One area of amateur radio that has really grown by a significant amount since the introduction of personal computers is the use of digital modes. These digimodes as they are sometimes called provide a very interesting way in which to communicate over ham radio, offering different challenges than common to the more traditional modes of transmission.
The first of the digimodes was RTTY. Originally this used mechanical teletypes. Eventually computers running terminal software replaced the noisy mechanical devices.
With further development in computer hardware and software it is possible to utilize many advanced transmission techniques and as a result quite a variety of digimodes have been invented and used. Each one of these digimodes has its own advantages and different types tend to be used for different applications.
The communications modes used on the HF ham bands must handle problems not encountered in other environments. HF signal paths tend to be noisy and distorted in ways that make the digital modes used successfully in other environments fail badly. A common voice grade land-line telephone circuit that many people used to use computer modems to transfer data will typically support 56 Kbits per second data rate. A HF SSB voice circuit can reliably support only a small fraction of that.
The variety of HF digital communication modes vary so widely in operating characteristics that measurements such as baud rate or bits per second are nearly meaningless. When comparing the various digimodes using different modulation methods and character encoding schemes it is often described as Words-Per-Minute (WPM) which comes from the common way of measuring of morse code (CW) transmission speeds.
Ham digital communication breaks down into two categories: keyboard-to-keyboard real time conversational and file transfer. File transfer can consist of sending things such as digital image files or text messages.
The basic way digital modes work is the HF transceiver’s microphone and speaker audio lines are connected to a computer’s sound card. And the transceiver is operated in SSB voice mode. Software decodes the receiver audio and generates transmit audio.
The program shown above provides an example of a current keyboard-to-keyboard digital communications program user interface. The main sections of the window above, starting at the top, are the receive text area, the (blank) transmit text area, and the control area. The control area includes a tuning audio spectrum display and a waterfall display. The display above shows several PSK31 signals in the receiver pass band, each digital conversation shows up as a vertical line. Selecting one of the signals for decoding is done by simply moving the mouse cursor and clicking on the desired signal.
Overview of popular digimodes:
- CW/Morse Code – Yes, computers can be used to send and receive CW. It works but receiving is not terribly reliable in the presence of noise, interference, or when signals are weak. (5 – 60 WPM, -10 dB S/N with experience operator)
- Hellschreiber or HELL – Nope, not that hot place with the sulfurous odor. This is the nickname of a communications mode invented in 1929 by a German whose last name happened to be Hell. There are variations of this mode but the most commonly used is FELD HELL that used simple On/Off keying to form visible characters by marking dots on a paper tape. PC programs simulate the paper strip as bands on the monitor screen. The PC program does not attempt to decode the characters as they are received. It merely makes light and dark marks on the simulated paper strip. It is up to the operator to make sense of what is shown. It is very much like watching a grainy strip of fax paper on one of the early fax machines scrolling by on the screen. If you hear something like high speed CW that you can’t make sense of, it is probably FELD HELL. (25 WPM, -10 dB S/N).
- Frequency Shift Keying (FSK) type of digimodes.
FSK overcomes the decoding problem of CW by transmitting a continuous carrier and shifting its frequency for indicating On/Off conditions, known as Mark/Space in FSK terms. This allows the decoder active redundant information to work with so is more reliably decoded.
- RTTY – This is the granddaddy of ham digital modes and is still very popular and used in large contests. Current practice is to use 170 Hz shift of the RF signal frequency with the higher frequency being the Mark and lower Space. The most common data rate used is 45.45 Baud. RTTY is typically used at high power: 100 watts to 1500 watts. (67 WPM upper case only, -5 dB S/N)
- AMTOR – This is the ham version of a commercial protocol based upon standard RTTY. It uses an automatic retransmission request protocol to achieve reliable communications. It uses 200 Hz shift at 100 baud. There is also a Forward Error Correction (FEC) mode. AMTOR is normally used with hardware modems. (0 – 67 WPM, -5 dB S/N)
- PACKET – Designed by hams in 1978 through 1984. It is used extensively on VHF, especially for APRS. HF Packet runs 200 Hz shift at 300 baud. It performs poorly on HF but is still used for automated messaging services. It operates in a dedicated connection mode that allows continuous transmission repeats until a message is finally delivered. It also has an unconnected mode that is used for general broadcast transmissions as with APRS position reports. (330 WPM, +20 dB S/N)
Here are some of the popular software used for running amateur radio digital mode transmissions.
- Fldigi - Very Popular open source program that can do most all of the popular digital modes. Considered by some to be the most accurate and reliable digital signal decoder (free, Windows, Linux, Mac).
- DM780 - Included as part of Ham Radio Deluxe suite, also very popular among the large group of hams using the HRD Suite. Uses the Fldigi decoding engine with a slightly more modern wrapped gui around it (free, Windows).
- DigiPan - A very simple program strictly used for PSK. One of the first available and still popular. (free, Windows)
- WinWarbler - Included as part of the DXLab suite of ham programs. (free, Windows)
- WSJT - A very new program focused on weak signal digital modes such as JT65 (free, Windows, Linux).
- WSPR - Growing in popularity, this program is focused on the WSPR(whisper) protocol which is designed to be used for very low power beaconing and the study of global radio propagation (free, Windows, Linux).
- MultiMode - Commercial program which runs on Mac OSX and supports many different digimodes including.
- iPad PSK31 - Psk31 app for the iPad (decode only)
- iPhone I-Psk31 - App for iPhone that both decodes and sends PSK-31 by connecting the iPhone to the radio through the mic/headset jack.
While it may seem that there is very wide variety of digimodes that can be used, this comes from the fact that people are experimenting and trying to improve the effectiveness of digital modes of transmission as technology moves forward.
While there are many digimodes to choose from, the situation may not be as difficult as many may imagine because it is often possible to receive and send a large number of types of transmission using a single interface. Most digital ham software supports many different modes so it is quite easy to send and receive many different transmission modes.