Digital Radio Primer - part 1
Disclaimer:
These are my comments on digital communications and are not necessarily all there is to know on the subject. As with everything computer related – there are at least six ways to do the same thing. Given this caveat, let me say this is opinion and not the complete story. I only relate to you my experience of 5 or more years using digital modes, and acting as the SATERN So. Terr. Digital Net Manager for almost as long, to give you the benefit of my experience. I will leave the rest for you to research the references we will provide at the end of this series.
In this series of articles we will take a cursory look at some terms used in ham radio while talking about common digital modes. We will also take a look at some of the software available for digital radio as well as the pros and cons of each. What we present is not all the software available nor does it attempt to be comprehensive. There is no substitute for experience with digital radio and the associated software and hardware.
If you have been interested in what digital communication is all about, this series may give you a deeper understanding of the terms and uses of common digital software in ham radio.
BASIC TERMS
DIGITAL COMMUNICATION refers to the translation of text or other data (images, PDFs, etc.) into digital information to be transmitted by conventional modulation methods allowed by amateur radio operators. Amateurs have about seven frequently used modulation methods including AM, FM, SSB, FSK, and CW to name only a few.
Some digital methods use CW. Yes CW Morse code is a digital mode! Similar to CW is FELD-HELL. This strange mode uses a rapid on-off CW like modulation to create digital information that is displayed for human readability. FELD-HELL is very spectrum efficient in that there is a single frequency used with very narrow spectrum use. Usually only 50-90 Hz wide. It is not a commonly used digital mode for emergency messages because there is no error correction and is very slow compared to other more widely used modes.
Some modes use Frequency Shift Keying (FSK). Addtionally, some modes like RTTY can use more than one method - FSK or AFSK. AFSK is a method where audio frequencies of AM or SSB are shifted instead of RF 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 less conventional cases, both).
TERMINAL NODE CONTROLLER
External hardware may be used to encapsulate multiple functions. This hardware is known as a TERMINAL NODE CONTROLLER or TNC, often it is just referred to as a modem, and combines a multitude of functions into a “black box” for the user.
You may not know that modem is an contraction of the terms modulator and demodulator. The TNC is the computer digital side of the hardware “black box”, the modem is the analog audio side to and from your radio. External hardware often combines both modem and TNC functions in one box.
The TNC will decode the demodulated incoming audio data stream and provide a digital signal that the computer will understand and can display. Or, on transmit it will encode the data for transmission. It is also responsible for the Transmit / Receive switching (PTT). If you have ever used VHF packet, you know it takes a TNC to hook up to the radio and to the computer. It is also possible to send and receive packet on HF, albeit at a much slower rate. The process is the same. Audio signals to and from the radio are converted to digital pulses in the modem, given to the TNC to provide the digitally decoded output to the computer for your packet display program. Often amateurs use the terms modem and TNC interchangeably when referring to external hardware.
The SCS TNC is of the same type modem/controller units as used for PACKET, only using the proprietary PACTOR II, III or PACTOR IV (in the case of the new Dragon hardware) protocol instead of PACKET and at a considerably higher cost. The most recent of additions to the SCS product line is a new Enhanced PACKET TNC with APRS capability for both HF and VHF/UHF. Enhanced PACKET provides a much more robust version of the historic PACKET protocol. It is widely used in Europe on both HF and VHF/UHF.
These days, powerful new computers often perform the same functions as external hardware. The software based, virtual solution, involves using a computer with a sound card. The computer becomes the modem/TNC. The computer sound card output (usually for speakers) is connected to the microphone input or aux transmit audio 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 by way of isolating hardware called a computer interface. The interface helps to match the dissimilar hardware aspects of connecting radio and computer. Some radios need a different voltage transition for rig control than is provided by the controlling port on the computer (a USB port or RS-232 Serial port). The computer interface may provide this voltage difference internally and provide the correct rig control output to the radio. In the case of at least one brand there may be a separate cable or hardware box to accomplish the voltage transition for CAT or CI-V control signals to the radio. This connection method is used for at least 20 of the 38 or so digital modes for ham radio. The digital computer interface to radio 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 separate digital connection from a serial port on the computer, to the direct data 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 higher speed 9600 baud PACKET mode on VHF/UHF, for example, requires this arrangement. The detection of FSK signals is taken directly from the frequency discriminating detector. These received digital signals are then interpreted for display by software on the computer providing the same functionality as the modem external hardware.
Recent software and hardware entries into ham radio allow computer interfacing with hardware that contains not only the radio interface components, but often a self-contained soundcard that functions independently of the computer. Products like the Tigertronics SignalLink, RigBlaster Advantage, and Timewave Navigator take advantage of this configuration to off-load the analog-to-digital sound conversion process to the internal soundcard of the product, and thus relieving the computer of this task. These products also incorporate a VOX PTT for transmit keying without having to have a separate PTT line.
While not necessary, having a separate soundcard for this dedicated, specialized purpose considerably enhances the quality of the audio conversion process while reducing the amount of work the computer is required to do. Computers with a second external soundcard or sound interface may be used to accomplish the same thing with some additional interconnection to the computer interface for the radio.
Error Correction
When digital signals are transmitted, the electrical signals of the digital information are used to modulate the amateur transmitter. When received, band conditions and signal strength become a factor in how accurately that information is received. The pulses of the digital information may be lost or interrupted by noise, precipitation crashes, fading, or other types of interference. Loss of data results in errors in the decoded digital data.
Many software methods have been employed to correct for this loss of data and restore the received data to the original content. The modern use of techniques like Forward Error Correction (FEC), longitudinal compression, Trelles Coding, encoding with Viterbi, NRZI or other encode schemes, may be used alone, or in combination, to attempt the error correction of data received and make the digital mode more robust.
These are sophisticated mathematical calculations that require considerable computer power. Not all digital mode software use all these methods on amateur radio digital modes. For instance PSK31 uses a Varicode character encoding to reduce the overall amount of data that has to be sent. But no error-correction is employed, so when noise or interference causes data loss, the result is text that is essentially garbage.
The opposite case is the OLIVIA digital mode. Sending data at about the same speed, but considerably more robust and employing heavy error correction, OLIVIA can decode digital signals when its' tones cannot even be discerned from the audio.
In or next lesson we will take a look at digital modes and make some comparisons.