The Digitizer: Converting the Waveform into the Language of Bars and Spaces |
Executive Summary |
This article provides a comprehensive, accessible exploration of the digitizer --- the critical circuit in a barcode reader that converts the analog signal from the photodetector into a digital pulse stream that represents the bars and spaces of the barcode. We examine the fundamental principle of threshold comparison, the use of differentiation for enhanced edge detection, and the various techniques employed to reject noise and ensure accurate digitization. Rather than focusing on abstract theory, we ground every concept in concrete design examples and real products from industry leaders including Symbol Technologies, Metrologic Instruments, and other pioneering companies. We explore the conventional comparator-based digitizer, the derivative-based approach that produces a digital signal where transitions correspond to barcode edges, and the multi-bit digitizer that measures edge strength to enable software-based multi-threshold processing. The article also covers practical implementations such as the use of gating signals to reject invalid data and specialized circuits like the margin threshold circuit for clean decoding. The closing summary synthesizes the key lessons and offers practical guidance for anyone designing or selecting digitizer circuits for barcode reading applications. |
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Chapter 1: The Last Analog Stage |
The digitizer is the final analog stage in the barcode reader signal chain. Its job is to take the conditioned, amplified analog signal --- which represents the varying reflectance of the barcode as it is scanned --- and convert it into a digital pulse stream where the widths of the pulses correspond directly to the widths of the bars and spaces. |
This conversion is essential because the decoder, which follows the digitizer, operates on digital signals. The decoder measures the widths of the pulses and compares them to the expected patterns for the barcode symbology being read. If the digitizer introduces errors --- for example, by missing edges, adding false edges, or distorting the pulse widths --- the decoder will be unable to decode the barcode correctly. |
The digitizer faces a significant challenge: the analog signal from the barcode is rarely clean. It contains noise from ambient light, the photodetector, and the amplifiers. It may have baseline wander due to varying DC offsets. The edges of the barcode pulses may be rounded due to the limited bandwidth of the optical system. The amplitude of the signal may vary dramatically from one scan to the next. The digitizer must handle all of these imperfections and still produce a digital signal that accurately represents the barcode. |
A patent from Symbol Technologies explains the digitizer's function in the context of a barcode scanner: 'The output of the filter stage 140 is sent to a digitizer stage 150, which has a particular threshold associated therewith, so as to detect portions of the return signal that are above the particular threshold. Based on these detections, the widths of the bars (i.e., those portions of the return signal that are above the particular threshold) and the widths of the spaces of the scanned bar code symbol can be determined' . |
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Chapter 2: The Comparator-Based Digitizer |
The simplest and most common type of digitizer is the comparator-based digitizer. In this approach, the analog signal is fed into one input of a comparator, and a reference voltage (the threshold) is applied to the other input. When the analog signal is above the threshold, the comparator output is high (representing a white space). When the analog signal is below the threshold, the comparator output is low (representing a black bar). |
The key challenge in comparator-based digitization is setting the threshold correctly. If the threshold is too high, narrow bars may be missed. If the threshold is too low, noise may be interpreted as bars. The threshold must be set to the optimal level for the specific barcode and scanning conditions. |
The patent from Symbol Technologies explains that the digitizer 'has a particular threshold associated therewith, so as to detect portions of the return signal that are above the particular threshold' . However, the patent also acknowledges that fixed-threshold digitizers can be problematic because 'once the bandwidth characteristics of the filter stage 140 are set (during the manufacturing of the bar code scanner), they cannot be adjusted during operation of the bar code scanner' . |
This fixed nature of many conventional digitizers makes them vulnerable to variations in the signal amplitude and noise. The development of adaptive thresholding and other advanced digitization techniques was driven by the need to overcome these limitations. |
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Chapter 3: The Derivative-Based Digitizer |
A more sophisticated approach to digitization uses the derivative of the analog signal. In this approach, the signal is first differentiated --- that is, the rate of change of the signal is calculated. The derivative is high when the signal is changing rapidly (at the edges of the barcode) and low when the signal is constant (in the middle of a bar or space). |
The derivative-based digitizer detects the zero-crossings of the derivative signal. A zero-crossing from positive to negative indicates a falling edge (a transition from white to black), and a zero-crossing from negative to positive indicates a rising edge (a transition from black to white). The time between zero-crossings corresponds to the widths of the bars and spaces. |
