DSP implementation of a multitone FSK modem for HF radio data systems

Multitone FSK modems are used in many commercial HF radio systems although serial modems with adaptive equalization can yield higher data rates. HF systems are often used in cost-sensitive applications where high quality radios are not available. Many radios are designed for voice communications and have a passband between 500 and 2300 Hz. In addition, most receivers have a manually adjusted local oscillator, commonly known as a "clarifier", which can be misadjusted by inexperienced users. Multitone FSK modulation was selected because of its ability to tolerate impairments introduced by the radios. The simplicity of the modulation selected also permits the complete modem to be implemented with standard commercial digital signal processing (DSP) chips. The advantages of DSP implementation include reduction in the capital cost of the system and easy installation and operation in remote areas where experienced personnel are not available.