Amateur Radio Digital Modes

Gary Sutcliffe, W9XTJanuary 14, 2015

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).

Some Common Digital Modes

Originally Analog● RTTY● SSTV● Fax● Hellscriber

Developed for PC

PSK31

JT65

FSK441

WSPR

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

V= 1.643094128441…

Infinite number of points between these points

Let’s digitize the signal

Measure the voltage at regular intervals

Samples are taken with an Analog to Digital (A/D) Converter which is essentially a voltmeter.

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.

A simple digital low pass filter

1. Digitize the signal2. For each point, add the value of the point, the previous point andthe next point and divide by 33. Convert the new values back to a voltage

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

Original Averaged

Noisy input Filtered output

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

Why is DSP Better Than Analog Circuits?

DSP• 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

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.

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

Enough theory!

On to the fun stuff!

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!

Computer-Radio Interface

Schematic and photo courtesy of Unified Microsystems www.unifiedmicro.com

Digital modes use SSB to transmit

But most of the digital modes are out of the phone band. Isn’t that illegal?

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

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

MMTTY - RTTY program

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

Digipan - PSK 31Program

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

JT-65 - Moon bounce program

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

FSK441 – Meteor scatter program

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