General/Chapter 6 Study Guide

Chapter 6: Digital Modes

Section 6.1: Intro to Digital Modes

Where to find digital:

• digital modes restricted to CW/data band allocation
• Calling frequencies are at lowest end, move up with activity
• On 20 m: PSK31 at 14.070 MHz, RTTY and other modes above that
• Table: Bands vs Frequency Ranges
• For the exam:
  • 20 m segment for digital:
    • 14.070 - 14.100 MHz
  • 20 m segment for PSK31: below RTTY segment, near 14.070 MHz
  • 80 m segment for digital: 3.570 - 3.600 MHz
  • 40 m segment for digital: 7.070 - 7.125 MHz
• Digital modes regulated as voice emissions by FCC
• Maximum bandwidth and data rates set by FCC to limit signal bandwidth
• Digital codes must be publicly specified and must comply with bandwidth/signal rate restrictions
• Rules may change to be based on bandwidth

Digital modes:

• RTTY - popular, mechanical teleprinters
• PSK31 - narrow bandwidth, weak signal, cheap hardware
• PACTOR - packet teletype over radio, improvement on RTTY, data transmitted as packets, PACTOR is proprietary mode
• Packet radio - common on VHF/UHF, not as common on HF due to higher error rates/sensitivity to noise
• Miscellaneous other modes:
  • MFSK16, GTOR, JT9, JT65, CLOVER, ALE, WSPR

Section 6.1: Summary

• 20 m band segment used for digital modes is 14.070 - 14.100
• The band segment on 80 m used for digital modes is 3.570 - 3.600 MHz
• The 20 m segment with PSK31 operations is below RTTY, around 14.070 MHz
• The communication system that sometimes uses the internet to transfer messages is WINLINK
• The digital mode that is designed to operate at extremely low signal levels is JT9 and JT65

Section 6.2: Digital Basics

Definitions:

• Air link - the part of the communication system involving RX/TX of radio signals
• Bit - 0 and 1, fundamental unit of data
• Bit rate - number of bits per second sent between 2 computers
• Baud - number of symbols per second
• Duty cycle - ratio of time spent transmitting to total time on/offline
• Protocol - rules controlling the data exchange method
• Mode - combination of protocol and modulation method
• Modem - translates bits into tones and back again

Bit rate vs baud rate:

• Modern modems can use complex audio signals to encode multiple pieces of information into single character
• As more information is packed into characters, the bit rate becomes faster than the baud rate
• Digital mode combines protocol (how information and characters are encoded) with modulation method
• OOK = on/off keying
• FSK = frequency shift keying
• PSK = phase shift keying
• I/Q = 2 phase-shifted AM signals

Frequency shift keying FSK:

• Bits of data encoded as tones; tone frequencies are represented by different bits
• Two-tone FSK signal: "mark" and "space"
• True, direct FSK: digital data controls VFO
• AFSK = audio frequency shift keying - convenient, uses mic/SSB, but prone to distortion
• More symbols means more frequencies means more time to differentiate between signals

Phase shift keying PSK:

• data is encoded as phase relationship between tones
• Rapid changes in phase sound like buzzing

Section 6.2: Summary

• An FSK signal is generated by changing an oscillator's frequency directly with a digital control signal
• The two separate FSK frequencies are identified by "mark" and "space"

Section 6.3: Character Based Modes

RTTY:

• RTTY - oldest, most popular, narrow band
• RTTY uses baudot code - 5 bits per character
• Stop/start bits to sync transmission
• 5 bits = 2^5 = 32 characters
• Letters and figures codes to get 62+2 characters
• Standard mark/space frequencies are 2125 Hz (mark), 2295 Hz (space)
• Difference between mark and space tones is called shift
• HF, most common speeds are 60, 75, 100 WPM corresponding to 45, 56, 75 baud
• Most conversations on HF are 45 baud, most common shift is 170 Hz

MFSK:

• MFSK - multiple-frequency shift keying, uses more than two tones to encode data
• MFSK16 - uses 16 separate tones, all 15.625 Hz apart
• This allows entire signal to be received with a 500 Hz CW filter
• Thru signal shaping and tone control, can sustain contacts through fading and distortion better than two-tone signals

DominoEX:

• DominoEX - modification of MFSK, uses differences betweeen successive tones
• Less sensitive to tuning error sand drift
• Data rates go from 4 baud to 21.5 baud, bandwidths up to 524 Hz

