Amateur Radio Digital Modes Gary Sutcliffe, W9XT January 14, 2015 1

What Are Digital Modes? The digital modes use a computer to generate and decode the transmitted signals. Traditionally this was done by circuits with capacitors, inductors, resistors, amplifiers, etc. Most of these use the sound card as a digital signal processor (DSP). 2

Some Common Digital Modes Originally Analog RTTY SSTV Fax Hellscriber Developed for PC PSK31 JT65 FSK441 WSPR 3

What is Digital? Let’s start with what is analog? It is continuous – there are an infinite number of points between any arbitrary two points In theory you can measure the voltage to any resolution 4

Let’s digitize the signal Use a device called an Analog to Digital converter. It is just a kind of voltmeter 5 5

With a digitized signal We can perform math on the data, and then convert the results back to a voltage with a Digital to Analog (D/A) converter. 6

A simple digital low pass filter Digitize the signal For each point, add the value of the point, the previous point and the next point and divide by 3 3. Convert the new values back to a voltage Original Averaged 0.185424 0.549383 0.651308 0.888572 0.831386 1.052738 1.033471 0.831724 0.849558 0.585576 0.459885 0.245268 -0.10869 -0.39857 0.244936 0.462038 0.696421 0.790422 0.924232 0.972532 0.972644 0.904917 0.755619 0.631673 0.430243 0.198822 -0.08733 -0.35708 This example was done with a spread sheet Started with a sin wave and added random noise to produce top waveform Ran the sample points through a simple averaging function then re-plotted Noisy input Filtered output 7 7

If You have fast enough A/D and D/A circuits with enough resolution You have a fast enough computer You are good enough with the math You can Create extraordinary filters, very narrow, no ringing Convert from the time domain to frequency domain (spectral analysis) Pull weak signals out of the noise Modulate/demodulate signals Do all sorts of other magic 8

Why is DSP Better Than Analog Circuits? Can change function with just a software change Frequency reference is the only part that might age Analog components Need to be tuned Change value with age, temperature, etc. Can be expensive and bulky 9 9

Some DSP rules of thumb If you sample the signal at least twice the highest frequency present, you can reproduce it (Nyquist Criteria) The more you know about the signal, the deeper you can pull it out of the noise. The slower the signal changes (in frequency and/or amplitude), the deeper you can pull it out of the noise. 10

Modulation designed for different needs PSK31 Very narrow bandwidth (slow character speed) Effective with low signal strength JT-65 Used for moon bounce and extremely weak signals Each character is a single tone about 5 Hz apart Very slow, each character takes ½ second Time synch, station’s clocks must be very accurate Error correction FSK 441 Used for meteor scatter High speed (exchange in 1/3 second) Lots of Doppler shift 11

Enough theory! On to the fun stuff! 12

What do you need to go digital? Stuff you probably already have Radio – SSB capable Computer with sound card Stuff you need to get Computer to radio interface (build or buy) Software – most is free! 13

Computer-Radio Interface Schematic and photo courtesy of Unified Microsystems www.unifiedmicro.com 14

Digital modes use SSB to transmit But most of the digital modes are out of the phone band. Isn’t that illegal? 15

Digital Modes Used at W9XT There are other modes I have not yet tried The following programs are the ones I use. There are others available that you might like better 16

RTTY Uses BAUDOT code Goes back to first mechanical printers Probably most popular digital mode Can be a plug-in to some logging programs 17

MMTTY - RTTY program 18

PSK31 Very narrow banded Excellent weak signal effectiveness Popular – set off the PC/sound card digital mode revolution in 1999 Sensitive to phase distortion on long paths Somewhat slow – a good typist can go faster than transmission rate 19

Digipan - PSK 31Program 20

JT-65 Developed for moon bounce by K1JT Extremely low signal levels, slow Doppler shift Excellent weak signal, detect ~28 dB below RX noise 65 tones, 5 HZ apart, ½ second dwell time Each tone is a character Very slow, about 1 minute to send calls & exchange Requires accurate clocks at both stations QSOs usually scheduled Structured QSO sequence Also being used on HF 21

JT-65 - Moon bounce program Decoded signal off moon of DK1CO sending W9XT his grid Notice the signal is 23dB below the noise level. 22 22

FSK441 Developed for meteor scatter Fairly strong signals Short infrequent bursts – exchanges sent fast, repeated Large Doppler shift Takes advantage of short pings, increasing success rate 30 second XMT/RX cycle Structured QSO sequence QSOs usually scheduled 23

FSK441 – Meteor scatter program 24

Summary The digital modes are the cutting edge of technology in ham radio They lower the bar for power and antenna requirements for some types of communications There are additional modes not covered here You can get on them for very little money 25