The Digitizer Output: The Barcode Signal Decoded |
Executive Summary |
This article provides a comprehensive, accessible exploration of the digitizer output --- the final product of the analog front end that carries the barcode information to the decoder. We examine the structure and characteristics of the digitized bar pattern (DBP) signal, the challenges of accurately representing barcode transitions, and the advanced techniques developed to enhance decoding performance. Rather than focusing on abstract theory, we ground every concept in concrete design examples and real patent disclosures from industry leaders including Symbol Technologies and other pioneering companies. We explore the traditional single-DBP signal that conveys timing and polarity, the multi-bit enhanced signal that adds strength information, the dual-DBP signal that combines high-sensitivity and low-sensitivity digitization, and the multiplexing techniques that allow enhanced signals to be transmitted over existing interfaces. The article also covers special cases such as inverse barcode digitization and the practical challenges of maintaining compatibility with legacy systems. 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 Final Product |
The digitizer output is the culmination of all the preceding analog processing. It is the signal that carries the barcode information from the analog front end to the digital decoder. Everything --- the photodetector, the transimpedance amplifier, the gain stages, the filters, the peak and valley detectors, the comparator with hysteresis --- exists to produce a clean, accurate digitized bar pattern (DBP) signal that faithfully represents the bars and spaces of the scanned barcode. |
The traditional DBP signal is a pulse width coded (modulated) wave that represents specific transition times of the analog signal from the detector . This signal is transmitted across a single signal line to the decoder, and its pulses indicate when the scanning beam transitioned from light to dark areas (and vice versa) that were sufficiently different in optical contrast to be detected by the digitizer circuit . |
The polarity of this signal indicates the presumed lightness or darkness of the surface as it is traversed by the scanning beam . A high level typically represents a white space, and a low level typically represents a black bar. The widths of the high and low pulses correspond to the widths of the spaces and bars, respectively. |
However, as we shall see, the traditional single-DBP signal has limitations. It provides no information about the strength of the transitions, making it difficult to distinguish true barcode edges from noise or print defects. Advanced digitizer designs have addressed these limitations by producing enhanced signals that carry additional information to the decoder. |
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Chapter 2: The Structure of the DBP Signal |
The digitized bar pattern (DBP) signal is a time-coordinated sequence of pulses. Each pulse in the signal corresponds to a transition in the analog signal --- a change from light to dark or from dark to light. |
The timing of the pulses is critical. The decoder measures the widths of the pulses and the gaps between them to determine the widths of the bars and spaces. These measurements are compared to the expected patterns for the barcode symbology being read. |
The polarity of the signal is also important. The polarity indicates whether the transition was from a white space to a black bar (a falling edge) or from a black bar to a white space (a rising edge). The decoder uses this information to reconstruct the bar-space sequence. |
The DBP signal is typically transmitted as a square wave, with well-defined high and low levels. The transitions are clean and sharp, ensuring that the decoder can accurately measure the pulse widths. |
The patent from Symbol Technologies explains that the digitizer output is a pulse width coded wave 'representing specific transition times of the analog signal from the detector' . This pulse width coded wave is 'transmitted across a single signal line to the decoder' . |
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Chapter 3: The Timing Signal and Its Limitations |
The traditional single-DBP signal provides only timing information. It tells the decoder when transitions occurred and their polarity, but it provides no information about the strength of those transitions. |
This limitation is significant for decoding poor-quality barcodes. When a barcode has print defects, the analog signal may contain spurious transitions caused by the defects. The digitizer cannot distinguish these false transitions from true barcode edges because it only sees the timing and polarity, not the strength. |
As the patent explains: 'No information is provided, however, as to whether a transition was relatively weak (barely crossing the threshold) or strong (representing a significant and sudden change in optical contrast)' . |
This lack of strength information is particularly problematic for low-contrast barcodes or for barcodes with defects that create small modulations in the analog signal. The digitizer may produce false edges that confuse the decoder. |
