[RTTY] ASCII Radioteletype

[Kok Chen]

Here are what I dug up about ASCII Radioteletype:

(a) FCC Part 97.309 (a)

> ... an amateur station may transmit a RTTY or data emission using the folowing specified digital codes:
> 
> (1) The 5-unit, start-stop, ITT Alphabet No.2 ... (commonly known as Baudot).
> (2) The 7-unit code, specified in CCIR 476-2 (1978)... (commonly known as AMTOR)
> (3) The 7-unit code, specified in ANSI X3.4-1977 or International Alphabet No. 5 defined in CCITT Recomendation T.50 or in ISO 646 (1983), and extension as provided for in CCITT Recomendation T61 (commonly known as ASCII).

(b) ARRL Handbook (2009) page 9.9 "ASCII (IA5) Radioteletype"

> ... Current FCC regulations provide that amateur use of ASCII shall conform to ASCII as defined in ANSI Standard X3.4-1977.  Its international counterparts are ISO 646-1983 and International Alphabet No. 5 (IA5) as specified in the ITU-T Recommendation V.3
> 
> ASCII uses 7 bits to represent letters, figures, symbols and control characters.  Unlike ITA2 (Baudot), ASCII has both upper- and lower-case letters....

The following is from http://f1ult.free.fr/DIGIMODES/MULTIPSK/ASCII_en.htm :

> Baud rate : 110. A character is composed of a « start » bit (1 « space »), 7 or 8 bits and a « stop » bit  (2 « mark »)
> Speed :  110 wpm (7 bits) or 100 wpm (8 bits)
> Modulation : FSK two tones (« mark » and « space » , "mark" high) with a shift of 170 Hz (or, sometimes, 200 Hz),
> Receive  mode : USB
> Character set : 7-bits ASCII (or 8-bit ASCII for some languages), no parity,
> Shape of pulse : rectangular
> Bandwidth : 700 Hz (due to rectangular shape),
> Demodulation : non coherent,
> Synchronization : asynchronous with start bit,
> Correction code : no
> Convolution code : no
> Interleaving : no
> Pmean/Ppeak : 1
> Lowest S/N : -2 dB

Basically, MultiPSK uses 170 Hz shift, 1 start bit, 7 or 8 data bits, no parity, 2 stop bits (with the additional note of "Character set : 7-bits ASCII (or 8-bit ASCII for some languages)").

His Baudot RTTY page states "Lowest S/N" as -5.5 dB.  So, F6CTE is giving a sensitivity difference between 110 baud ASCII and Baudot RTTY as 3.5 dB.

Note that his program assumes that you use USB instead of LSB (identical with RTTY mode in his program).   So, just keep the normal "mark is higher tone on RF spectrum" that we use for Baudot 45.45 baud RTTY.

(I.e., MultiPSK uses a high mark audio tone, USB will move his Mark to the higher of the two RF carriers.  If you use LSB today for RTTY, mark is your lower audio tone, and the LSB system maps Mark also to the higher frequency carrier).

ANSI X3.4-1977 code can be found here: http://en.wikipedia.org/wiki/ASCII .

73
Chen, W7AY