PDF417 ( PDF stands for " Portable Data File " ) is a 2D - barcode and was developed by Ynjiun Wang at Symbol Technologies in 1991.
The stacked bar code types require in reading the complete congruence of scan and code symbol or line of code. This restriction reduces the attainable information density, as larger code heights must be printed, as appears necessary from a theoretical perspective.
The code PDF417 takes this into account by the code structure, the automatic registration and processing of newlines considered and controlled by helical scan makes. This makes it possible to achieve a drastic reduction in the height of the printed code encoding, and accordingly, a significant increase of the information density in spite of the actually existing angular tolerances during the scanning.
The properties of this coding are summarized:
- Automatic discrimination of skips,
- Possibility of error correction,
- Separation of channel and source coding separate mapping tables,
- Application- specific configuration of the redundancy code ( security levels ).
PDF417 code defines the three standard modes: ASCII, binary and numeric mode and includes other nine user-specific modes. In the ASCII mode, a code word encoded two alphanumeric characters in the numeric mode even three digits.
Basis of the PDF417 is a ( 17,4,6 ) source coding with four bars and four spaces, divided into 17 modules. The maximum element width is six modules. A PDF417 impression can thus contain a maximum of 2000 8- bit characters. A typical mold is 3-4 square inches in size and achieves a typical density of 100 to 300 data bytes per square inch (equivalent to 15.5 to 46.5 bytes per cm ²).
The patterns are available in the three groups, so-called divided cluster. Codewords, which are arranged in the same row come from the same group. With every third line is a type of cluster is repeated. Through these lines marking the automatic line discrimination is realized. The decoder of the reading device registered with the change of the helical scan code group (cluster ), and takes this into account in the interpretation of the read information. The restriction a complete scan a line of code is thus reduced to the scanning of a complete character.
As a further technical characteristic PDF417 code has the possibility of the error detection and correction. A PDF417 symbol reader for this purpose contains two check words for error detection and correction of a variable number of words. The backup can be configured in nine steps. Level 0 does not allow correction (ie only error detection ), Level 8 allows the complete reconstruction of up to 510 destroyed codewords in a read symbol consisting of 925 codewords. This security level due to the insertion of check words, however, a reduction in the density of information.
Type of code is used, inter alia, of the Agency, in order to prevent tampering ( on date of shipment ) of questionnaires, as well as boarding passes in air traffic.