Advanced digitizer designs address this limitation by generating enhanced signals that include strength information, allowing the decoder to make more intelligent decisions about which transitions are valid. |
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Chapter 4: The Multi-Bit Enhanced Signal |
The multi-bit enhanced signal is an advanced digitizer output that provides both timing and strength information. It typically comprises two time-coordinated signals: timing and strength . |
The timing signal again indicates when (relative to a Start of Scan signal) transitions from light to dark (and vice versa) occurred. The strength signal (whether area strength or edge strength) provides additional information to the decoder that can be used to determine which transitions were due to print defects or noise . |
The relative strength of the transitions can also provide clues about relative depth of modulation, which can aid in deblurring the signal . The strength information is typically conveyed using multiple parallel or serial pulses to convey an 8-bit value . |
This additional information allows the decoder to perform multiple-threshold processing on a single scan. The decoder can try different thresholds and use the strength information to decide which edges are valid and which are noise. |
The patent notes that 'the relative strength of the transitions can also provide clues about relative depth of modulation' . This is particularly valuable for decoding barcodes that are slightly out of focus, where the edges are blurred and the transitions are gradual. |
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Chapter 5: The Dual-DBP Signal |
The dual-DBP signal is another advanced digitizer output. It typically comprises two time-coordinated signals, each a traditional DBP signal, but one signal was generated using a more sensitive digitizer threshold than the other . |
The dual-digitizer circuit has been designed so that a transition on the high-sensitivity digitizer that is confirmed by a transition that also occurred on the low-sensitivity digitizer is indicative of a relatively strong analog signal (having a relatively large amplitude) . |
Transitions on the high-sensitivity digitizer that are not so confirmed by the low-sensitivity digitizer are indicative of a relatively weak analog signal . Therefore, the output signal of the high-sensitivity digitizer may contain more pairs of transitions than the output signal of the low-sensitivity digitizer . |
For example, a single bar pulse on a noisy analog signal may be 'broken up' into a bar/space/bar triplet of pulses on the high-sensitivity digitizer's signal due to a subtle print defect in the bar . The bar may be properly detected by the low-sensitivity digitizer, thus facilitating proper decoding of the overall pattern. |
This dual-threshold approach provides a powerful method for distinguishing valid edges from print defects and noise. |
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Chapter 6: Multiplexing Enhanced Signals |
One of the challenges of enhanced digitizer signals is that they cannot be sent over the conventional single signal line interface between scan engines that provide a single DBP signal to a decoder . |
The patent addresses this problem by multiplexing the enhanced signals (either multi-bit or dual-DBP) onto a single DBP channel, so that no hardware modifications are required . Neither the connectors nor the acquisition ASIC need to change. |
The multiplexed-DBP engine-output option can be enabled and utilized by a scanner driver that is aware of the new feature, but older versions of the terminal software will by default receive the standard single-DBP signal . |
Thus compatibility is maintained between the hardware and software of current and future terminals so that new engines can be dropped into existing terminals without modification, and when desired, the terminal's software can be upgraded to take advantage of the enhanced signals . |
The patent describes a system where the multiplexer 'receives the timing signal and strength signals for a given transition and encodes the timing and strength signals into a single signal by transmitting a pulse track for the given transition that includes a coded indicator pulse indicating the polarity of the given transition followed by a strength pulse that correlates to the strength of the given transition' . |
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Chapter 7: The Multiple Sensitivity-Mode Digitizer |
Another approach to enhancing the digitizer output is the multiple sensitivity-mode digitizer. This circuit generates a first digitized signal using a first threshold and a second digitized signal using a second threshold, and then selects between them based on the decoding requirements . |
The patent describes a digitizer circuit that includes 'a first thresholding circuit for generating a first digitized signal in response to a comparison between the first derivative signal and a first threshold representing a first detection sensitivity' . |
A second thresholding circuit generates a second digitized signal in response to a comparison between the first derivative signal and a second threshold representing a second detection sensitivity. A mode selector then selects as the output digitized signal one of the first digitized signal and a third digitized signal, where the third digitized signal represents the first digitized signal modified by the second digitized signal . |
