Published: 10. 10. 2005   Category: Hamradio station OK2MNM

Spectroscope view of RTTY (Baudot)

This signal was recorded from 14 MHz so there is a strong noise. RTTY (Radio TeleTYpe) is digital mode used by hams to transmit text over short waves. Used modulation is FSK (Frequency Shift Keying). Two frequencies matching two logic states. Zero (space) has 1445 Hz and one (mark) has 1275 Hz. So frequency shift used is 170 Hz. For receiving of these signal is used lower sideband (LSB) so the frequency assignment is normal. When upper sideband (USB) is used the signal is inverted. Modulation speed is 45.45 Bauds such that one modulation state duration is 1 / 45.45 = 22 miliseconds.


Obr.: Hamradio RTTY.
Rec. 1111110 1011110 1111100 1110010 1110010 1111110 1111100 1001110 1001000 1011110 1111100 1110110 1110010
Dem. 11111 11110 01111 10011 10011 11111 01111 11100 00100 11110 01111 11011 10011
Data LTRS V K FIGS 2 LTRS K M SP V K FIGS 2

The transmitted code use International Telegraph Alphabet No. 2 (ITA-2) which was introduced by CCITT in 1924. One letter contains 7 symbols. First symbol is start (mark) followed by 5-bit ITA-2 code and last symbol is stop (space) and it may from 1 to 1.5 times longer than other symbol.

International Telegraph Alphabet No. 2 (ITA-2)

bin.hex.letterfigure
0000000BLKBLK
0001103A-
1100119B?
011100EC:
0100109D$
0000101E3
011010DF!
110101AG&
1010014H#
0011006I8
010110BJBELL
011110FK(
1001012L)
111001CM.
011000CN,
1100018O9
   
bin.hex.letterfigure
1011016P0
1011117Q1
010100AR4
0010105S'
1000010T5
0011107U7
111101EV;
1001113W2
111011DX/
1010115Y6
1000111Z"
0100008CRCR
0001002LFLF
0010004SPSP
111111FLTRSLTRS
110111BFIGSFIGS

Signals:

LTRS — Letter shift
FIGS — Figure shift
BLK — Blank
SP — Space
CR — Carriage Return
LF — Line Feed