A patent describes the use of a derivative circuit that 'performs a first (and sometimes also a second) derivative function on the voltage value, in order to enhance transitions in the amplified voltage output which correspond to dark bar edges and white space edges of the scanned bar code symbol' . |
Another patent describes a barcode reader that 'converts an electric signal generated from reflection light received from a barcode to a differential signal by a differential processing unit, and detects an extreme of the differential signal by an extreme detection unit' . The extreme of the differential signal corresponds to the edges of the barcode. |
The derivative-based approach has several advantages over direct comparison. It is less sensitive to variations in the DC level of the signal, and it can provide more precise edge detection, especially for high-density barcodes with narrow bars. However, it is also more sensitive to high-frequency noise, which can produce false zero-crossings. |
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Chapter 4: The Peak Locating Comparator and False Transition Gating |
A patent from Symbol Technologies describes an advanced derivative-based digitizer that uses a peak locating comparator and a false transition gating circuit to reject noise . The circuit includes an amplifying circuit for amplifying a differentiated signal, a delay circuit for producing a delayed signal, and a peak locating comparator for comparing the differentiated signal to the delayed signal. |
The operation of the circuit is as follows: the differentiated signal is delayed to produce a delayed version. The peak locating comparator compares the differentiated signal to the delayed signal. When the two signals cross, a peak is detected. The time of the crossing corresponds to the peak of the differentiated signal, which is the edge of the barcode. |
The patent describes a 'false transition gating comparator' that 'only changes state upon the first transition of the peak comparator following a transition of the gating comparator. In this way, noise that may cause spurious transitions on the output of the peak location comparator do not cause false transitions on the latch comparator output unless the noise is large enough to trip the gating comparator' . |
This circuit effectively rejects noise that is not large enough to trip the gating comparator, ensuring that only valid edges are digitized. |
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Chapter 5: The Multi-Bit Digitizer: Measuring Edge Strength |
A significant advancement in digitizer design is the multi-bit digitizer, which does not just detect the presence of edges but also measures their strength. This additional information allows the decoder to perform multiple-threshold processing on a single scan, greatly improving the chance of decoding poorly printed or damaged barcodes. |
A patent from a barcode reader manufacturer describes a multi-bit digitizer that detects 'the presence of edges of a scanned barcode symbol, and also measures the strength of each detected edge' . The digitizer outputs a timing signal that is representative of the timing of each edge, and an edge direction signal (polarity) that indicates whether the edge is black-to-white or white-to-black. |
The patent explains that 'the relative strength of successive edges are also determined by the software routines, which make use for that purpose of the relative widths of each respective timing pulse: the timing pulse stays high for a time that is proportional to the magnitude of the bar code element edge strength' . |
This is a clever technique: the width of the timing pulse encodes the edge strength. A stronger edge produces a longer pulse. The decoder can then use this edge strength information to decide which edges are valid and which are noise. |
The patent notes that the advantage of this arrangement is that 'the decoder algorithm can attempt many different thresholds on a single scan (excellent for aggressive wands)' . This is a significant improvement over prior art digitizers that only had a single threshold. |
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Chapter 6: Edge Direction and Timing Signals |
The multi-bit digitizer described in the patent produces two output signals: a timing signal and an edge direction signal . The timing signal consists of pulses, with each pulse corresponding to a detected edge. The width of each pulse represents the strength of that edge. The edge direction signal indicates the polarity of the edge --- whether it is a transition from white to black or from black to white. |
The patent describes how the decoder uses these two signals: 'The software routines determine the direction of the edge by means of the edge direction signal 34 and the timing (position) of the edge by means of the timing signal 32' . |
The decoder then 'constructs the detected bar code pattern from the timing signals and the digital edge strength signals' . This allows the decoder to perform multi-threshold processing on a single scan, greatly improving the chances of decoding poorly printed or damaged barcodes. |
This multi-bit approach is particularly valuable for 'aggressive wands' where only a single scan is available for processing . |
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Chapter 7: The Window Comparator and Variable Threshold |
Another approach to digitization, described in a patent for a single-chip custom linear circuit, uses a window comparator with a variable threshold . The threshold 'tracks the analog signal's amplitude, to maintain the highest possible signal-to-noise ratio for all possible amplitudes' . |