This design provides flexibility for handling different barcode qualities. The high-sensitivity mode is useful for reading symbols with a large depth of focus, but it also makes the digitizer sensitive to print defects. The low-sensitivity mode is less sensitive to defects but may miss weak signals . |
The mode selector allows the reader to choose the optimal mode for the current barcode, or to combine the two modes to get the best of both. |
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Chapter 8: The Trade-off Between Sensitivity and Defect Immunity |
The multiple sensitivity-mode digitizer patent explains the fundamental trade-off in digitizer design. The first category of digitizers, using diode thresholding, can digitize low levels of modulation of the analog signal, making them useful for reading symbols with a large depth of focus for a given symbol density . |
However, the features that allow these digitizers to provide a large depth of focus also make the digitizers sensitive to print defects on the symbol that cause relatively low levels of modulation of the analog signal . |
This is particularly a problem when a poor quality symbol is located near the 'waist' of the laser beam (typically at about one to three inches from the nose of the scanner) where small defects are more easily resolved . As a result, hand-held laser scanners cannot read dot matrix symbols or other symbols with defects in this range using the first category of digitizers . |
The second category of digitizers, high-thresholding digitizers, will not respond to modulations of the first derivative of the analog signal unless that modulation exceeds some predetermined threshold . That threshold is a percentage of the height of modulation caused by the wide bars or spaces in the symbol being scanned . |
The multiple sensitivity-mode digitizer provides a way to balance these competing requirements. |
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Chapter 9: The Inverse Barcode Challenge |
A special challenge for digitizers is the inverse barcode. A direct or normal barcode has black bars on a white background. An inverse barcode has white bars on a black background. The digitizer must produce the correct output for both types. |
A patent describes the problem: 'An inverse bar code is a code where the bars are more reflective than the background on which they are disposed including the spaces therebetween' . |
The difficulty is that the digitizer's restore-to-white function, which forces the output to the white state when no barcode is being scanned, can cause erroneous digitization when an inverse barcode is scanned. The patent explains that 'an erroneous digitization often occurs when the background changes from black to white initially as the code is scanned. This is called the first transition of the code' . |
Such a first transition will be digitized as a black bar, particularly in a dual diode digitizer. This erroneous black bar signal is distorted and invariably will either not be decoded or result in erroneous data representing the code upon decoding . |
The solution is to provide means for changing the state to which the restoring means restores the output when the inverse bar code is being digitized, thereby inhibiting a premature change of state at the onset of scanning of the code . |
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Chapter 10: Restoring the Output State |
The restore-to-white function is a common feature in digitizers. It forces the output to the white state when no barcode is being scanned, preventing the digitizer from producing spurious pulses due to noise. |
For inverse barcodes, this function must be modified. The digitizer must be restored to a state corresponding to the background of the inverse code, which is black. |
The patent explains that 'the digitizer is operated, automatically under computer control on different ones of a series of successive scans of the code, which are carried out until the code is successfully read, to provide an output, which is of different states when reading white sections and black sections of the code, and to be restored, respectively, to a state corresponding to the reading of a black section of the code and to a state corresponding to the reading of a white section of the code, when a direct code is to be digitized and when an inverse code is to be digitized' . |
In effect, the output is prevented from being restored to a black representing state when a direct code is to be digitized and to a white representing state when an inverse code is to be digitized . In addition, the digitizer has circuitry for inverting the state of the digitized bar code output of the digitizer when inverse codes are digitized . |
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Chapter 11: Inverting the Output |
When an inverse barcode is being digitized, the polarity of the output must be inverted. If the digitizer outputs a high level for white bars and a low level for black spaces (which is the opposite of a normal barcode), the decoder will be confused. |
