[RTTY] The magic of TU

[Kok Chen]

On Oct 3, 2013, at 11:59 AM, Vladimir Sidarau wrote:

> but I am not an expert in ASCII codes and cannot prove it at all.

Vlad,

Over the air, it would be ASCII code, not Baudot.

Baudot characters like R and Y are most critical and most prone to reception error, in terms of intersymbol Interference (ISI) and multipath, since they have alternating bits.  And that is why you see RYRYRYRY used to tweak RTTY reception (not really needed nowadays with reasonable software modems).

You can find the Baudot table here

http://en.wikipedia.org/wiki/Baudot_code

For the bit sequence, you can use Fig 16-1 of the ARRL Handbook (from the 2010 edition on).

You are anecdotally correct about TU, at least the T part.

An RTTY start bit is a 0 followed by the LSB of the Baudot character, the rest of the 5 bits and followed by the stop bit (1).

So a T (including the start and one stop bit) would be keyed as 0000011.  That has minimal bit transitions (and thus less affected by ISI and multipath). 

The letter O (0000111) is probably a tad better than T, and has even lower keying sidebands than a T.  The RY sequence produces the widest keyclicks, and most prone to multipath.

73
Chen, W7AY