[Moon] call3.txt why?

[Giorgio Marchi]

----- Original Message ----- 
From: "Peter Sundberg" <sm2cew at telia.com>
To: <moon at moonbounce.info>
Sent: Tuesday, April 22, 2008 9:06 PM
Subject: [Moon] call3.txt why?


>
> I appreciate the fact that people using JT65 take pride in not using DS, I
> would just like to know why they say that??
>
> 73 de Peter, SM2CEW
>
Hi Peter and all, I am an emer who uses DS. I have studied accurately what 
it is.
When JT65 was born I translated the WSJT Manual in Italian, partly as a 
service to the Italian ham community, but partly because so doing I was 
obliged to understand all the fine points, which I would have overlooked in 
a quick perousal of the text. The deciding point was when I translated the 
document specifically on JT65. Regarding possible false decodes it says:
................................
For JT65 I chose the Reed Solomon code RS(63,12), which encodes each 72-bit 
user message into 63 six-bit "channel symbols" for transmission.  Every 
codeword in this code differs from every other one in at least 52 places - 
which, in a nutshell, is why the code is so powerful.

.............................

As an example, the encoded sequences for three nearly identical messages are 
illustrated in Figure 2.

Message #1:  G3LTF DL9KR JO40

Message #2:  G3LTE DL9KR JO40

Message #3:  G3LTF DL9KR JO41

.........................

the three fully encoded sequences of channel symbols appear to be almost 
entirely different from one another - so different that there is virtually 
no chance whatsoever that, if it is decodable at all, a noise-corrupted 
version of one of these messages would ever be misconstrued as one of the 
others.



What convinced me of the validity of the help that DS could give was this 
last sentence. If I receive a cq with a call decoded only by DS (as can be 
understood from the numbers that appear at the end) and answer this call, 
either it is correct or it is completely different, so it would be 
impossible for the other side not to understand this. If the qso procedes, 
this means that the decode was correct, AND THE SUCCEDING DECODES GIVE THE 
NECESSARY COMPLETENESS TO THE QSO. If I see a question mark and a low vote, 
then I do not answer, awaiting however an "average" decode before sending 
ROs. My experience shows me that when I got votes 8 to 10 the decode was 
always right, so I was justified in proceeding.

My eme activity started with cw skeds. My cw is very rusty, so some hours 
before the sked I used to send to myself the characters of the call in order 
to get the feel of the sound. This helped me a lot during the sked, and I 
now realize that I was mentally making a sort of DS.

What DS offers as a decode is not the final decode but only a suggestion of 
a probability, it is up to each of us how we use it.

73, Gio IK1UWL