[RTTY] 2Tone Chirp

[Ron Kolarik]

Thanks for the explanation David. The chirp I hear is probably
the ramp up on transmit, I'll look at it when I have more time.

73,
Ron
K0IDT


----- Original Message ----- 
From: "David G3YYD" <g3yyd at btinternet.com>
To: <rtty at contesting.com>
Sent: Monday, December 16, 2013 4:27 AM
Subject: [RTTY] 2Tone Chirp


> As you can see I have joined this reflector.
> 
> The start up sequence of 2Tone is just as Chen, W7AY, describes. This is 
> intentional as I will explain.
> 
> When receiving using 2Tone in normal mode it asses the envelope value of 
> both the mark and space tones. It uses this along with its noise 
> assessment to generate a threshold value for each tone. These threshold 
> values are used as part of the way 2Tone works out whether mark or space 
> is being received at any instance in time.
> 
> At the moment a new transmission starts up 2Tone receive envelope and 
> noise values will be incorrect as they will either be a value of the 
> previous transmission or the value of the noise/QRM being received. In 
> order to optimise 2Tone decode there is a need to send a space tone and 
> a mark tone before characters are sent.
> 
> Being that mark tone is required to ensure the asynchronous decoder is 
> ready for the start bit of the first character then the logical sequence 
> is space tone for one character time; setting the space threshold value 
> and mark noise value. This is then followed by mark tone for one 
> character time setting the mark threshold value and space noise value 
> and resetting the asynchronous decoder to wait for  first start bit (one 
> bit time of space tone).
> 
> Most (all?) other means of sending RTTY normally start with a long, in 
> many cases unnecessary long, mark tone and then sends the first 
> character. This means the space tone threshold will be in error. But 
> 2Tone is a bit more sophisticated. It deliberately delays assessment by 
> one character time so it can also use information from the second sent 
> character to establish a space (and improved mark) threshold value for 
> the first character. So not all is lost but for best results the 2Tone 
> sequence is that fastest you can go and get the best decode accuracy.
> 
> As Chen says 2Tone transmission has a slow amplitude ramp up at the 
> start and ramp down at the end of transmission so there are no clicks to 
> cause QRM to other band users. Very few AFSK implementations  do this 
> and no rig based FSK does it causing unnecessary QRM on adjacent channels.
> 
> 
> 73 David G3YYD
> 
> 
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