HF Digital Comms

The widespread availability of powerful Personal Computers that include a digital sound card for Digital Signal Processing (DSP), is allowing radio amateurs to use free software for digital communication. The distinguishing features of HF digital modes today are the use of lower power, small or attic antennas. Today the TNC is replaced by an on-board sound card in the personal computer making it easy to try digital modes.

Ham digital mode decoder software… all freeware
Sample of Freeware from eHAM Reviews

Most like on Ham Radio Deluxe (HRD) because it decodes well, easy to use and has interfaces to the past and present transceivers. YES all with only your mouse clicks! You will need the keyboard only for RAG CHEWING. Try them all they are free you may like HAM SCOPE or DIGIPAN better than HRD.

Ham Radio Deluxe

Ham Radio Deluxe (HRD) is a suite of free programs that includes computer control for transceivers, mapping, satellite tracking and logging functions. HRD works on Windows XP, Vista and 7 operating systems. HRD supports most of the VHF, HF, and UHF amateur radio transceivers; Yaesu, ICOM, Elecraft, Ten-Tec FlexRadio, Kenwood. HRD includes a digital mode program called the Digital Master 780 (DM780) that includes built-in logbook, Favorites, Integrated DX cluster, Customizable band layouts, Satellite tracking interface, Scanning. The software is free for use by radio amateurs. If you like HRD you are asked to make a donation to promote Amateur Radio in schools.
HRD Radio Control Screen

HRD Digital Modes
  • PSK, QPSK, Contestia, CW, Domino, Hell, MFSK, MT63, OLIVIA, RTTY, RTTYM, THROB.
  • SuperBrowser decodes and displays all PSK QSOs in one screen
  • SSTV with Scottie and Martin modes.
  • RSID (automatic mode detection and tuning).
HRD World QSO Map Screen

HRD Logbook Screen

PSK31 is a DIGITAL mode using keyboard to keyboard QSO’s on the HF bands.

PSK31 is created by shifting the phase of the carrier. In the most-commonly-used variant (BPSK31), binary information is transmitted by either imparting a 180-degree phase shift (binary zero) or no phase-shift (binary one) in each 32ms symbol interval. The boundaries between character codes are marked by two or more consecutive zeros. Since no character code contains more than one consecutive zero, the software can therefore instantly detect the 'space' between characters. Martinez arranged the character alphabet so that, as in Morse code, the more frequently occurring characters would have the shortest encodings, while rarer characters used longer encodings. He gave the name 'varicode' to this encoding scheme. Usually short QSO’s exchanging station info, weather, name and report etc. PSK-31 is a full duty cycle mode.

HRD PSK Super Browser Screen

PSK31's bandwidth of 31.25 Hz was chosen because a normal typing speed of about 50 words per minute requires a bit rate of about 32 bits per second, and specifically because 31.25 Hz could easily be derived from the 8 kHz sample rate used in many DSP systems, including those used in the computer sound cards commonly used for PSK31 operation (31.25 Hz is 8 kHz divided by 256, and so can be derived from 8 kHz by halving the frequency eight times).


Colloquial usage of the term 'PSK31' in amateur radio usually implies the use of the most commonly-used variant of PSK31: binary phase shift keying. BPSK uses no error control, but an allied mode, QPSK31, uses four phases instead of two, to provide a degree of forward error correction. It is very simple to switch from BPSK to QPSK if difficulties arise during a contact.

PSK31's efficiency and narrow bandwidth make it highly suitable for low-power and crowded-band operation. PSK31 contacts can be conducted at less than 100Hz separation, so with disciplined operation at least twenty simultaneous PSK31 contacts can be carried out side-by-side in the bandwidth required for just one SSB voice contact.

Radio Teletype (RTTY) “RAT”

RTTY, using either AFSK or FSK modulation, is moderately resistant to vagaries of HF propagation and interference, however modern digital modes, such as MFSK, use Forward Error Correction to provide much better data reliability. Landline teleprinter operations (TTY) began in 1849 when a circuit was put in service between Philadelphia and New York City. Émile Baudot designed a system using a five unit code in 1874 that is still in use today. Teleprinter system design was gradually improved until, at the beginning of World War II, it represented the principal distribution method used by the news services.


Slow-scan television (SSTV) is a picture transmission

SSTV is narrowband television takes up to a maximum of 3 kHz of bandwidth providing still pictures transmission, usually taking from about eight seconds to a couple of minutes, depending on the mode used, to transmit one image frame. SSTV uses analogue frequency modulation, in which every different value of brightness in the image gets a different audio frequency. In other words, the signal frequency shifts up or down to designate brighter or darker pixels, respectively. Color is achieved by sending the brightness of each color component (usually red, green and blue) separately. This signal can be fed into an SSB transmitter, which in part modulates the carrier wave. A transmission consists of horizontal lines, scanned from left to right. The color components are sent separately one line after another. The color encoding and order of transmission can vary between modes. The modulating frequency changes (between 1500 and 2300 Hz) corresponding to the intensity. The modulation is analogue, so even though the horizontal resolution is often defined as 256 or 320 pixels, they can be sampled using any rate. The image aspect ratio is conventionally 4:3. Lines usually end in a 1200 Hz horizontal synchronization pulse of 5 milliseconds.


Computer Interfacing to Digital Modes (Transceiver to Personal Computer)

Interfacing to your PC sound card interface to provide all modulate / demodulate functions as well as operator interface and control. The sound card is attached to an HF SSB rig using the PC speaker out and microphone in ports on the sound card. Typically, there is an interface installed between the sound card and the rig to adjust signal levels and provide good isolation between the two devices. On the SSB rig side, typically the microphone input and external speaker out are used. PTT control is done via a control signal on a PC serial port (RTS or DTR) and isolated with an opto isolator to drive the rig PTT line. In some transceivers the data port of the rig can be used in lieu of the microphone and external speaker ports. You can buy or build your interface.

Homebrew Digital Mode Interface by G4KQO

My Review of Digital Mode Interface Vendors

HF Digital Frequencies

My Shack with RIG Connected to Flat Screen TV
Relax in the recliner with a Wireless Keyboard & Mouse!