The patent describes a system that 'includes means for inverting the states of the bar code representing digitized output when the inverse bar code is being digitized to as to provide a digitized output having like states whether representing direct or inverse bar codes' . |
The inversion ensures that the decoder always receives the same polarity, regardless of whether the barcode is direct or inverse. This simplifies the decoder design and improves reliability. |
The inversion is typically controlled by the microprocessor, which can detect whether the barcode is inverse based on the characteristics of the signal. |
Chapter 12: The Inhibitor Circuit |
The patent mentions an inhibitor circuit that prevents false transitions in the digitizer output. The inhibitor circuit is a type of false transition gating circuit that ensures that only valid edges are passed to the decoder. |
The patent refers to a 'bar code inhibitor circuit of the type described in the Bremer application' . This circuit is used to prevent false transitions when the scanner is in the border areas of the barcode. |
The inhibitor circuit works by gating the output of the digitizer when the signal is below a certain threshold. This ensures that small noise signals do not produce false transitions. |
The inhibitor circuit is particularly important for reading inverse barcodes, where the background is black and the signal may be noisy. |
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Chapter 13: The Digitizer Output to the Decoder |
The digitizer output is connected to the decoder, which measures the pulse widths and decodes the barcode. The decoder typically includes a timer/capture unit that can measure the duration of pulses with high precision. |
The decoder analyzes the sequence of pulses, measuring the widths of the bars and spaces. It compares these measurements to the expected patterns for the barcode symbology being read. |
The patent describes the digitizer output being 'directed to a bar pattern decoder circuit, which it can drive directly in some embodiments' . The decoder then decodes the signal and presents the results. |
The quality of the digitizer output directly affects the decoding performance. If the digitizer output contains false edges or distorted pulse widths, the decoder will be unable to decode the barcode correctly. |
Chapter 14: Compatibility with Legacy Systems |
A key requirement for enhanced digitizer outputs is compatibility with legacy systems. The digitizer must be able to produce a standard single-DBP signal for systems that do not support the enhanced signals. |
The patent explains that 'the multiplexed-DBP engine-output option can be enabled and utilized by a scanner driver that is aware of the new feature, but older versions of the terminal software will by default receive the standard single-DBP signal' . |
This ensures that new engines can be dropped into existing terminals without modification. When desired, the terminal's software can be upgraded to take advantage of the enhanced signals. |
This backward compatibility is essential for commercial barcode readers, where the installed base of systems must be supported. |
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Chapter 15: The Role of the Microprocessor |
The microprocessor plays a key role in controlling the digitizer output. It can select the mode (standard, multi-bit, or dual-DBP), control the multiplexing, and manage the restore-to-white function. |
In the inverse barcode digitizer, the microprocessor controls the restoration state and the output inversion. It can detect that an inverse barcode is being scanned and adjust the digitizer accordingly. |
The patent describes that 'the digitizer is operated, automatically under computer control on different ones of a series of successive scans' . This automatic control ensures that the reader can handle both direct and inverse barcodes without user intervention. |
The microprocessor's involvement in the digitizer output is a key advantage of modern barcode readers. It allows for flexible, adaptive digitization that can handle a wide range of barcode qualities and types. |
Chapter 16: The Start of Scan Signal |
The Start of Scan signal is an important reference for the digitizer output. It indicates when a scan of the barcode begins, allowing the decoder to synchronize with the digitizer. |
The patent mentions that the digitized bar pattern signal is 'transmitted across a single signal line to the decoder' and that the timing signal indicates 'when (relative to a Start of Scan signal) transitions from light to dark areas (and vice versa)' occurred . |
The Start of Scan signal is typically generated by the scanning mechanism. It provides a reference point for measuring the timing of the transitions. |
The decoder uses the Start of Scan signal to determine the absolute timing of the barcode elements, which is important for decoding. |
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Chapter 17: The Strength Signal Encoding |
The strength signal in the multi-bit digitizer encodes the strength of each detected edge. The strength information is typically conveyed using multiple parallel or serial pulses to convey an 8-bit value . |
The strength pulse 'correlates to the strength of the given transition' . A stronger edge produces a longer pulse or a higher value. |
