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

2. 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

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

4. 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

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

6. 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

7. 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
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

8. 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

9. 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

10. 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

11. 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

12. Enough theory!
On to the fun stuff! 12

13. 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

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

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

16. 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

17. 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

18. MMTTY - RTTY program 18

19. 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

20. Digipan - PSK 31Program 20

21. 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

22. 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

23. 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

24. FSK441 – Meteor scatter program24

25. 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