Message-ID: <62970E9E74CE47129482C1C415B6B635@AGB>
From: "Graham" <g8fzk@g8fzk.fsnet.co.uk>
To: <rsgb_lf_group@blacksheep.org>
References: <555CCC21.4060901@gmx.com> <555E100C.25784.C7C6D2@mike.dennison.ntlworld.com> <E2BCDE73F4D64D958A6E7D980A8125DE@gnat>
In-Reply-To: <E2BCDE73F4D64D958A6E7D980A8125DE@gnat>
Date: Thu, 21 May 2015 19:39:50 +0100
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Subject: Re: LF: MF 630m: False Decode or Real?
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Status: O

Graham thinks that because there were no more false decodes in the previous
24 hours this one MUST be good.

Its not  Just  Good Alan  ...  Its  'Ruddy MARVELLOUS '

As in  any  data  system, there  is  no  actual  limit  to the  sensitivity 
,  where  there  is  a  limit  to  is the  acceptable  %  error  rate

Opera  data  decode  s/n can be  lowered , but  then  comes  the  % false 
decode  increasing , you  could  counter argue that  false  decodes  are 
purely  random  , so  are  of no  consequence .. some may

The  design  criteria  for  a  correlation  system  remains  the  same , 
but  where  as in the  data  decode , logic  decisions  are  called  on to 
extract the  data  from the  coding ,  as in the  case  of  Opera  a 
minimum  of  40% random  capture .

A correlated  detection  'Operas Dynamic' , is a  pattern  match  against 
time , sharing  the  40% minimum level , the   information  is  presented 
as  a  fixed  pattern  and the  system  desires  a  correlated  match  , 
much the  same as  the  bombe  and  colossus    at  station  X , the  system 
also  seeks  to  minimise the retained  call  list ,  the  8 day  period 
was  set to  compliment  the  band  activity ,

The  design  challenge  is  to  produce a  match  with  enhanced 
sensitivity, whilst  maintaining  a  'credible'   false  detection  level 
compared to  a  data  decode. In the   case  of  Opera  dynamic ,  this  is 
set at   -5 bB with  a commensurate   low  level  of  false  detections ,

Zero false detections  from a group of  14  stations in a 4  hour  period , 
subjected  to  high band noise  conditions , is a  indication  of  the 
level  of  the system  design

As correctly noted  random  events  do  conspire to  produce  false 
detections ,  the  confidence  level  for  a  stand alone  non  web-linked 
RX  is  indicated by the  recovered  s/n  level, the  user should  refer to 
tx  time  start  etc

However, being  a  dynamic  system,  once  more than  a single  Rx is 
linked  via the  web,   then cross  correlation  of the  detection  times  , 
enables a  very  robust  statement  to  be  made , as to  the  authenticity 
of the  result ... noting  unfortunately, the  psk-map shows  all 
detections , where as  the  web-linked  local  copy  is  provided  with the 
system  evaluation   ''??''  for  a  false  or  only 'single' detection

The  evaluation of the  Dynamic detection , is based  on the  system 
parameters  and detection  algorithms performance,  providing   a  modern 
robust fully  automated  addition  to  the  established  Opera  system . The 
only  auto  configuring  dual  mode low  s/n  system  available  to the  MF 
and  LF user  group.

73-G,
--------------------------------------------------
From: "Alan Melia" <alan.melia@btinternet.com>
Sent: Thursday, May 21, 2015 5:47 PM
To: <rsgb_lf_group@blacksheep.org>
Subject: Re: LF: MF 630m: False Decode or Real?

> A couple of comments on the statistics......this has little to do with the 
> decode process itself.
>
> The timing only goes to show that the "decoded" call sign was active at 
> the time NOT that he was really received, but might have been.
>
> Graham thinks that because there were no more false decodes in the 
> previous 24 hours this one MUST be good. Statistics dont work like that ! 
> If it is a million to one for a false decode you must expect at least near 
> to a million decodes before another false one. On the contrary 2 
> questionable decodes of the same call would be extremely unlikely to be a 
> random correlation so would provide good evidence the call was actually 
> received and decoded properly. The probability of one true decode but not 
> a second due to fading is very (very) low but not impossible.
>
> Next though many do not seem to appreciate it, statistics dont "prove" 
> anything, they provide a measure of the probability of an expected 
> outcome. You determine both the probability of the expected outcome and 
> also the probability it could be just random chance. The bigger the 
> difference then the more likely the expected event occured. This is what 
> makes a second decode so powerful.
> People DO win the lottery every week! So false decodes do happen by 
> chance.....but the same lottery winners rarely win two weeks in 
> succession.
>
> Alan
> G3NYK
>
>
>
>
> ----- Original Message ----- 
> From: "Mike Dennison" <mike.dennison@ntlworld.com>
> To: <rsgb_lf_group@blacksheep.org>
> Sent: Thursday, May 21, 2015 5:04 PM
> Subject: Re: LF: MF 630m: False Decode or Real?
>
>
>> Very likely a false decode. The clue is the two question marks. In my
>> experience, almost every decode with question marks is false, and
>> there are very few cases where these particular decodes are useful. I
>> believe it would be helpful not to display these at all.
>>
>> Having said that, the deep search facility in Opera is very useful
>> and has revealed some very interesting propagation information. But
>> only using the decodes without the question marks which, again in my
>> experience, are almost always genuine.
>>
>> Mike, G3XDV
>> ===========
>>
>>> Just popped up at my RX:
>>>
>>> 17:52    477 VK3ELV de DF1VB/3 Op8 Deep Search ?? 16348 km -37 dB in
>>> Dortmund with 140w + Top loaded L 18m vert 80m horz
>>>
>>> Any comments welcome
>>>
>>> 73, Jochen
>>>
>>>
>>>
>>> -- 
>>>   -= DF1VB =-
>>>   -= KH2MM =-
>>> Jochen Althoff
>>> +49 171 2020206
>>>
>>> "The wireless telegraph is not difficult to understand.
>>> The ordinary telegraph is like a very long cat.
>>> You pull the tail in New York, and it meows in Los Angeles.
>>> The wireless is the same, only without the cat."
>>> (Albert Einstein)
>>>
>>>
>>>
>>> -----
>>> No virus found in this message.
>>> Checked by AVG - www.avg.com
>>> Version: 2015.0.5941 / Virus Database: 4347/9830 - Release Date:
>>> 05/21/15
>>>
>>
>>
>>
>
>