[RTTY] MMTTY Macros

[Kok Chen]

I wrote earlier:
> It is better to choose Baudot letters with fewer bit transitions.  The
> reason why RYRY is used for testing is that there are maximal bit
> transition.

This reminds me of something else... I hope I can explain one of the  
minor mysteries of RTTY.

RTTY'ers who have done enough DX'ing probably have at various times  
seen a bunch of sporadic V printed on their screen.

I remember one time many aeons ago, I was watching a pileup and Garry  
NI6T sent a quick "whats all the V?" :-) :-).

I didn't see any V from that particular DX station that Garry  
commented on, but I have often seen it with other stations.

The answer is very simple actually -- notice that

1) most programs/TNC use LTRS as the diddle character.

2) the LTRS character in Baudot is 0x1f (5 bits of '1' )

	'1' is sent as a mark, a start bit is sent as a space, and stop is  
sent as a mark -- so LTRS is transmitted as a space, followed by 6.5  
bits of mark

3) the V character in Baudot is 0x1e.  LSB is transmitted first, so V  
is a '0' bit followed by four '1' bits.

	including the start and stop bits, a V is transmitted as two spaces,  
followed by 5.5 marks

Therefore if multipath causes the start bit of a diddle to start  
overlapping into the first data bit, that diddle character can print  
as a V if the multipath is severe or if  the demodulator's multipath  
countermeasure is weak.

The next time you see a bunch of V printed on the screen,  it is not  
the station at the other end that is at fault.  The ionosphere is  
smearing the start bit sufficiently into the first data bit to cause a  
decoding problem.

This only happens if the DX station is manually typing.  Most software  
generated exchanges don't have embedded diddles.

73
Chen, W7AY