). The multiplier 34 multiplies _{one of the} coefficients K ₀ , K ₁ , K ₂ , and K ₃ . The outputs of the shift register thus multiplied are all added in an adder 36 to generate an output signal that is then supplied to the processing unit 20. This process is performed continuously in real time by efficiently convolving the input signal from the ADC 18 with the characteristics of the digital filter 30. One skilled in the art will recognize that this is equivalent to multiplying the input signal characteristics by the filter characteristics in the frequency domain. In other words, this is basically the same as treating the filter characteristic as a transfer function and obtaining the output signal by the filtering action on the input signal.

  The voltage controlled oscillator (VCO) 40 supplies a clock signal to the shift register 32. The voltage controlled oscillator is driven by the distance detector 50. The distance detector 50 generates a voltage signal that increases with the distance of the barcode, thereby increasing the output frequency of the VCO 40. This effectively increases the bandwidth of the filter 30 while retaining the same characteristic waveform.

  FIG. 5 shows waveforms that help to further understand the operation of the digital filter 30. Waveform A shows the input waveform to the filter. Waveform B shows the Gaussian characteristic of filter 30 and waveforms C and D show the convolution waveform between waveforms A and B when different frequencies are given by VCO 40. In this case, the frequency of the waveform D is increased from the waveform C, and as a result, the S / N is improved. Of course, the waveforms C and D correspond to the differential form of the signal received from the preamplifier 10. A signal corresponding to the detected barcode is obtained by integrating these waveforms.

  While the preferred embodiment of the invention has been described, those skilled in the art can make various additions, modifications and substitutions without departing from the scope and spirit of the invention as defined by the appended claims. You will understand.

Claims (20)

A method for improving the signal to noise ratio in a signal resulting from its transition from a ray encoded with digital data, comprising:
Before processing the signal to derive the digital data, the digital data is input to a filter whose bandwidth is controlled in direct relation to the distance from the light source to the target to be read. To improve the signal-to-noise ratio. The signal to noise ratio improving method according to claim 1,
A method for improving a signal-to-noise ratio, wherein the filter is a low-pass filter, and its bandwidth is determined by a cutoff frequency. The signal to noise ratio improving method according to claim 1,
The method for improving a signal-to-noise ratio, wherein the filter has a waveform matched to a pulse waveform of the signal. The signal to noise ratio improving method according to claim 1,
A signal to noise ratio improving method, wherein the light beam is a laser beam. The signal to noise ratio improving method according to claim 1,
A method for improving signal to noise, wherein the light beam is reflected from an optical code and is laser light. The signal to noise ratio improving method according to claim 1,
The filter is a digital filter, the bandwidth of which is controlled by the frequency of the variable oscillator, further detecting the distance of the light source of the light beam, and adjusting the frequency of the oscillator in direct relation to the detected distance. A method for improving a signal-to-noise ratio. The signal-to-noise ratio improving method according to claim 6,
The method for improving a signal-to-noise ratio, wherein the filter has a waveform matched to a pulse waveform of the signal. The signal-to-noise ratio improving method according to claim 6,
The filter has a shift register that receives the signal and is shifted by the variable oscillator, and each output has a predetermined output so that the shift register generates an output of the filter necessary for processing to extract the data. A method for improving a signal-to-noise ratio, comprising a plurality of stages added with a weight of The signal-to-noise ratio improving method according to claim 6,
A method for improving the signal-to-noise ratio, characterized by using a distance detector for detecting the distance and thereby controlling the frequency of the oscillator. The signal to noise ratio improving method according to claim 9,
The method of claim 1, wherein the oscillator is a voltage controlled oscillator that responds to the voltage output of the distance detector. A filter for improving the signal-to-noise ratio in a signal resulting from its transition from a ray encoded with digital data, disposed before the signal is input to a processing device to extract the digital data;
A filter unit having a control input for controlling the bandwidth in direct relation to the value of the control signal;
A filter comprising: a distance detection unit for determining a distance of a light source of the light beam and generating a signal applied to the control input directly related to the distance. The filter of claim 11.
The filter, wherein the filter unit is a low-pass filter, and a bandwidth thereof is determined by a cutoff frequency. The filter of claim 11.
The filter, wherein the filter unit has a waveform matched to a pulse waveform of the signal. The filter of claim 11.
The filter, wherein the light beam is a laser beam. The filter of claim 11.
A filter, wherein the light beam is laser light reflected from an optical code. The filter of claim 11.
The filter unit is a digital filter having a bandwidth control input and responsive to a frequency of an input signal to adjust the bandwidth in direct relation to the frequency, and a variable oscillator connected to the control input; And a distance detector for detecting the distance of the light source of the light beam and adjusting the frequency of the oscillator in direct relation to the distance. The filter of claim 16,
The filter, wherein the filter portion has a waveform matched to a pulse waveform of the signal. The filter of claim 16,
The filter unit includes a shift register that receives the signal and is shifted by the variable oscillator, and further includes a weighter that gives a predetermined weight to each output stage of the register, and the processing necessary for extracting the data. A filter comprising: an adder that combines the weighted outputs to produce the output of the filter. The filter of claim 18.
The filter, wherein the oscillator is a voltage controlled oscillator that is responsive to the distance detector. The filter of claim 18.
The filter, wherein the predetermined weight is related to the pulse waveform of the signal.

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