Yes 2Tone and Fldigi use the same start of RTTY transmission. I originated
the idea and discussed it with the Fldigi team, who implemented as well.
Both 2Tone and Fldigi decoders (and W7AY Mac cocoaModem) unlike MMTTY make
use of the absence of tone as well as the presence of tone. This gives a
distinct advantage when it comes to coping with frequency selective fading
when one tone is weaker than the other. They also make use of the noise base
for each tone as this optimises the combining of the two tones just before
the mark/space decision is made.
To get the best from these advanced decoders then on start up sending one
character time of space tone followed by one character time of mark tone is
ideal. This enables the RX AGC value to stabilise, the space tone amplitude
and the mark tone noise to be evaluated with the space tone character time.
The mark tone character time allows the space tone noise and the mark tone
amplitude to be evaluated plus the asynchronous decoder is now looking for
the start bit. It is the minimum start up time possible just the ticket for
contesting.
At the end of transmission 2Tone will send letters shift and then
immediately after this will ramp down on mark tone so the transmission
ceases 44mS later. This raised cosine ramps ensure minimum "click" at the
end of transmission and is the shortest possible. The start of transmission
also uses a raised cosine ramp to avoid a wideband click.
Unfortunately many other transmission implementations have very long mark
start up tones then send variable stop bit lengths and also often send at
45baud rather than the 45.4545 baud required. At the end of transmission
they often have a very long mark tail. All of this is sub-optimum increasing
the receive error rate plus wasting time with unnecessary long start and
ends of transmission.
73 David G3YYD
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