Digital Communication

1. Digital Communication How it started, what it is & we can do with it today!

2. How It Started • 1831 Insulated Wire • 1836 Morse Code • 1843 Fascimile • 1870 Baudot Code

3. Wires & Solenoids • Current Loop Systems • Ranges in Miles • Easy Repeaters • Multiple Stations

4. Morse Code 1836 • International Morse code composed of five elements: • 1. short mark, dot - "dot duration" is one unit long • 2. longer mark, dash is three units long • 3. inter-element gap within a character —one unit long • 4. short gap (between letters) — three units long • 5. medium gap (between words) — seven units long • Only non time sensitive digital scheme

5. Morse Code 1836 • Manual Send, Paper Tape Receive • First was soft paper & pin to crease tape • Operators learn to “hear” the code, tape is abandoned • (If you understand click and clack)

6. Morse Code 1836

7. Fax Machine 1843 • Write message in non-conductive ink on metal plate • Scan sheet with a needle • Remote solenoid makes inverse image • Requires synchronized clockworks

8. Baudot Code • 5 Element “byte”, two hand keyboard • 2 & 3 Fingers, 30 WPM

9. Baudot Code • Manual or punched tape on send • Used punched paper tape on receive • Sender “clockworks” switches current • Receiver steps tape, activates 5 solenoids • 5 bits means 32 symbols (letters) • Still used by TDD phone devices • Still used by hams today for RTTY

10. Baudot Code • Shift codes select alternate meanings

11. Digital Radio Modes • There are 10 kinds of people in the world - those who understand binary and those who don’t. • 1980 FCC allows ASCII for hams • PC’s and sound cards make it practical • Hams invent new modes – endlessly! • JT-9 Announced last November – for 160M

12. What is ASCII? • American Standard Code for Information Interchange • Originally a 7 bit code • Later 6 bit (half ascii) and 8 bit (extended) • 6 bits means 64 values (no lower case) • 7 bits means 128 values • 8 bits allows 256 values • Shift In, Shift Out, Escape extends further (DEC screen positioning codes)

13. What’s a Byte? • 0 or 1, hi or low, 1270 / 1070 Hz, ??? • 1 start bit • 6,7,or 8 data bits • 1 parity bit (Odd, even, or none) • 1 stop bit • Allows unsynchronized clocks • Allows simple error checking

14. Why All These Modes? • Radio is an analog medium • Propagation is variable • Bandwidth is limited • Speed vs reliability • Because we can!

15. Basic Principles • Input data in binary form used to “modulate” • Can change amplitude, frequency, phase • Can use multiple tones • Mathematical functions used to generate an analog signal • Same math functions used to recover data from analog signal • Can include compression and error recovery • Everyone can invent a better function!

16. The Big Modes – RTTY 1922 • Usually 45 Baud, 170 Hz Signal, some 75 baud • At 45 baud about 60 words per minute • Surplus Teletype Model 26 in 1946 • FSK / AFSK audio frequency shift keying • PCs now function as “glass teletype” • Popular contest mode

17. The Big Modes – The ‘TOR’s • SITOR - Simplex Telex Over Radio • AMTOR – Amateur SITOR (1970) 100 baud • PACTOR – AMTOR / Packet combo 200 baud • Data is sent in groups of 3 characters • Receiver responds to each 3 character group ACK/NAK • NAK causes retransmission of group • Implies two station comms only

18. The Big Modes – Packet • 1200 bps AFSK TNCs used on 2-meters • AX.25 Protocol specifies channel access (ability to transmit on the channel) to be handled by CSMA (Carrier Sense Multiple Access). • Digipeaters retransmit if its call is in the digipeater field • APRS is a packet system

19. The Big Modes – PSK31 • Developed by Peter Martinez G3PLX in 1995 • Uses phase shift keying (BPSK or QPSK) • Throughput is 31 baud, bandwidth is 31Hz • CW transmitter must put out 15 to 18 times more power than a PSK31 transmitter, to achieve the same S/N ratio at the Rx • PSK31 more susceptible to frequency changes than CW or RTTY but better under weak signal conditions

20. The Big Modes – MT63 • MT63 distributes the encoding of each character over a long time period, and over several tones. 100 WPM • 64 tones 15.625Hz apart, in a 1kHz bandwidth • Even if 25% of the character sent is obliterated, it will give perfect copy • Wide bandwidth (1Khz for the standard method) makes this mode less desirable on crowded ham bands such as 20 meters

21. Some lesser modes - Clover • PSK mode which provides a full duplex simulation • Suited for HF operation (especially under good conditions) • Clover’s key characteristics are band-width efficiency with high error-corrected data rates • Clover adapts to conditions by constantly monitoring the received signal • Hardware based mode and limited use

22. Some lesser modes - Hellschreiber • A method of sending and receiving text using facsimile technology • This mode has been around a long time; the recent use of PC sound cards as DSP units has increased the interest in Hellschreiber • The single-tone version (Feld-Hell) is the method of choice for HF operation • 35 WPM text rate, with a narrow bandwidth (about 75 Hz). • Text characters are "painted" on the screen, as apposed to being decoded and printed • As a "fuzzy mode" it has the advantage of using the "human processor" for error correction.