The patent explains that the multiplexer 'receives the timing signal and strength signals for a given transition and encodes the timing and strength signals into a single signal by transmitting a pulse track for the given transition that includes a coded indicator pulse indicating the polarity of the given transition followed by a strength pulse that correlates to the strength of the given transition' . |
This encoding scheme allows the strength information to be transmitted over a single signal line, along with the timing and polarity information. |
Chapter 18: The Pulse Track Encoding |
The pulse track encoding used in the multiplexed-DBP signal is a key innovation. It allows the timing, polarity, and strength information to be combined into a single signal. |
The encoding works by sending a coded indicator pulse that indicates the polarity of the transition, followed by a strength pulse that indicates the strength of the transition . |
This approach is efficient because it uses a single signal line to transmit all the information. It is also backward compatible because older decoders can ignore the strength information and just use the timing information. |
The patent notes that the encoded signal is 'transmitted across a single signal line' . This is important for compatibility with existing hardware. |
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Chapter 19: The Decoder's Use of Strength Information |
The decoder uses the strength information to make more intelligent decisions about which transitions are valid. Strong edges are more likely to be valid than weak edges. |
The patent explains that 'the relative strength of the transitions can also provide clues about relative depth of modulation, which can aid in deblurring the signal' . This is particularly valuable for barcodes that are slightly out of focus. |
The decoder can also use the strength information to distinguish between true barcode edges and print defects. Defects typically produce weaker edges than the true barcode edges. |
By using the strength information, the decoder can decode barcodes that would be impossible to decode with a traditional single-DBP signal. |
Chapter 20: The Decoder's Use of Dual-DBP Information |
The dual-DBP signal provides another type of enhanced information. The decoder can use the confirmation between the high-sensitivity and low-sensitivity digitizers to distinguish valid edges from noise. |
The patent explains that 'a transition on the high-sensitivity digitizer that is confirmed by a transition that also occurred on the low-sensitivity digitizer is indicative of a relatively strong analog signal (having a relatively large amplitude)' . |
Transitions that are not confirmed by the low-sensitivity digitizer are indicative of weak signals, which are likely due to noise or print defects . |
The decoder can use this information to reject false edges and improve decoding performance. |
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Chapter 21: The Hand-Held Scanner Application |
The digitizer output is particularly important for hand-held scanners, where the signal can vary dramatically due to changes in distance and scanning angle. |
The patent notes that hand-held laser scanners 'cannot read dot matrix symbols or other symbols with defects in this range using the first category of digitizers' . The sensitivity of the digitizer must be carefully controlled to handle these variations. |
The multiple sensitivity-mode digitizer provides a solution. By allowing the reader to switch between high-sensitivity and low-sensitivity modes, the digitizer can adapt to different signal conditions. |
This adaptability is essential for hand-held scanners, which must handle a wide range of barcode qualities and scanning conditions. |
Chapter 22: The Symbol Density Detection |
A key feature of the multiple sensitivity-mode digitizer is the ability to detect the density of the barcode symbol. The symbol density determines the appropriate digitizer mode. |
The patent describes a 'symbol density determined 4' that determines the density of the barcode and selects the appropriate mode . This is part of the control circuit/mode selector. |
The symbol density detection allows the digitizer to automatically select the optimal mode for the current barcode. This simplifies the operation of the reader and improves performance. |
The decoder also provides feedback to the digitizer, indicating whether the decoding was successful. This feedback can be used to adjust the digitizer mode for subsequent scans. |
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Chapter 23: The Laser Waist and Defect Sensitivity |
The 'waist' of the laser beam is a critical region for digitizer performance. At the waist, the beam is most tightly focused, making it sensitive to small defects on the barcode. |
The patent explains that 'small defects are more easily resolved' at the waist . This can be a problem for digitizers that are sensitive to low levels of modulation. |
The high-sensitivity digitizer mode is particularly problematic at the waist. The small defects can produce false transitions that confuse the decoder. |
The multiple sensitivity-mode digitizer addresses this by allowing the reader to switch to a less sensitive mode when operating at the waist. |
Chapter 24: The Moving Scanner Solution |
