[RTTY] VVVVV

[Kok Chen <chen@mac.com>]

For those who are mystified by printing bunches of V's in the middle of 
a QSO, I think I have stumbled on the root cause.

I happened to be implementing a modem with multiple simultaneous ATC 
(adaptive threshold correction) units, each one with a different 
attack/decay AGC in the quest to better decode signals with very rapid 
QSB (e.g. fluttered signals).

While sorting through the LTRS characters, I see a sprinking of V 
characters among them.  V happens to be Baudot hex 1E, and LTRS happens 
to be Baudot hex 1F.

Notice that LTRS is often used as a diddle character.  So, if there are 
a lot of least-significant-bit noise hits, you are going to get a lot 
of Vs.

The LSB of Baudot is transmitted right after the start bit.  With LTRS, 
you are essentially transmitting a long mark that is occasionally 
broken by a single space bit (start bit of the LTRS).  If the AGC times 
of the threshold correction stage is not optimally set for this case, 
it would throw the next bit off, especially in noisy conditions.  If 
there is any group delay between mark and space tones (as when there is 
multipath), it can also easily turn a LTRS into a V character.

I am beginning to think that the LTRS character is a poor choice as a 
diddle, but then a NULL/Blank (0x00) character is only slightly better, 
(the duty cycle is just slightly less skewed for systems which use 1.5 
and 2 stop bits).

73
Chen, W7AY


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