BASIC TERMS:

DIGITAL COMMUNICATION refers to the translation of text or data into digital information to be transmitted by conventional modulation methods. Amateurs have about seven frequently used modulation methods including AM, FM, SSB, FSK, and CW to name only a few. That is to say digital pulses of digital information are used to modulate the ham radio transceiver using modes common to ham radio. Some digital methods use CW. Yes CW Morse code is a digital mode! Some modes use Frequency Shift Keying (FSK). Some modes like RTTY can use more than one method - FSK or AFSK. AFSK is a method where audio frequencies are shifted instead of carrier frequencies.

Regardless of mode, digital pulses are used to send information (data or text) by conventional modulation methods.

The conversion of digital data to transmitted data is performed either by hardware or software (and in rare cases both).

The external hardware is known as a TERMINAL NODE CONTROLER or TNC. Often it is just referred to as a modem. Remember that modem is an acronym for modulator-demodulator. The TNC function is the digital side of the hardware black box the modem is the analog audio side.

The TNC will decode the incoming audio data stream and provide a digital signal that the computer will understand and can display. If you have ever used packet, you know it takes a TNC to hook up to the radio and to the computer. Audio signals to and from the radio are converted in the TNC and provide the digital output to the computer for your packet program. The SCS Pactor III TNC is of the same type modem/controller units only using the proprietary PACTOR III protocol instead of packet and at a considerably higher cost.

The software TNC equivalent involves using a computer with a sound card. The computer becomes a modem/TNC. The computer sound card output (usually for speakers) is connected to the microphone input of the radio and the audio output of the radio (often the headphone or ext spkr) is connected to the line input of the computer sound card. This connection method is used for at least 20 of the 37 or so digital modes for ham radio. This interface arrangement provides the hardware connections for PSK, MFSK, AFSK and other audio based digital modes.

Modes such as RTTY and digital FAX can use FSK and must have a digital connection from a serial port on the computer, to the FSK input to the radio (if it has one). The radio provides the carrier frequency shift given digital data signals at the FSK port. The received digital signals are then interpreted for display by software on the computer in the same way as AFSK signals are used.