The patent explains that this approach 'allows highly accurate reads every single scan, when the signal levels are high enough, but will adapt itself for a lower signal-to-noise ratio, for those times when the signal is feeble and a few scans might be required to obtain a read' . |
The window comparator's output pulses are used to trigger the Set and Reset inputs of a flip-flop, 'thus producing a square wave that is representative of the symbol being read' . For precise timing information, an additional D-type flip-flop is used, clocked from the output of an exclusive-OR gate. |
This adaptive threshold approach provides excellent performance across a wide range of signal amplitudes, from strong to feeble. |
Chapter 8: The Margin Threshold Circuit |
A margin threshold circuit is a specialized circuit that ensures clean digitization by rejecting signals that are below a certain amplitude. This is useful for preventing false transitions caused by noise when the scanner is not reading a barcode. |
A patent describes a margin threshold circuit that 'gates off the output of the digitizer circuit ... when the amplitude of the first derivative signal is less than 0.7 V to allow a digitized bar pattern with clean margins' . |
The circuit uses a comparator with a diode in the feedback path. The diode prevents 'the two inputs of the comparator from crossing due to noise on the first derivative signal that is less than 0.7 V' . |
This ensures that the digitizer only produces an output when a valid barcode signal of sufficient amplitude is present, rejecting noise and other spurious signals. |
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Chapter 9: The Gating Signal and Speed Validation |
Some digitizers incorporate a gating signal that indicates when the data is unreliable. This signal is typically used to reject scans that are too fast or too slow, which can produce distorted signals. |
A patent describes a multi-bit digitizer that includes a 'gating signal (200) indicates that the data are unreliable and invalid if the measured voltage applied to the scanner motor of the barcode reader is higher or lower than defined limit values. Any scans made at a speed which were too high or too low are rejected' . |
The gating signal is determined by measuring the voltage applied to the scanner motor, which is proportional to the scan speed. If the voltage is outside a defined range, the gating signal indicates that the data is invalid. |
This ensures that the decoder only attempts to decode scans that were made at an appropriate speed, reducing the chances of errors. |
Chapter 10: The Restore-to-White Function |
A restore-to-white function is a feature that forces the digitizer output to the white state (high) when no barcode is being scanned. This prevents the digitizer from producing spurious pulses due to noise. |
A patent describes a digitizer that includes 'a restore-to-white circuit is connected between an output port of the comparator and the second port of the comparator' . |
The restore-to-white circuit detects when the signal falls below a threshold and forces the comparator output to the white state. This ensures that the digitizer output is clean and well-defined, even in the absence of a barcode signal. |
This feature is particularly valuable in handheld scanners, where the user may move the scanner away from the barcode between scans. |
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Chapter 11: The Limiter Circuit for Controlled Swing |
The digitizer described in the patent includes a limiter circuit that controls the output voltage swing. The limiter circuit provides a maximum voltage as the output signal when the reference voltage is greater than the sum signal, and a minimum voltage when the reference voltage is less than the sum signal . |
The patent explains that the limiter circuit provides 'a full peak-to-peak voltage swing capability for a digitizer for a barcode scanner' . |
This controlled swing ensures that the digitizer output is a clean digital signal with well-defined high and low levels, suitable for interfacing with the decoder. |
Chapter 12: The Peak Hold / Bottom Hold Detection |
Another technique for digitization uses peak hold and bottom hold detection to set the threshold dynamically. The peak hold circuit captures the maximum value of the signal (corresponding to the white spaces), and the bottom hold circuit captures the minimum value (corresponding to the black bars). The threshold is set between these two values. |
A patent describes a barcode reader that uses a 'peak hold/bottom hold detection unit' to generate a peak hold value and a bottom hold value. These values are then used by a threshold value setting unit to generate a threshold signal that serves as the determination standard . |
The patent explains that 'the threshold signal is input to the comparison unit, and compared with the above-mentioned differential signal. As a result of this comparison, a signal having a level equal to or more than a threshold value out of the differential signal is determined to be a valid signal, and output as a plurality of comparison signals' . |
This adaptive threshold technique ensures that the digitizer is always operating at the optimal threshold for the specific signal amplitude. |
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Chapter 13: The Extreme Validity Determination |
A barcode reader patent describes a sophisticated technique for determining whether a detected extreme of the differential signal is valid or just noise. The technique uses the timing between extremes to distinguish valid edges from noise . |
