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GENERAL CLASS Chapter 6.1~6.6 Digital Modes Chapter 6 Digital Modes 6.1 Intro to Digital Modes 6.2 Digital Basics 6.3 Character-Based Modes 6.4 Packet-Based Modes 6.5 Receive & Transmit Digital Modes 6.6 Digital Operating Procedures 1 6.1 Introduction to Digital Modes page 6-1 ∗ Digital communications modes exchange information using individual characters encoded as digital bits . ∗ “A” using CW is “di dah” ∗ “A” using ASCII is “01000001” ∗ Digital communications consists of two basic steps • Information encoding [FCC – 97.309] • Modulation formats ∗ Examples of Digital Communications Modes • RTTY, Packet (VHF/UHF), PSK31, JT-65/JT-9/FT-8/JS-8 …. Keyboard • PACTOR,WINMOR , Winlink…. Email and messaging • DSTAR (ICOM), System Fusion (Yaesu), AOR digital voice, WinDRM, FreeDV …. Voice via digital methods2 6.1 Introduction to Digital Modes page 6-1 ∗ Digital nodes are restricted to CW/Data segments of the HF bands • Usually found at the top end of the CW segment • Band plans define where digital modes may be found • Calling frequencies are typically at the lower end of the band and activity moves up with increased activity [G2E04, G2E08] ∗ 20 Meter band examples for digital mode operating frequencies • PSK-31 – 14.070 MHz; JT-65 – 14.076 MHz; JS8Call – 14.078 MHz • RTTY – 14.080 MHz ∗ Digital Modes are limited in the maximum data rates and signal bandwidths [FCC – 97.307] ∗ Information encoding and signal transmission protocols must be defined by FCC rules or be a publicly available method. ∗ Digital recording of Modes – http://www.kb9ukd.com/digital 3 6.1 Band Plan page 6-1 ∗ Table 6.1 Digital Signal Band Plan [G2E07] 4 6.1 Digital Mode Overview page 6-2~3 ∗ Radioteletype (RTTY ) sound similar to fax machine sound ∗ RTTY pronounced “ritty” is the original mode designed to copied and printed off the air by a mechanical teletype device. CW band above JT-65 and PSK-31 typically. ∗ PSK-31 sound , PSK-63 faster, bad PSK31, ALC and overdriven ∗ Narrow bandwidth <100 Hz ∗ Good for weak signal on HF ∗ Sound card based (external or internal PC card) ∗ PACTOR and WINMOR ∗ G-TOR, forerunner of PACTOR using FSK in short bursts at varying data (100 – 300 baud) rate depending on conditions ∗ Packet Radio ∗ VHF/UHF mode ∗ Not tolerant of HF transmission errors from noise and fading ∗ CW sound at 20 wpm (CQ CQ CQ)5 6.1 Digital Mode Overview page 6-2 Winlink System (http://www.winlink.org ) • Email and file transfer • Uses the internet to connect a system of email servers with gateway and mailbox stations around the world on HF, VHF and UHF frequencies [G2E13] • No direct connection to the internet • Provides a means for providing email access where it is not available locally or the internet is not available due to disasters or local outages. • Can act as stand alone mailboxes 6 6.1 Digital Mode Overview page 6-3 ∗ Other Digital Modes ∗ G-TOR : forerunner of PACTOR using MFSK in short bursts at varying data (100 – 300 baud) rate depending on conditions ∗ MFSK16 : 16 tones FSK… sounds like whistles in a narrow band ∗ JT65/JT9 : Part of WSJT family of protocols by Joe Taylor, Nobel Laureate, Princeton University. Effective in noisy and unstable band conditions at very low signal levels. [G8C01] ∗ CLOVER : MFSK family of modes developed by HAL communications. Designed for high bit rates on HF by structuring data in blocks and paying careful attention to the transmitted waveform. ∗ ALE (Automatic Link Establishment): Searches through a set of frequencies to find the best frequency to communicate with a desired station. ∗ WSPR : Propagation path experimentation mode similar to JT65. 7 6.2 Digital Basics page 6-4 ∗ Definitions ∗ Air Link – the part of the communications system involves radio transmission and reception of signals ∗ Bit – the fundamental unit of data; 0 or 1 representing all or part of a binary number ∗ Bit Rate – the number of digital bits per second sent from one computing system to another ∗ Baud or Bauds – the number of symbols per second sent from one computing system to another; also known as symbol rate ∗ Duty Cycle – the ratio of time that a transmitter is on to the total of on time plus off time ∗ Protocol – the rules that control the method used to exchange data between two systems ∗ Mode – the combination of a protocol with a modulation method 8 6.2 Digital Basics page 6-4~5 ∗ Digital Data Encoding ∗ Digital signals include both information and overhead bits to control signal flow and provide error correction ∗ The total count of all information and overhead bits is the bit rate ∗ A modem is a device which encodes information and translates the bits into audio