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Przetwarzanie sygnału
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Before the scanner's computer decoder can process the encoded information contained in a bar code, it must first receive a digital signal which represents the width of each of the bars and spaces. A signal processor circuit is used to convert the image of the bar code to a digital signal representing this image. In order to understand the process by which this conversion is performed, we must first understand how the scanner "sees" a bar code.
It starts by projecting a laser light beam into the scan field. Whenever this beam falls upon an object, some light bounces off the object. The light that bounces off the object is referred to as the reflected light. All objects absorb a portion of the projected light and reflect the remaining portion.
The amount of reflected light varies from different objects and the amount depends upon the color and the surface texture. On a bar code, the surface texture is usually constant, so the printing and background determine the amount of reflected light.
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Normally a bar code consists of a light color background comprising spaces upon which dark colored bars are printed (a). This produces very distinguishable transitions from high levels of reflected light to low levels of reflected light. This is important because it is the transition in level that the scanner detects not the amount of reflected light.
The signal processor section of the scanner identifies the transitions and converts them to a digital signal. In order to do this, some of the reflected light from the bar code is received by a lens and focused on a device known as a photodiode. The photodiode converts the light into electrical current that is proportional to the level of reflected light. Because this electrical signal is extremely small, it is necessary to pass it through several amplifier stages to reach usable signal levels (b). These stages also have the effect of filtering the signal so that only the information contained in the frequency range of a bar code is amplified.
The transition detector then identifies the point at which the signal level had a large change (c) in a short period of time. This would indicate a transition from a space (high level of reflected light) to a bar (low level of reflected light). A transition from a bar to a space is revealed when the signal level quickly changes from low to high (d). The detected transitions are then merged together to form the digital representation of the bar code (e) which is passed to the computer for decoding. Poorly printed edges, holes in the bars and inconsistent surfaces are all conditions that the signal processor must compensate for. It is therefore obvious that a bar code with crisp, black bars printed on a clean white background will be most easily detectable.
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