The patent explains that 'an inter-extreme time difference is measured to determine whether the extreme is a valid extreme or noise, and the extreme determined to be noise is not stored' . |
The validity of the extreme is determined 'in accordance with the size of the inter-extreme time data' . If the time between extremes is too short (indicating rapid noise), the extreme is rejected. If the time is within a valid range, the extreme is stored. |
This technique reduces the storage requirements and processing time, because only valid extremes are stored and processed. |
Chapter 14: The Adaptive Bandwidth Filter |
The digitizer's performance can be improved by using an adaptive bandwidth filter that adjusts its characteristics based on the signal conditions. A patent describes a filter stage with a 'fixed bandwidth,' but notes that this can be a limitation . The patent explains that 'once the bandwidth characteristics of the filter stage are set (during the manufacturing of the bar code scanner), they cannot be adjusted during operation of the bar code scanner' . |
More advanced digitizers use adaptive filters that can adjust their bandwidth based on the signal conditions. This allows the digitizer to reject more noise when the signal is weak, while preserving the signal's high-frequency components when the signal is strong. |
The use of adaptive filters is a key trend in modern digitizer design, enabling reliable reading of barcodes in a wide range of conditions. |
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Chapter 15: The Single-Chip Custom Linear Circuit |
A patent describes a single-chip custom linear circuit that integrates many of the digitizer functions, including the front end with automatic gain control, the digitizer, the motor drive circuit, and the laser drive circuit . This level of integration reduces the component count and improves performance. |
The patent explains that the single-chip circuit 'achieves a high performance scanner with much fewer components' . The front end transforms the electrical current analog signal from the photosensor into a voltage signal, filters, amplifies and readies it for use by the digitizer. Conditioning of the signal includes proper amplification via the AGC cell as well as removal of noise and the effects of ambient light' . |
The digitizer itself is described as 'the heart of the scanner. It is here that the conditioned analog signal is analyzed and broken down to create the electronic equivalent of the bar code symbol being read' . |
Chapter 16: The Peak Detector for AGC |
The digitizer described in the single-chip patent includes a peak detector that provides a feedback signal for the AGC and provides a tracking threshold for the window comparator . |
The patent explains that 'the peak detector 410 is used to feedback a control voltage signal for the AGC cell; to provide a signal tracking threshold for the digitizer's window comparator; and to feed signal status information to the digitizer's MARGIN circuit, so in those applications where the AGC cell is not used' . |
The peak detector circuit includes a phase splitter/buffer input stage and two full wave and a single wave peak detector stage. The peak detector's output transistors are capable of supplying the surge current needed to quickly charge the loading capacitor . |
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Chapter 17: The Voltage Divider for Threshold Generation |
A patent describes the use of a voltage divider to generate the threshold for the digitizer. The voltage divider is connected to the output of a peak detector, and it produces a scaled-down voltage that is a percentage of the peak voltage . |
The patent explains that 'the voltage at the node between resistors R1 and R2 is a percentage of the voltage across capacitor C1 and, with the value shown, is about 27%' . |
The scaled-down voltage is used as a threshold for comparators that compare it to the first derivative signal and an inverted version of the first derivative signal. This provides a dynamic threshold that tracks the signal amplitude. |
The voltage divider is also used in the margin threshold circuit: 'Resistor R3 in the voltage divider 318 prevents the peak detector 316 from discharging all the way to zero when there is no bar code symbol or other graphics being scanned' . |
Chapter 18: The D-Type Flip-Flop for Phase Locking |
The digitizer described in the single-chip patent uses a D-type flip-flop to lock the output of the digitizer to the edges of the signal. The flip-flop changes state only on the first pulse into its clock input after the main flip-flop changes state. |
The patent explains that 'the output of flip-flop 324 is connected to the data 'D' terminal of flip-flop 308. This causes the output of flip-flop 308, as shown by its output digital signal (wave form KK) to change state only upon the first pulse into its clock input after flip-flop 324 changes its state. Additional clock pulses, such as those caused by noise, are ignored' . |
This phase-locking technique ensures that the digitizer output is stable and does not have spurious transitions due to noise. |
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Chapter 19: The Inter-Extreme Time Measurement |
The extreme validity determination in the barcode reader patent uses a time measurement technique to distinguish valid extremes from noise. The technique measures the time difference between successive extremes and compares it to a threshold . |