OLIVIA:

• wider bandwidth MFSK mode, 125-1000 Hz bandwidth, variable number of tones
• Uses forward error correction, can be used at low signal levels

Others:

• MT63: Multi-tone, data has 64 tones, advanced DSP techniques allow it to perform well under noisy conditions
• PSK31: Phase shift keying, 31 is the symbol rate (31.25 baud)
  • Variable-length character codes (like Morse code)
  • PSK63 at 63.5 baud also popular
  • Very narrow bandwidth, so usually close to calling frequency
• PSK31 sends single tone, reversals of phase sent at regular intervals
• Reversal = 0, no reversal = 1
• 1 symbol = 2 transmission intervals

Section 6.3: Summary

• Most common frequency shift for RTTY is 170 Hz
• In a single PSk31 character, the number of bits used is variable
• Baudot code is a 5-bit code with additional start/stop bits
• For PSK 31, capital letters and punctuation take more time/more bits than lowercase
• In PSK31, the 31 represents the approx. transmitted symbol rate
• To send characters with a PSK31 signal, use varicode

Section 6.4: Packet-Based Modes

• Packet refers to transmission of data in structured groups
• Common structure:
  • Header - bit patterns allowing receiver to sync with packet structure, control and routing information
  • Data - data to be exchanged, usually ASCII, often compressed
  • Trailer - additional control/status information, error detection
• Packets from one protocol can be used as data for another
• Error detection provides reliable transport
• Checksum is weaker version of error detection
• CRC cyclic redundancy check is stronger error detection
• ACK message - message contents match
• If NACK message, re-send data until it is received
• ARQ - automatic repeat request
• ARQ - broadcast mode, monitor mode

Packet:

• VHF/UHF exclusively, based on X.25, AX.25 packets exchanged using VHF FM phone at 1200 or 9600 baud
• HF: easily corrupted data, b/c of fading, even at loewr 300 baud rate
• RTTY: no check for transmission errors
• TOR - teletype over radio
• PACTOR - packet based TOR
• WINMOR - Windows TOR
• Apply microprocessors to TOR

• PACTOR 1 - FSK modulation
• Pactor 2/3/4 - PSK modulation
• WINMOR - FSK or PSK
• TOR modes employ error detection and ARQ protocols
• Most popular on HF to exchange large amounts of information
• HF data:
  • TOR modes (PACTOR/WINMOR)
• Winlink ham radio email system accessed via these TOR modes

Section 6.4 Summary

• A PACTOR modem/controller can determine if a channel is in use by putting modem/controller into monitor mode
• Joining a contact between 2 stations using PACTOR protocol is not possible (limited to 2 stations)
• The part of the data packet that contains routing and header information is the header
• In PACTOR protocol, NACK packet means receiver requests packet be retransmitted
• A receiving station responds to an ARQ data mode packet with errors by requesting retransmission of packet
• Forward error correction allows receivers to correct errors by transmitting redundant information with the data

Section 6.5: Receiving/Transmitting Digital Modes

• Most digital modes are LSB
• JT65 and JT9 are USB
• Receivers must have match between LSB or USB, baud rates, and correct tone frequencies
• FCC rules define digital mode bandwidth the same as other signals:
  • 26 dB below mean power
• As baud rate increases, bandwidth increases
• Most common protocols offer multiple rates that can be adjusted for conditions
• Common mode bandwidths: 50 Hz (PSK31) to 2.3 kHz (PACTOR 4)
• Stay in the band: using LSB with frequency modulation signal will go below edge of band
• SSB: duty cycle 20-25%
• Baudot: duty cycle 100%
• PSK31: duty cycle 100%
• ARQ modes: less, but still 50%
• If operating high-duty-cycle mode, turn down transmit power to keep from cooking your amp (50% of max. power)

Signal quality:

• For AFSK, most common problem is too much, too little audio
• Mic input may be overdriven, causing splatter
• Some radios have direct digital input
• Waterfall display will show splatter as strong sidelobe signals
• distorted waveforms harder to detect by self

ALC and digital modes:

• ALC - automatic level control, prevents excessive drive to transmitter, works inside of amplifier to protect amplifiers
• ALC can interface with digital modes
• ALC can make signal harder to decode (like overmodulation0
• disable ALC system, or turn mic gain down to where ALC will not activate
• Monitor ALC action on same transciever meter that monitors power output and SWR