CCIR 476 Character Set and Technical Specifications

CCIR 476 Character Set and Technical Specifications provide a standardized framework for data transmission. This character set includes 96 characters, which are essential for encoding and decoding data.

The technical specifications of CCIR 476 are based on the use of a 5-bit code, which allows for the representation of 32 possible characters. This is achieved through the use of a combination of binary digits.

The 96 characters in the CCIR 476 character set are divided into two categories: alphabetic and control characters.

CCIR 476 has 35 possible valid binary code values, calculated using the binomial coefficient, which is the same as the number of ways to choose 4 marks for 7 bit positions.

This is 3 more code points than ITA2's 32 code points, giving CCIR 476 a slight edge in terms of coding capacity.

The additional 3 code points are used for specific purposes, including idle and phasing requests.

Some ordinary characters are reused as control signals, adding another layer of functionality to the protocol.

For more insights, see: Did the Telegraph Used Morse Code

The Navtex Protocol is a specific set of rules that must be followed for transmitting messages using the NAVTEX system.

The protocol requires that any message be initiated by transmitting the characters "ZCZC".

A 4-character "header" is transmitted after the initiating "ZCZC" characters, containing a transmitter identity character, a subject indicator character, and two message serial number characters.

My example code uses "SA00" as the header, where "S" stands for the DWD transmitter in Pinneberg Germany, "A" identifies the message as a navigational warning, and "00" is the serial number of the message.

The Navtex Protocol also requires that messages be terminated by the characters "NNNN".

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To test the CCIR 476 encoded bitstream, we compiled and flashed an example sketch to an Arduino Uno. The Arduino Uno was then connected to a Siglent 1032X function generator.

The function generator was set to a frequency of 517.915 kHz, and the inbuilt FSK modulation feature was enabled. The modulation source was set to "External" and the FSK hop frequency was set to 518.085 kHz.

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The bitstream from the Arduino Uno was connected to the "Aux In/Out" connector of the Siglent 1032X function generator. The bitstream was output on digital pin 2 of the Arduino.

The NAVTEX test signal was successfully received on a NAV4 Navtex receiver. The "SBY" LED of the Navtex receiver started blinking, and the thermal printer printed the transmitted test message without errors.

CCIR 476 has a unique character set that's built on top of the ITA2 (Baudot code) character code. This five-bit code has 32 possible code points.

Four code points are used for null (BLANK), space (SPACE), carriage return (CR), and line feed (LF). Two code points are used for a letter shift (LTRS) or a figure shift (FIGS).

The remaining 26 code points are used for characters in the letters and figures sets. This allows for 2×26 = 52 additional characters.

CCIR 476 recasts ITA2 into a 7-bit code, which has 4 marks (ones) and 3 spaces (zeros) in each valid character code. This ensures that each valid character code has a Hamming distance of at least 2 from every other character.

Here's a breakdown of the valid binary code values in CCIR 476:

This means that CCIR 476 has 3 additional code points over ITA2. SITOR uses these additional code points for idle, phasing, and repeat requests.