The patent explains that 'the extreme validity determination unit 100A includes the memory 101A, the inter-extreme time measurement unit 102A and the time comparison unit 103A' . The inter-extreme time measurement unit measures the time between successive extremes, and the time comparison unit compares this time to a predetermined threshold. |
If the inter-extreme time is too short, the extreme is determined to be invalid (noise). If the time is within a valid range, the extreme is stored in the memory. |
This technique is particularly effective for rejecting high-frequency noise that produces rapid, spurious extremes. |
Chapter 20: The Decoder Algorithm for Multi-Bit Processing |
The multi-bit digitizer patent describes a decoder algorithm that takes the timing signal, the edge direction signal, and the edge strength information and attempts to decode the barcode . The algorithm performs multiple-threshold processing on a single scan by processing the data with different thresholds. |
The patent explains that 'the software routines make use of the edge direction polarity signal, the timing signal, and the information on the strength of the edges (derived from the widths of the timing signal pulses) to attempt to decode the incoming analog signal as a bar code symbol. The output of the decoder algorithm is then evaluated for the validity of the decoded bar code symbol. If the decoded bar code symbol is not a valid bar code symbol, a different threshold is selected in software, and the process is repeated' . |
This ability to try multiple thresholds on the same scan data is a significant advantage, especially when only a few scans cross the barcode symbol during a single swipe. The patent notes that 'the ability to try different noise thresholds in the same scan data greatly improve the chance of decoding' . |
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Chapter 21: The Data Transformation to Standard Format |
The multi-bit digitizer patent describes a software routine that transforms the multi-bit data to a standard format so that a standard decoder algorithm can decode the data . This allows the multi-bit digitizer to be used with existing decoder software, reducing development effort. |
The patent explains that the transform software 'converts the multi-bit data to a standard format so that a standard decoder algorithm can decode the data to a bar code, and also for modifying the threshold used during processing of the multi-bit memory data' . |
This is a practical approach that allows the benefits of multi-bit digitization to be realized without requiring a complete redesign of the decoder software. |
Chapter 22: The Gating Signal and Motor Voltage |
The multi-bit digitizer patent describes a gating signal that is generated based on the voltage applied to the scanner motor . The gating signal indicates when the data are unreliable and invalid if the measured voltage is higher or lower than defined limit values. |
The patent explains that 'a gating signal (200) indicates that the data are unreliable and invalid if the measured voltage applied to the scanner motor of the barcode reader is higher or lower than defined limit values' . |
This is a simple and effective way to reject scans made at speeds that are too high or too low. The patent notes that the gating signal 'may also be known as the 'start of scan' signal' . |
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Chapter 23: The Fuzzy Logic Barcode Reader |
A patent describes a fuzzy logic barcode reader that uses fuzzy logic processing to determine the ratios of barcode element widths . This is a more sophisticated approach to decoding that can handle variations in the element widths due to printing errors, scanning angle, and other factors. |
The patent explains that 'the fuzzy logic barcode reader of the present invention uses fuzzy logic to determine ratios that fall within a predetermined error tolerance. Consequently, ratios can be accurately determined despite variations in the width data' . |
The fuzzy logic processor includes a Min/Max comparator for successively comparing the fuzzified input signals to each other in accordance with predetermined fuzzy logic rules. The processor then determines the optimum rule for execution and executes it to decode the barcode. |
Chapter 24: The Latch Array for Division |
The fuzzy logic barcode reader patent describes a latch array that divides the width data by a predetermined ratio . The latch array outputs the divided-latched data as quotient data to the fuzzy microcontroller. |
The patent explains that 'the latch array 16 latches the width data \( D_{IN} \) into a series of latches that divide the latched data by a predetermined ratio. The latch array 16 thereafter outputs the divided-latched data as quotient data to the fuzzy microcontroller 50' . |
The purpose of the latch array is to normalize the width data so that the fuzzy logic processor can determine the ratios of the element widths. The patent notes that 'for a given barcode pattern, the output of the barcode scanner 10 will vary in a ratiometric manner. ... the ratio of one barcode element to another barcode element remains the same' . |
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Chapter 25: The High Impedance Clamp Circuit |
A patent describes a 'false bar code inhibitor circuit' that includes a high impedance clamp circuit . The clamp circuit suppresses noise by preventing the signal from exceeding a certain amplitude. |
The patent explains that the false bar code inhibitor circuit includes a detector circuit, a signal restore circuit, an externally adjustable threshold comparator circuit, a software controller, and a high impedance clamp circuit . |
