Symbology
The Intelligent Mail barcode is a height-modulated barcode that encodes up to 31 decimal digits of mail-piece data into 65 vertical bars.
The code is made up of four distinct symbols, which is why this barcode was once referred to as the 4-State Customer Barcode. Each bar contains the central "tracker" portion, and may contain an ascender, descender, neither, or both (a "full bar").
The 65 bars represent 130 bits (or 39.13 decimal digits). Those 130 bits are grouped as ten 13-bit characters. Each character will have 2, 5, 8, or 11 of its 13 bits set to one. The Hamming distance between characters is at least 2. Consequently, single-bit errors in a character can be detected (adding or deleting a one bit results in an invalid character). The characters are interleaved throughout the symbol.
The number of characters can be calculated from the binomial coefficient.
Total number of character are 2 times 1365 for 2730. Log2(2730) is 11.41469. So the 65 bars (or 130 bits) encode a 114-bit message.
The encoding includes an eleven-bit cyclic redundancy check (CRC). Subtracting the 11 CRC bits from the 114-bit message leaves an information payload of 103 bits (the specification sets one of those bits to zero). Consequently, 27 out of the 130 bits are devoted to error detection.
The encoding scheme does not use error correction.
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