![]() It is up to the operator to make sense of what is shown. ![]() It merely makes light and dark marks on the simulated paper strip. The PC program does not attempt to decode the characters as they are received. PC programs simulate the paper strip as bands on the monitor screen. 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. This is the nickname of a communications mode invented in 1929 by a German whose last name happened to be Hell. Hellschreiber or HELL – Nope, not that hot place with the sulfurous odor.(5 – 60 WPM, -10 dB S/N with experience operator) It works but receiving is not terribly reliable in the presence of noise, interference, or when signals are weak. CW/Morse Code – Yes, computers can be used to send and receive CW.Selecting one of the signals for decoding is done by simply moving the mouse cursor and clicking on the desired signal. The display above shows several PSK31 signals in the receiver pass band, each digital conversation shows up as a vertical line. The control area includes a tuning audio spectrum display and a waterfall display. 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 program shown above provides an example of a current keyboard-to-keyboard digital communications program user interface. ![]() Software decodes the receiver audio and generates transmit audio. And the transceiver is operated in SSB voice mode. The basic way digital modes work is the HF transceiver’s microphone and speaker audio lines are connected to a computer’s sound card. File transfer can consist of sending things such as digital image files or text messages. Ham digital communication breaks down into two categories: keyboard-to-keyboard real time conversational and file transfer. 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. 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. A HF SSB voice circuit can reliably support only a small fraction of that. 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. HF signal paths tend to be noisy and distorted in ways that make the digital modes used successfully in other environments fail badly. The communications modes used on the HF ham bands must handle problems not encountered in other environments. Each one of these digimodes has its own advantages and different types tend to be used for different applications. 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. ![]() Eventually computers running terminal software replaced the noisy mechanical devices. Originally this used mechanical teletypes. 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. 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.
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