Friday, December 7, 2007


Narrow Band Direct Printing (NBDP)
NBDP is the printing of all messages which will give us a printed record of the communications. If the ship is involved in a distress situation it would be nice to have a printed record of all communications that took place during the operation.

Narrow Band Direct Printing (NBDP) is a term we use to describe a method of sending information over the radio and having it printed. In some publications it is called TELEX, a system used on land line communications between offices.
One of the disadvantages of using NBDP for communications is that a skilled keyboard operators are required. An advantage is that there is a written hard-copy of all communications. NBDP communications uses digital signals on the links between communicators.

A NBDP circuit uses teleprinters, like an electric typewriter, or now days using a computer terminal. The message is formatted in the word processing program and then transferred to the "line" section for transmitting. Teleprinters use the CCITT ITA NO.2 code (International Telegraph Alphabet Code) for communicating. Each keyboard character is composed of a 5-element code. This code is only able to provide us with 32 combinations, and the "shift" key allows 26 more combinations.

All data communications uses some form of error checking so we use a 7 -unit code. We add a "space" element and a "mark" element to form this 7 bit code. Space is generally accepted to mean a logic zero (0) and Mark as a logic one (1). These special codes are used to provide a system of checking for errors. We use this as a "parity" checking system.

To further aid in error checking we can use the FEC (forward error checking) system to assist us in providing non-error communications. FEC frames are sent over the air and they include a "parity" information and this is check on receipt at the receiving station. If an error in the checksum is noted then the receiving station will not accept the frame .