The clamp circuit is used to prevent false bars from being detected due to noise or other anomalies in the signal. This is particularly important for reading low-quality barcodes where the signal-to-noise ratio is poor. |
Chapter 26: The Software Controller |
The false bar code inhibitor circuit described in the patent includes a software controller that dynamically sets a window for the barcode read . The window allows the bar code reader to mask false bars in the output bar code from the digitizer. |
The patent explains that 'with this system, a window for a bar code read is dynamically set each time, and with the window the bar code reader is able to mask false bars in the output bar code from the digitizer' . |
The software controller adds a layer of intelligence to the digitizer, allowing it to adapt to different signal conditions and reject false bars that would otherwise cause decoding errors. |
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Chapter 27: The Front End with AGC |
The single-chip custom linear circuit patent describes a front end that includes an automatic gain control (AGC) circuit for controlling the gain of the analog signal prior to digitization . The front end transforms the current signal from the photosensor into a voltage signal, filters, amplifies and readies it for use by the digitizer. |
The patent explains that 'conditioning of the signal includes proper amplification via the AGC cell as well as removal of noise and the effects of ambient light' . |
The front end ensures that the digitizer receives a signal that is within its optimal operating range, regardless of the amplitude of the original signal from the photosensor. This is essential for reliable digitization. |
Chapter 28: The Layout Considerations for the Front End |
The patent notes that the layout of the front end is critical for performance . The current input node is 'critically dependent on the integrated circuit as well as printed circuit board layout to minimize undesirable feedback and/or oscillation.' |
The patent explains that 'the equivalent input noise current and voltage of the first amplifier (INI) is most critical since it determines the minimum detectable signal and therefore limits the dynamic range of the whole system. Layout which minimizes crosstalk for this first stage is paramount' . |
This highlights the importance of careful PCB layout and shielding in the design of high-performance barcode readers. |
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Chapter 29: The Input Bias Current and Droop |
The patent notes that 'the input bias current of the AGC control input (PKAGC), be minimized to reduce drooping on the peak detector capacitor voltage between scans which is typically 27 msec' . The capacitor is held to 1 microfarad or less for the purposes of maintaining fast AGC attack times and a physical size that is consistent with the scanner's application. |
The droop issue is a practical challenge in the design of peak detector circuits. The charge on the capacitor slowly leaks away through the input bias current of the following stage. Minimizing this current is essential for maintaining the peak voltage between scans. |
Chapter 30: The Custom Linear Circuit Integration |
The patent describes a single-chip custom linear circuit that incorporates many functions, achieving a high performance scanner with much fewer components . The chip includes a digitizer, a motor drive circuit, a laser drive circuit, and a front end including an AGC. |
This level of integration is a significant advantage for barcode readers. It reduces the component count, the PCB size, and the cost. It also improves reliability by reducing the number of interconnections. |
The patent notes that the digitizer is 'the heart of the scanner. It is here that the conditioned analog signal is analyzed and broken down to create the electronic equivalent of the bar code symbol being read' . |
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Chapter 31: The Full Wave Peak Detector |
The single-chip circuit uses a full wave peak detector to capture the amplitude of the analog signal . The peak detector is used for AGC and for generating the tracking threshold for the digitizer. |
The patent describes a full wave peak detector that charges a capacitor to the absolute value of the peaks of the signal. The capacitor discharges through resistors, and the voltage across the capacitor is used as a reference for the threshold generation. |
The full wave peak detector provides a more accurate measure of the signal amplitude than a half-wave detector, as it captures both positive and negative peaks. |
Chapter 32: The Window Comparator with Tracking Threshold |
The digitizer in the single-chip circuit uses a window comparator with a tracking threshold . The tracking threshold follows the amplitude of the analog signal, providing optimal performance across a wide range of signal levels. |
The patent explains that 'the threshold tracks the analog signal's amplitude, to maintain the highest possible signal-to-noise ratio for all possible amplitudes' . |
The window comparator produces output pulses that trigger a flip-flop, producing a square wave that represents the barcode. |
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Chapter 33: The Set-Reset Flip-Flop |
The window comparator output triggers the Set and Reset inputs of a flip-flop, producing a square wave that represents the barcode . The flip-flop is set when the signal rises above the upper threshold and reset when it falls below the lower threshold. |
The use of a flip-flop provides hysteresis, which is a form of positive feedback that prevents the output from chattering when the signal is near the threshold. Hysteresis is essential for clean digitization of noisy signals. |
