Amateur Radio Digital Modes

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

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Some Common Digital Modes

• Originally Analog
• RTTY
• SSTV
• Fax
• Hellscriber

Developed for PC PSK31 JT65 FSK441 WSPR
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What is Digital?

• Lets 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

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Lets digitize the signal
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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.
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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
Noisy input
Filtered output
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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

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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

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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.

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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, stations clocks must be very
  accurate
• Error correction
• FSK 441
• Used for meteor scatter
• High speed (exchange in 1/3 second)
• Lots of Doppler shift

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Enough theory! On to the fun stuff!
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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!

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Computer-Radio Interface
Schematic and photo courtesy of Unified
Microsystems www.unifiedmicro.com
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Digital modes use SSB to transmit
But most of the digital modes are out of the
phone band. Isnt that illegal?
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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

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RTTY

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

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MMTTY - RTTY program
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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

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Digipan - PSK 31Program
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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

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JT-65 - Moon bounce program
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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

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FSK441 Meteor scatter program
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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