HF Selcall CCIR 493-4 Information - HFLINK.COM

HF Selcall
This is an informational resource for HF Selcall. Selcall CCIR 493-4 is the primary focus of this resource, and the variations of it that have been developed for modern HF selective calling as used in HF SSB Land Mobile applications. It functions as incidental tones for selective calling and alerting for voice communications. References for ITU-R M.493-n (Marine GMDSS DSC) types of digital selective calling also are included. The long term objective of this resource is to provide information about the use of these types of HF Selcall in various radio communication services. Please refer to the original documents of the various standards and recommendations for more detail and updates.

Background
HF Selcall using FSK 100 baud has been in use for many years in land mobile service in the Australia region and remote areas of the world. It is common among international disaster relief and aid organizations' HF vehicle and base communications. This selcall system is mainly based on CCIR Recommendation 493 (XIIIth Plenary Assembly Geneva 1974) for a Digital Selective-Calling System for use in the International (Terrestrial) Maritime Mobile Service. It has gone through many revisions such as 1978 Kyoto CCIR 493-1 and later CCIR 493-4, which provide the coding of the FSK signal and the structure of the format for signalling. The protocols and format of this system were enhanced to provide various features and services, including: selective calling, telephone number calling (phone patch call), group call, all call, and remote control. It has been expanded over the years to include: location reporting (GPS), emergency call, text messaging (HF paging), and SMS-texting, text email, and more.

Ham Radio Use of Selcall
The availability of Aussie selcall in various types of commercial HF radios, especially Codan, has led to ham operators using it for ham radio selective calling applications, primarily for voice SSB and data communications. The Amateur Radio HF Selcall Channels are available for all ham operators to use. A database of Ham Radio Selcall Address is maintained by HFLINK group.
4-digit and 6-digit Selcall Address
A selcall address is like a phone number. The earliest systems of this type of Selcall used unique identification addresses of 4 digits in length. The 4-digit type is the most common Selcall system in service throughout the world for land mobile HF service. But, due to the limitations of the number of possible unique addresses (9,999) in the 4-digit system, the protocol for land-based HF was expanded to 6 digits, capable of 999,999 unique addresses. Much of the earlier CCIR 493 format, coding, and signalling standards have been retained in the modern marine DSC. The maritime system selcall ITU-R 493.9+ has expanded the addressing and features in what is known as marine Global Marine Distress Safety System Digital Selective Calling or (GMDSS DSC) for use in ships and boats on HF and VHF throughout the world. However, most HF land-based services have remained with the 4-digit and 6-digit system. In most implementations by HF SSB Land Mobile equipment manufacturers (Codan, Barrett, QMAC, Micom, Icom, Vertex, Jenal, etc) the 6-digit is backward compatible with 4-digit. In other words, a 6-digit selcall radio can also call a 4-digit address, but not vice-versa. Most 4-digit radios cannot decode a 6-digit selcall.
Example of a 4-digit selcall address: 1234
Example of a 6-digit selcall address: 456789

