An automatic mail-processing center (AMPC) handles mail essentially in two stages

(1) Coding and

(2) Decoding

In the first stage, a mail is passed through coding desks (technically called advance compact suite –ACS) where the mail once fed into input module comes into viewing position of each coder (manual operator) automatically. The operators keys in the pin code of the mail through a keyboard. Barcode corresponding to the pin code is printed on the bottom 15 mm of the mail leaving a gap of 20 mm from the right edge, by a barcode printer attached to each coding desk. Orange fluorescent ink is used for barcode printing.

The barcode pattern used is a 3 out of 5 coding system wherein three vertical lines (bars) out of five possible bars corresponding to 0,1,2,4 and 7 gets printed sequentially. The sum of the space corresponding to the two non -printed bars gives the digit of the pincode. Each digit of the pincode is separated from another by a start bit. This means, for every digit of the pincode is a six-digit number, there will be totally24 bars. Besides pincode , the operator identification code is also printed on the mail with a gap of 21mm. A bar-coded mail gets collected in one of the sixteen stackers attached to the ACS. This depends on the pincode and the computer scheme in operation at the time.

In the second stage, bar-coded mail collected from each stacker is kept at the input module of a letter sorting machine (LSM). The mail moves in ACS and LSM through roller driven belts. The function of various parts of the machine is regulated and controlled by printed circuit boards with microprocessors. Each letter passes in front of an ultra violet fluorescent index reader at a speed of more than 30000 per hour. Under the influence of UV light, the fluorescent index is excited. The radiated light project onto photodiode is converted into a current and amplified by a voltage converter producing a voltage signal. Thus, pulses are generated at the output in accordance with the coded bars. These pulses regulate and control the opening and closing of deviators in each bin and this guides the mail to the correct bin. Totally there are 200 bins in each LSM. Mails collected in each bin are for a particular post office (to which post office, is determined by the computer scheme in operation in the LSM at that time).

Normally, for practical purposes, the computer scheme for sorting are designed in such a way that preliminary segregation of mail into different region is done at ACS level and detailed sorting to 200 post offices for each region is done at LSM level.