tones for transmission over the air ∗ Symbol Rate ( Baud Rate ) ∗ Baud rate refers to the number of symbols per second ∗ A symbol is the combination of signal characteristics that make up each distinct state ∗ CW has on symbol as an on or off state of the transmitter ∗ In RTTY there are two tones per symbol of the transmitter ∗ For simple codes such as Baudot or ASCII each symbol is one bit 9 6.2 Digital Basics page 6-4 Data Modems, 9600 baud transmitter ∗ 2 bits, 19.2 bits/sec (9600 x 2) ∗ 4 bits, 38.4 bits/sec (9600 x 4) ∗ 6 bits, 56.6 bits/sec (9600 x 6) AX.25 Packet ∗ The AX.25 protocol defines only the packet construction and data flow rules ∗ Data modem (TNC) output (audio tones) use a 1200 baud modem standard ∗ Operating practices according to the band plan determines the modulation method (SSB or FM) Digital Modulation Modes ∗ On-Off Keying (OOK) for CW or Pulse code Modulation (PCM) ∗ FSK and PSK – Frequency Shift Keying, Phase Shift Keying ∗ I/Q Modulation – two carriers with 90˚ phase difference and vary the amplitude and phase independently 10 6.2 Digital Basics – Modulation page 6-5 Digital Signal Principles ∗ Symbol rate directly effects the required frequency shift for FSK and the tone spacing for AFSK. The receiver requires higher shift or tone spacing to be able to determine the symbol sent Frequency Shift Keying (FSK) ∗ Frequency shift keying is a method of digital communication in which the individual bits of data are encoded as tones ∗ The two tones are called a “ mark ” and ” space ”[G8C11] ∗ Space represents a “0” and mark represents a “1” ∗ Direct FSK involves controlling the transmitter VFO directly by the digital signal from the computer [G8A01] ∗ AFSK (audio frequency shift keying) uses two audio tones which modulate the SSB or FM transmitter. AFSK is more convenient but requires careful control of audio level to prevent distortion or interference to nearby stations. Phase Shift Keying (PSK) ∗ The most common type of phase shift keying is to invert one of the waveforms, shifting its phase by 180 degrees. PSK31 is an example. 11 6-5 6.2 Digital Basics – Modulation page 6-5 Phase Shift Keying (PSK) ∗ Bussing at low rates or noise as bit rate increases ∗ Most common is BPSK which shifts the phase 180˚ from one interval to the next 12 6-5 6.2 Digital Basics – Modulation page 6-5 Which sideband to use in digital modes? ∗ Soundcard digital data mode use USB - PSK31, PSK63, JT65, JT9, FT8, JS8Call and other modes ∗ True FSK modes use LSB - Clover, RTTY, PACKET, AMTOR, PACTOR 13 Review Questions – 6.1 to 6.2 G2E04 20 meter frequencies for digital comm. G2E08 where is 20 meter PSK31 operations found G2E07 where is 80 meter digital communications G2E13 what communication system is used for messages G8C01 digital mode for low signal strength operation G8C11 how are FSK frequencies identified G8A01 how is an FSK signal generated 14 OPERATING PROCEDURES G2E04 What segment of the 20-meter band is most often used for digital transmissions? A. 14.000 - 14.050 MHz B. 14.070 - 14.100 MHz C. 14.150 - 14.225 MHz D. 14.275 - 14.350 MHz 15 OPERATING PROCEDURES G2E08 In what segment of the 20-meter band are most PSK31 operations commonly found? A. At the bottom of the slow-scan TV segment, near 14.230 MHz B. At the top of the SSB phone segment, near 14.325 MHz C. In the middle of the CW segment, near 14.100 MHz D. Below the RTTY segment, near 14.070 MHz 16 OPERATING PROCEDURES G2E07 What segment of the 80-meter band is most commonly used for digital transmissions? A. 3570 – 3600 kHz B. 3500 – 3525 kHz C. 3700 – 3750 kHz D. 3775 – 3825 kHz 17 OPERATING PROCEDURES G2E13 Which communication system sometimes uses the Internet to transfer messages? A. Winlink B. RTTY C. ARES D. Skywarn 18 SIGNALS AND EMISSIONS G8C01 Which of the following digital modes is designed to operate at extremely low signal strength on the HF bands? A. FSK441 and Hellschreiber B. JT9 and JT65 C. Clover D. RTTY 19 SIGNALS AND EMISSIONS G8C11 How are the two separate frequencies of a Frequency Shift Keyed (FSK) signal identified? A. Dot and Dash B. On and Off C. High and Low D. Mark and Space 20 SIGNALS AND EMISSIONS G8A01 How is an FSK signal generated? A. By keying an FM transmitter with a sub-audible tone B. By changing an oscillator’s frequency directly with a digital control signal C. By using a transceiver’s computer data interface protocol to change frequencies D. By reconfiguring the CW keying input to act as a tone generator 21 6.3 Character Based Modes page 6-6 ∗ Modes which transmit individual characters between stations are referred to as keyboard-to keyboard or unstructured modes .
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