23. Some lesser modes – MFSK-16 • An advancement to the THROB mode and encodes 16 tones • Uses Fast Fourier Transform technology to decode the ASCII characters, and Constant Phase Frequency Shift Keying to send the coded signal. • Continuous Forward Error Correction (FEC) sends all data twice with an interleaving technique to reduce errors from impulse noise and static crashes • The relatively wide bandwidth (316 Hz) for this mode allows faster baud rates (typing is about 42 WPM) and greater immunity to multi path phase shift • Becoming a standard for reliable keyboard to keyboard operation and is available in several popular programs

24. Some lesser modes – WSPR • For sending and receiving low-power transmissions to test propagation paths on the MF and HF bands • Can decode signals with S/N as low as -28 dB in a 2500 Hz bandwidth • Standard message: callsign + 4-digit locator + dBm (i.e. K1ABC FN20 37) • Duration of transmission: 110.6 seconds • Transmissions nominally start one second into an even UTC minute: i.e., at hh:00:01

25. NBEMS • Narrow Band Emergency Messaging System • Consists of four programs: • Fldigi – Fast Light Digital modem application • Flarq – Fast Light Automatic Repeat Request • Flwrap – embed a checksum in a file • Flmsg – easily send ICS forms and Radiogram

26. NBEMS philosophy • Keep it cheap. • Keep it simple. • Use Open Source software. • Don't depend upon infrastructure. • Make it fun to use between drills and disasters. • Any computer, any radio.

27. How NBEMS Works • Fldigi uses your computer's sound card to generate and decode digital signals. • All work is done by your computer, don't need an external Terminal Node Controller (TNC). • Audio from your computer speakers go into your radio's mike input for transmission. • Audio from your radio goes into your computer's mike or line-in for decoding. • Don't need an extremely powerful new computer, older machines work just fine.

28. Interfacing with computer • Rigblaster • SignaLink • But, if necessary, hold radio mike up to computer speaker and... • Hold radio speaker up to computer mike! • In an emergency, don't really need hardwired interface. • Disable all DSP “enhancement” programs on mic

29. Acoustic Interface • Easiest way to interface radio to computer is to... • Hold radio mike up to computer speakers. • Hold radio speaker up to computer mike. • You do PTT manually. • Works especially well with VHF/UHF FM. • Real gamesaver during emergencies. • Allows you to easily send data using any radio. • Hams can participate who do not have a soundcard interface. • MT63 is sufficiently robust to deal with background noise, even in a noisy EOC or field site.

30. Signalink USB

31. SignaLink Configuration • SignaLink is very easy to configure. • Just connect to computer via USB. • Configure Fldigi to use SignaLink USB sound card. • Generate just enough audio from computer to trigger SignaLink vox. • Use volume controls on SignaLink and don't touch computer audio settings

32. What Modes ? • Operating Mode • MT63 – 2000 • Olivia 16/500 • If you have a decent FM voice channel start with MT63-2000 • When MT63-2000 fails, switch to Olivia 16/500 • When Olivia fails, no communications.

33. ARRL Radiogram Calculate Check Today Pick ARRL Message

34. ARRL Radiogram Delivery

35. ICS 213 Form

36. IC 213 HTML Delivery

37. Where to find digital modes?? • 6 Meter Band: 50.290 - 50.292 Mhz • 10 Meter Band: 28.110 - 28.125 Mhz • 12 Meter Band: 24.920 - 24.930 Mhz • 15 Meter Band: 21.060 - 21.090 Mhz • 17 Meter Band: 18.100 - 18.110 Mhz • 20 Meter Band: 14.065 - 14.090 Mhz • 30 Meter Band: 10.130 - 10.145 Mhz • 40 Meter Band: 7.060 - 7.080 Mhz • 80 Meter Band: 3.620 - 3.640 & 3.575 - 3.585 Mhz

38. And What About D-Star? • All digital, all the time • Combined voice & data in 20 ms chunks • Data stream about ½ voice, ½ data • Voice input is digitized, compressed, merged with data – calllsigns, repeater, routing, text • GMSK modulation to create analog signal– like cell phones • Limited forward error correction

39. What does D-Star Add? • Call Routing – repeater to repeater • Reflectors – Hub for repeater “spokes” • Allows simple text & small file exchange • Now HF as well as VHF / UHF

40. What Next? • Club event to set up PC’s and radios • NBEMS net on W1SYE • Perhaps start a “Digital Team” of Elmers • Could we build a Rhode Island Net?