Microbit RRC-1258 MkII(s) Manual do Utilizador descarregar pdf (Página 5)

Datamodes

receive and decode with a mechanical machine. This is primarily be-

cause, irrespective of the sending speed, not all the characters take the

same length of time to transmit. Emile Baudot overcame this problem in

the 1840s, by developing a code in which all the characters were the

same length. This permitted receiving equipment to be synchronised for

each character sent.

The construction of RTTY data is shown in Fig 2.2. The standard

Mark condition tone is 2125Hz (logic ‘1’) and the Space condition tone

is 2295Hz (logic ‘0’). The grey areas are 1’s or 0’s, depending on the

characters being sent. Transmission of each character commences with

a ‘start’ bit. The length of the start bit – indeed all the bits – depends

upon the rate of transmission. For a 50 baud transmission (as used by

the Telex network), the length is 20 milliseconds. For 45.45 baud trans-

mission (as commonly used by radio amateurs), the length is 22 milli-

seconds. The start bit is followed by five data bits (10010 for a letter D),

then 1.5 stop bits. The extra length of the stop element of the character

is to give mechanical teleprinters the opportunity to come to a halt

before the next character arrives.

Five data bits give a possible 32 combinations, which is not

sufficient for transmission and reception of the 26 letters of the alphabet,

the numbers 0-9, and common punctuation marks and control charac-

ters such as a bell, new line, carriage return, etc. This problem is over-

come by dedicating two of the 32 possible character combinations for

Letters Shift and Figures Shift. In Letters Shift, the 26 letters of the

alphabet plus some common functions (carriage return, new line, space)

are possible. In Figures Shift, numbers, punctuation, a few symbols and

the same common functions are available.

RTTY is normally transmitted by a process known as Frequency

Shift Keying (FSK) or Audio Frequency Shift Keying (AFSK). In a loud-

speaker this results in a wobbling sound, because the transmitted signal

is being

switched

between two

frequencies.

During peri-

ods when no

data is being

transmitted

but the

transmitter is

still on, there

is a steady

tone. Practi-

cally any two

audio tones

can be used,

the standard

Fig 2.2: How

RTTY characters

are constructed

and transmitted.

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