subject: Technical Seminar: Analysis Of Two [print this page] Expression information, is not expressed in the vertical direction, any information, which a certain height is usually aligned to facilitate readers.
One-dimensional bar code information into the application can improve the speed and reduce the error rate, but one-dimensional bar code, there are some shortcomings:
* Smaller data capacity: 30 characters left
* Can only contain letters and numbers
* Bar code size is relatively large (lower space utilization)
* Can not read the bar code after being damaged
In the horizontal and vertical two-dimensional space to store information, bar codes, known as two-dimensional bar codes (2-dimensionalbarcode).
And one-dimensional bar code, two-dimensional bar code, there are many different encoding method, or code system. Code system for coding on these principles, it is usually divided into the following three types
1. Linear stacked two-dimensional code is a one-dimensional bar code in coding theory, based on the number of one-dimensional code generated in the vertical stack. A typical code system, such as: Code16K, Code 49, PDF 417 and so on.
2. Matrix is a rectangular two-dimensional code space by black and white pixels in the matrix encoded with different distribution. A typical code system, such as: Aztec, MaxiCode, QR Code, Data Matrix and so on.
3. Post Code Article by encoding different length, mainly used for message encoding, such as: Postnet, BPO4-State.
In many types of two-dimensional bar codes, the commonly used code system are: Data Matrix, MaxiCode, Aztec, QR Code, Vericode, PDF 417, Ultracode, Code 49, Code16K, among which:
* DataMatrix is mainly used for small parts marking the electronics industry, such as Intel's Pentium processor back on the publication of this code.
* MaxiCode by United Parcel Service (UPS) company developed for the package sorting and tracking.
* Aztec by the U.S. Welwyn (WelchAllyn) launched, and up to 3832 numeric or 3067 alphabetic characters or 1914 bytes of data.
The following, we PDF 417 , for example, describes the features and characteristics of two-dimensional bar code.
1) PDF 417 Introduction PDF 417 code is from the Chinese Wang Yinjing in America (audio) Dr. invention. PDF is to take the three words in English PortableDataFile the acronym means "portable data file." Because each bar code symbol character formed by four bars and four empty form, if the composition of the most narrow bar or space is called a module, the above four bars and four empty the total number of a certain module 17, 417 yards or so called PDF417 code.
2) PDF 417 features 1. Information content than can be said PDF417 code letters, numbers, ASCII characters, but also can express a binary number. In order to make code more compact and improve information density, PDF417 encoding of up to three formats:
* Compression formats Extended alphanumeric characters up to 1850;
* Binary / ASCII format can hold 1108 bytes;
* Digital compression format can accommodate 2 710 figures.
2. Error correction is usually a one-dimensional bar code verification feature to prevent the wrong time and place soiled will be refused once the bar code reading. The two-dimensional bar code can not only prevent errors, and can correct mistakes, even if partially damaged bar codes, but also can restore the correct information out.
3. Do not ask general printing device can print to print, fax can be read.
4. Can be a variety of reading devices can read PDF417 code reader with laser grating, linear style and facial image scan reader to read.
5. Size adjustable to accommodate different print space
6. Code system has become an open international standard, China has established 417 yards of the national standard.
3) PDF417 error-correction function Two-dimensional bar code error correction function is expressed through some of the information repeated (redundant) to achieve. For example, in PDF417 code, a line in addition to containing information on the Bank, there are other positions that reflect the character (error correction code) information. Thus, even when the bar code of a partially damaged, you can also exist in other locations through the error correction code to restore the information out.
PDF417 error correction capability in accordance with error correction code words were divided into 0 to 8 of 9, Figure 4, the higher the level, the more correct the code words, the stronger the ability to correct, the greater the bar code. When the correct level 8, even though 50% of bar codes can also be defaced read correctly as shown in Figure 5.
4) PDF 417 several deformation PDF 417 code system, there are several forms of distortion:
* PDF417 truncated codes in the relatively "clean" environment, the possibility of damaged bar codes is small, the line indicator may be omitted to the right and reduce terminator.
* PDF 417 microcode to further reduce the PDF code.
* Macro PDF417 code when the document is too long, can not be said that when a PDF417 code can be used multiple
