From charlesreid1

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:

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


  • 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 - 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 - 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 - 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


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


  • 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


  • 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
  • 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

Section 6.5: Summary

  • When sending RTTY via AFSK with SSB transmitter, the usual mode to use is LSB (RTTY = LSB)
    • Most digital modes on HF are LSB signals, except JT65 and JT9
  • The usual sideband to use for JT65 and JT9 signals using AFSK is USB
  • One or more vertical lines adjacent to PSK31 signals (on waterfall) indicate overmodulation
  • If a transceiver's ALC system is not set properly when transmitting AFSK signal in SSB mode, the ALC will distort the signal and cause spurious emissions
  • Approximate bandwidth of PACTOR 3 signal is 2.3 kHz
  • Important to know the duty cycle of a digital mode because some high-duty cycles can exceed a transmitter's average power rating
  • The relationship between transmitted symbol rate and bandwidth is, higher symbol rates require more bandwidth

Section 6.6: Operating Procedures

  • Listen first!
  • Protocol for calling CQ follows CW and RTTY:




  • If signals are clear, fewer repeats; if weak, more repeats
  • Follow CW for ending contacts, too: K = end transmission, SK = signing off
  • PACTOR/WINMOR - have specific disconnect message or command (BYE /D)
  • Unmanned gateway and mailbox stations monitor a frequency and wait to be contacted
  • FCC classification: automatically-controlled digital stations
  • Automatically-controlled stations only allowed to use RTTY and data modes below 220 MHz (1.25 meters) in specific band segments

Table: Band vs. Automatic Control Frequency Range

Exam question:

  • Automatically controlled stations may communicate with other automatically controlled stations anywhere on 1.25 m and shorter wavelengths (higher freq.)
  • Auto controlled stations communicating with other auto controlled stations on specified segments for 80 m - 2 m
  • In order to contact an automatically controlled station outside those allocations (80 m - 2 m), if you are operating under FCC rules, you must contact the station from a station under local or remote control
  • To connect, depends on mode or equipment, but usually send CONNECT message, then send training sequenc to determine protocol/type
  • Once a connection is established, data can be exchanged

During contact:

  • Waterfall display, shows signals
  • Tuning aids for RTTY signals: one shows waveform with mark/space frequencies
  • Another shows constellation/crossed ellipse display
  • Ellipses should be same size and be perpendicular, indicates signal is tuned

Third party traffic:

  • Same rules apply
  • All info: email, digital images, web pages
  • For this reason, internet access through gateways must be very limited

Interfering signals:

  • Hidden transmitter problem: if you are in a skip zone for a transceiver, you will not hear them but they will hear you
  • RTTY/PSK31 - tolerate interference fairly well
  • PACTOR/WINMOR - effect of interference
    • Try to automatically recover data
    • In presence of interference, they will:
      • Fail to connect (connect request not decoded by receiver)
      • Frequent retries/transmission delays (error being introduced)
      • Timeouts/dropped connections (number of requested retransmissions will cause drop)
  • Listen to channel or watch operating display to look for an interfering signal
  • Use different frequencies or point a directional antenna elsewhere

Section 6.6 Summary

  • A third party message cannot be transmitted if the third party's amateur license has been revoked
  • To conduct communication with digital station operating under automatic control outside of automatic control segments, station initiating contact must be under local/remote contrl
  • An unattendedf digital station transferring message from/to internet is an automatically controlled station
  • Messages sent via digital modes are never exempt from third party rules
  • Automatically controlled RTTY stations are allowed to communicate with other digitally controlled stations anywhere in 1.25 meters or shorter, and on specified segments for 2 m - 80 m
  • A PACTOR station can detect if frequency is in use by going into monitor mode
  • When signals interfere with PACTOR/WINMOR, results in all of the following:
    • Frequent retries/timeouts
    • Long pauses in message transmission
    • Failure to connect
  • To establish contact with a digital messaging system gateway, transmit a connect message on the gateway station's published frequency
  • Waterfall display:
    • Frequency is horizontal
    • Time is vertical
    • Intensity is signal strength
  • A failure to exchange information due to excessive transmission attempts will result in a dropped connection (for PACTOR/WINMOR)