Modulation: 2aryFSK (FSK), with 170 Hz shift. The frequency shift point of waveform inter-symbol transition is not specified in the protocol; in practice it does not affect the performance very much, so the transition can be at the negative or positive peak or zero-crossing of the signal, or in between. Only a single tone is present at any time interval (like traditional RTTY FSK). Fixed baseband audio frequency of the FSK encoder and decoder is used with an SSB transceiver on HF channels. Upper Sideband (USB) is most common.
Baseband FSK Frequencies:
The Australian standard (Codan or U.N. Open Source Selcall) is the most common for HF SSB Land Mobile.
CCIR 493-4 HF Land Mobile International Standard shift FSK is 1700Hz=0 and 1870Hz=1 (Center frequency= 1785Hz).
[Note: Marine GMDSS DSC CCIR 493-9+ for HF SSB is 1615Hz=0 and 1785Hz=1 (Center frequency=1700Hz)]
Symbol: The shift symbol is represented in both the CCIR and ITU-R tables of bit coding as B=0 and Y=1 (1700Hz is the B-state and 1870Hz is the Y-state of the signal elements).
[Note: Marine GMDSS DSC: 1615Hz is the B-state and 1785Hz is the Y-state]
Code: 10-bit words allowing error detection, with a 128 characters set. The character symbols 0 though 99 are used to transmit numerical values, and the number of the character symbol is equivalent to the value.The meaning of service command character symbols 100 to 127 depends on their position in the message and on the message format.
Speed:100 baud (100 symbols/sec or 100 bits per second at RF) for HF signalling. Each bit is 10ms (10 milliseconds) in length. Raw 10 Characters/second.
Protocol: A "dot pattern" of 0101010101... frequency shift keying for at least 2 seconds (or 6 seconds or 20 seconds) starts the signal. The purpose of the pattern is two-fold: 1. To capture all channel-scanning receivers on the transmitted signal. 2. To provide sync lock for the decoding. After the initial dot pattern, a phasing sequence is sent consisting of a set of known service command characters, in a "countdown" for the purpose of aligning the decoder for character word sync. Characters are transmitted by packets in a sequence. Each character of the messaging is normally transmitted twice, for time spreading. The repetition of a character occurs 4 characters after its first transmission in the sequence (the same as in SITOR-B). A character is 100milliseconds long. Thus the repetition of a character occurs after 400ms.
Frequency accuracy: The radio-frequency design tolerances of the resultant RF signal for both transmission and reception should be ƒA10 Hz according to the protocol. However, in practice, field calibration of radios to about ƒA25Hz (or worse) is sufficient for good success in common applications where typical PLL decoders are used. For very weak signal decoding, frequency accuracy is more important. Most HF SSB Selcall operation is used with voice Upper Sideband (USB) and the SSB transceiver dial display frequency (VFO frequency or SSB zero beat suppressed carrier frequency) defines the listed RF channel frequency.

Selcall System Process Description

The encoding of the character symbols for the Selective Calling function is based on ASCII. It is necessary for the digital decoder to be synchronized to the incoming signal before the transmitted information can be correctly decoded. A sequence of special signals is transmitted as a preliminary to each call signal for th purpose of establishing correct phasing (including the identification fo those characters which form the initial and repeated part of the message).

Due to the the presence of noise in an HF system, an error detection system is part of the protocol. Error detection is achieved by adding a 3-bit parity check to each 7-bit ASCII character; this enables multiple bit errors to be detected. Also, each character in the message is transmitted twice, separated in time by more than the mean length of HF noise bursts. At the receiver, all characters are checked for parity and fed into a buffer store: those with incorrect parity being flagged. This process continues until end of sequence characters are received or the decode is aborted as a result of too many errors. The buffer store is then read by examining its content character by character. If a character has no bad-parity flag, it is copied to an output buffer, otherwise its repeat is examined. Should both initial and repeat characters be flagged, a null character is placed in the output buffer, since it is better to totally discard an obvioiusly wrong signal than to accept a character known to be in error.

The data in the output buffer is finally assembled and tested to see if it has a format corresponding to a valid call type. If any of the special symbols in the message are misplaced or in error, then again the message is discarded. If the message is valid, the system provides the proper result for the call for alerting, display, and controlling functions.
Call Sequence:

• Dot Pattern
• Phasing Sequence
• Specifier Symbol
• Address Signal
• Message Signal
• End Of Sequence signal

The Dot Pattern locks the FSK demodulator and establishes bit sync.

The Phasing Sequence establishes bit, character, and direct/repeat signal phases. Direct is called DX and repeat is called RX in the symbol notations of sequence diagrams.

The Specifier Symbol signal format establishes the general nature fo the call; these basic options are: ALLCALL and SELECTIVE CALL.

The Address Signal consists of a special symbol in the case of ALLCALLS and the identification symbols for the required station for SELECTIVE CALLS.

The Message Signal is divided into a number of parts:
The category (priority) of the message. A self identification message in the same format as the called station address.

The End of Sequence signal signifying the completeion of the call is part of the message signal.