Microbit RRC-1258 MkII(s) Uživatelský manuál stáhnout pdf (Strana 6)

Computers in Amateur Radio

Mark-Space shifts being 85Hz, 170Hz, 425Hz or 850Hz. The most

popular shift used by radio amateurs on HF is 170Hz. 85Hz tends to be

used at VLF, because the bandwidth of antennas is extremely small,

while sending RTTY at higher rates requires a bigger frequency shift and

consequently greater bandwidth. When FM or SSB is being used, the

two tones are usually 2125Hz and 2295Hz. These two frequencies ensure

that any audio harmonics are outside the passband of amateur transmit-

ters, because their audio cut-off is about 3kHz. Data throughput is about

60 words per minute at 45.45 bauds.

Although RTTY is limited by the fact that it contains no error

detection or correction, it remains an extremely popular datamode

amongst radio amateurs.

ASCII

The original ASCII (American Standard Code for Information Exchange)

used electromechanical teletypes that worked at 110 bauds. This

equated to an element length of 9ms.

ASCII was a significant improvement on Baudot, because it con-

tained seven data bits. The 128 possible combinations meant that upper

and lower case letters could be sent, plus a lot more symbols, punctua-

tion and control characters.

As standard, ASCII does not incorporate error detection or correc-

tion, but it is possible to add a measure of error detection by adding a

parity bit.

Although there is no reason why it should not be used in its basic

form by radio amateurs, it tends not to be. Rather, it tends to be used

as the core code for some other datamodes.

PSK31

PSK31 was invented by Peter Martinez, G3PLX in 1998. It employs

Phase Shift Keying at 31.25 bauds and has the advantage of requiring a

very narrow bandwidth (31Hz). It was designed primarily for real-time

keyboard-to-keyboard QSOs, is capable to transmitting all the charac-

ters on a keyboard and has even been used to transmit small pictures.

Rather like Morse, where not all characters take the same time to

transmit (with the most commonly used ones taking the least amount of

time), PSK31 uses a ‘varicode’ system. More commonly used letters

such as ‘e’, ‘n’ and ‘s’ employ less bits and take significantly less time to

be transmitted. If transmitting a passage of text in lower case letters, a

throughput of about 53 words per minute can be achieved, but if the

same text is transmitted in upper case letters the throughput drops to

about 39 words per minute.

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Microbit RRC-1258 MkII(s) Uživatelský manuál Strana 6