The patent describes 'a flip-flop 412, thus producing a square wave that is representative of the symbol being read' . |
Chapter 34: The D-Type Flip-Flop for Phase Locking |
An additional D-type flip-flop is used for precise timing information . This flip-flop is clocked from the output of an exclusive-OR gate, which is part of a delay loop branch of the circuit. |
The D-type flip-flop locks the output of the digitizer to the edges of the signal, ensuring that the transitions are clean and well-defined. This is important for accurate measurement of the bar and space widths. |
The patent explains that 'for precise timing information (i.e., phase), an additional D-type flip-flop 414 is used and is clocked from the output of the exclusive-OR gate 416 which is part of a delay loop branch of the' circuit. |
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Chapter 35: The Noise Filtering in the Digitizer |
The digitizer circuit includes various noise filtering techniques to reject spurious signals. These include the margin threshold circuit, the peak detector, and the gating comparator. |
The patent explains that the margin threshold circuit 'prevents the two inputs of the comparator from crossing due to noise on the first derivative signal that is less than 0.7 V' . The peak detector provides a signal tracking threshold, and the gating comparator rejects spurious transitions. |
The combination of these techniques provides a high degree of noise immunity, ensuring that the digitizer output is a clean representation of the barcode signal. |
Chapter 36: The Restore-to-White Circuit |
The restore-to-white circuit described in the patent forces the digitizer output to the white state when no barcode is being scanned . This prevents the digitizer from producing spurious pulses due to noise. |
The patent explains that the digitizer includes 'a restore-to-white circuit is connected between an output port of the comparator and the second port of the comparator' . |
The restore-to-white circuit is an important feature for handheld scanners, where the user may move the scanner away from the barcode between scans. It ensures that the digitizer output is clean and well-defined. |
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Chapter 37: The Digitizer as an ASIC Core |
The fuzzy logic patent describes a system where the fuzzy logic processor is implemented as an ASIC core . This allows the digitizer and decoder functions to be integrated into a custom chip. |
The patent explains that the 'fuzzy input processor, fuzzy output processor and fuzzy memory interface are designed for maximum flexibility and may be used separately with other digital systems as cores for Application Specific Integrated Circuit (ASIC) designs' . |
The use of ASIC cores reduces the cost and power consumption of the barcode reader, making it suitable for high-volume applications. |
Chapter 38: The Concept of Edge Strength |
The multi-bit digitizer is based on the concept of edge strength. Not all edges are equal; some edges are strong (sharp transitions) and some are weak (gradual transitions). The edge strength provides valuable information about the quality of the barcode signal. |
A patent explains that the digitizer 'detects the presence of edges of a scanned barcode symbol, and also measures the strength of each detected edge' . The edge strength is encoded in the width of the timing pulse: 'the timing pulse stays high for a time that is proportional to the magnitude of the bar code element edge strength' . |
The edge strength information is used by the decoder to determine which edges are valid and which are noise. This is a significant improvement over conventional digitizers that only detect the presence of edges. |
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Chapter 39: The Software and Hardware Partitioning |
The multi-bit digitizer patent describes a partitioning of functions between hardware and software . The sensor, the signal edge detector, and the means for detecting edge direction are implemented in hardware, while the decoder is implemented in software. |
The patent explains that 'the sensor, the signal edge detector, and the means for detecting edge direction are all implemented in circuits in hardware, while the decoder (including the routines for determining relative edge strengths) are implemented in software' . |
This partitioning allows the digitizer to be implemented efficiently, with the high-speed functions performed in hardware and the more complex functions performed in software. |
Chapter 40: The Dynamic Window Setting |
The false bar code inhibitor circuit described in a patent uses a dynamic window to mask false bars . The window is set each time a barcode is read, based on the characteristics of the signal. |
The patent explains that 'a window for a bar code read is dynamically set each time, and with the window the bar code reader is able to mask false bars in the output bar code from the digitizer' . |
This dynamic window setting is a form of adaptive thresholding that allows the digitizer to adapt to different signal conditions, improving decoding accuracy. |
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Chapter 41: The Differentiator and Filter Combination |
The digitizer described in a patent includes a differentiator and a filter in combination . The differentiator enhances the edges of the signal, and the filter removes noise. |
The patent explains that 'the output of the derivative circuit 130 is sent to a filter stage 140, which typically has a fixed bandwidth' . The bandwidth characteristics of the filter stage are set based on the typical scanning range of the bar code scanner, as well as the typical bar widths that are to be scanned by the bar code scanner. |
