Selcall

What it is

Selcall is a desktop application for CCIR 493-4 selective calling — the digital calling protocol used extensively on HF radio by maritime, aeronautical, and land-mobile services worldwide. The application encodes and decodes CCIR 493-4 selcall tones in software, using the computer's sound card as the radio interface.

CCIR 493-4 is the standard that underlies Digital Selective Calling (DSC) on maritime HF and MF/VHF, as well as the proprietary selcall systems used by CODAN and Barrett for land-mobile and emergency HF networks. A single software implementation that speaks the standard protocol is interoperable with all of them.

The selcall protocol

CCIR 493-4, standardized by the International Telecommunication Union, defines a narrow-band FSK signaling protocol for addressed calling on HF radio. A selcall transmission is a short burst of ten audio tones — each drawn from a defined set of frequencies — that encodes a destination address, message category, and originating address. The receiver monitors the channel continuously and alerts the operator only when a call addressed to its own identity is received.

The protocol operates at 100 baud using FSK modulation and is designed for the noisy, selective-fading conditions typical of HF propagation. Error detection is provided by the protocol structure; each address digit is transmitted twice, with the second transmission inverted as a parity check.

DSC (Digital Selective Calling) — the standard built into every modern marine VHF, MF, and HF radio — is a direct application of CCIR 493-4, extended with additional message categories for distress, urgency, and routine traffic. Selcall equipment from CODAN, Barrett, and others uses the same underlying signaling but with vendor-specific address conventions for land-mobile networks.

Use cases

• —Amateur radio — selcall provides a quiet channel-monitoring capability: the radio stays silent until a call arrives addressed to your station identity.
• —Maritime interoperability — monitor DSC distress and routine calls alongside commercial DSC equipment without a dedicated controller.
• —CODAN / Barrett network monitoring — receive and decode selcall traffic from existing land-mobile HF networks using standard equipment and a PC.
• —Testing and training — generate selcall test transmissions to verify coverage, equipment configuration, and operator response procedures.

Platform support

Development status