One solution to the defect sensitivity problem is to move the scanner away from the symbol. This causes the laser spot to grow larger, rendering the defects unresolvable . |
The patent notes that 'these digitizers can read defective symbols by moving the scanner away from the symbol, causing the laser spot to grow larger' . |
This is a practical solution for hand-held scanners, where the user can adjust the scanning distance. However, it is not ideal, as it reduces the depth of focus. |
The multiple sensitivity-mode digitizer provides a more elegant solution, allowing the reader to adapt to the symbol quality without changing the scanning distance. |
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Chapter 25: The Dot Matrix Symbols Challenge |
Dot matrix symbols are a particular challenge for digitizers. These symbols are formed by small dots, and they often have print defects that can confuse the digitizer. |
The patent notes that 'hand-held laser scanners cannot read dot matrix symbols or other symbols with defects in this range using the first category of digitizers' . |
The low-sensitivity mode is better suited for dot matrix symbols because it is less sensitive to the small modulations caused by the dots. However, the low-sensitivity mode may also miss weak signals from low-contrast barcodes. |
The multiple sensitivity-mode digitizer provides a way to balance these competing requirements, allowing the reader to handle both dot matrix symbols and low-contrast barcodes. |
Chapter 26: The Digitizer Output and Decoding Performance |
The quality of the digitizer output is the single most important factor in decoding performance. If the digitizer output is clean and accurate, the decoder can reliably decode the barcode. If the digitizer output contains false edges or distorted pulse widths, the decoder will fail. |
The patent emphasizes that 'the output signal of the high-sensitivity digitizer may contain more pairs of transitions than the output signal of the low-sensitivity digitizer' . This illustrates the trade-off between sensitivity and accuracy. |
The enhanced digitizer outputs (multi-bit and dual-DBP) provide the decoder with additional information that can be used to reject false edges and improve decoding performance. |
This is the key advantage of modern digitizer designs: they provide the decoder with more information, allowing it to make smarter decisions. |
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Chapter 27: Summary --- The Digitizer Output in Perspective |
The digitizer output is the bridge between the analog front end and the digital decoder. It carries the barcode information from the photodetector to the decoding engine, and its quality directly affects decoding performance. |
We have examined how different companies and technologies have approached the challenges of digitizer output: |
Symbol Technologies developed the multi-bit enhanced digitizer that provides both timing and strength information, and the dual-DBP digitizer that combines high-sensitivity and low-sensitivity signals. They also developed the multiplexing technique that allows enhanced signals to be transmitted over existing interfaces . |
The multiple sensitivity-mode digitizer patent describes a circuit that generates first and second digitized signals using different thresholds, with a mode selector that chooses between them. This addresses the trade-off between sensitivity and defect immunity . |
The inverse barcode digitizer patent describes a system that modifies the restore-to-white function and inverts the output for inverse barcodes, ensuring that the decoder receives the correct polarity regardless of the barcode type . |
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The key lessons from our exploration are: |
The digitizer output is a pulse width coded signal. The widths of the pulses correspond to the widths of the bars and spaces. The polarity indicates whether the transition was from white to black or black to white. |
Traditional DBP signals provide timing and polarity only. They do not provide strength information. This limits the decoder's ability to distinguish valid edges from noise. |
Enhanced signals provide strength information. Multi-bit signals carry an 8-bit strength value for each transition. Dual-DBP signals combine high-sensitivity and low-sensitivity digitization. |
Multiplexing allows enhanced signals to be transmitted over existing interfaces. This maintains compatibility with legacy systems while enabling improved performance. |
Inverse barcodes require special handling. The restore-to-white function must be modified, and the output must be inverted to provide the correct polarity. |
The choice of digitizer mode depends on the barcode quality. High-sensitivity mode provides large depth of focus but is sensitive to defects. Low-sensitivity mode is less sensitive to defects but may miss weak signals. |
In the end, the digitizer output is a testament to the importance of the analog-to-digital interface in barcode readers. It is the signal that carries the barcode information from the physical world into the digital domain. The art of digitizer output lies in the careful balance of sensitivity, noise immunity, and compatibility, creating a signal that the decoder can reliably interpret. |
cs@easiersoft.com