The combination of differentiation and filtering provides a clean signal for digitization, with enhanced edges and reduced noise. |
Chapter 42: The Digitizer in the Full Signal Chain |
The digitizer is the final stage in the analog signal chain, following the transimpedance amplifier, the amplifier stages, and the filter. The signal chain prepares the photodetector signal for digitization. |
The patent explains the complete signal chain: 'the return light from a scanned bar code symbol is received by a photodetector 100, which converts the received light into a current value. The current value is sent to a transimpedance amplifier 110, which converts the current value into a voltage value. The voltage value is input to an amplifier stage 120, which provides a constant gain to the voltage value. The amplified voltage output from the amplifier stage 120 is sent to a derivative circuit 130, which performs a first (and sometimes also a second) derivative function on the voltage value. The output of the derivative circuit 130 is sent to a filter stage 140. The output of the filter stage 140 is sent to a digitizer stage 150' . |
This signal chain represents the classic architecture of an analog front end for a barcode reader. |
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Chapter 43: The Practical Implementation of the Digitizer |
The digitizer can be implemented using discrete components, integrated circuits, or as part of a custom ASIC. The choice of implementation depends on the performance requirements, cost constraints, and volume of the application. |
The single-chip custom linear circuit described in a patent is an example of a highly integrated implementation . The digitizer is integrated with the front end, the AGC, the motor drive, and the laser drive on a single chip. |
This level of integration is a significant advantage for barcode readers, reducing the component count, the PCB size, and the cost. |
Chapter 44: The Waveshare Barcode Scanner Module |
The Waveshare Barcode Scanner Module is a modern example of a commercial barcode reader that incorporates advanced digitization and decoding capabilities . The module uses advanced image recognition algorithms to decode various barcode formats. |
The module supports UART, USB, and Keyboard emulation modes, and can be configured for various parameters including LED indicators, buzzer alerts, exposure, sensitivity, and scanning modes . |
This demonstrates the trend toward highly integrated, software-configurable barcode readers that are easy to integrate into systems. |
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Chapter 45: Summary --- The Digitizer in Perspective |
The digitizer is the critical bridge between the analog world of reflected light and the digital world of the decoder. It converts the conditioned analog signal into a digital pulse stream that represents the barcode, enabling the decoder to extract the encoded information. |
We have examined how different companies and technologies have approached the challenges of digitization: |
Symbol Technologies developed a derivative-based digitizer that uses a peak locating comparator and a false transition gating circuit to reject noise . The digitizer obtains all of its threshold information from the first derivative of the analog signal, providing robust digitization even with varying ambient light. |
Metrologic Instruments developed a single-chip custom linear circuit that integrates the digitizer with the front end, AGC, motor drive, and laser drive . The digitizer uses a window comparator with a tracking threshold and a margin threshold circuit for clean digitization. |
A barcode reader manufacturer developed a multi-bit digitizer that detects the presence of edges and measures their strength . The digitizer outputs timing and edge direction signals to a decoder, which can perform multiple-threshold processing on a single scan. |
Another patent describes a digitizer that uses peak hold/bottom hold detection and extreme validity determination based on inter-extreme time measurement . |
A fuzzy logic patent describes a decoder that uses fuzzy logic processing to determine the ratios of barcode element widths . |
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The key lessons from our exploration are: |
The digitizer converts analog signals to digital pulses. The pulse widths correspond to the bar and space widths of the barcode. |
Threshold setting is critical. The threshold must be set correctly to ensure accurate digitization. Adaptive thresholding techniques improve performance. |
Differentiation enhances edges. The derivative of the analog signal emphasizes the edges, making them easier to detect. |
Edge strength provides valuable information. Strong edges are more likely to be valid than weak edges. Multi-bit digitizers encode edge strength for the decoder. |
Noise rejection is essential. Digitizers use various techniques, including gating circuits, margin thresholds, and peak detectors, to reject noise. |
Digitizers are increasingly integrated. Modern barcode readers integrate the digitizer with other functions on a single chip. |
In the end, the digitizer is a testament to the ingenuity of engineers who have found ways to convert a noisy, varying analog signal into a clean digital representation of a barcode. The art of digitization lies in the careful balance of threshold setting, noise rejection, and edge detection, creating a signal that the decoder can reliably interpret. |
cs@easiersoft.com