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Fw: LF: DJ8WX 8.97 kHz

To: <[email protected]>
Subject: Fw: LF: DJ8WX 8.97 kHz
From: "Graham" <[email protected]>
Date: Sun, 24 Mar 2013 21:17:38 -0000
Importance: Normal
Reply-to: [email protected]
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From: Graham
Sent: Sunday, March 24, 2013 8:49 PM
Subject: Re: LF: DJ8WX 8.97 kHz

Ok Marcus
 
That would be  an interesting  'test'  , the   s/n levels  seem  to  be a little  varied , depending  on the   reference, I know  when ros  was first coded , Jose used  fldigi  as  the  comparison ,   but  later  changed  to  align  with the  path-sim  propagation  simulator , which then  changed the  base line , Op is also  aligned  with   the  same  simulator http://www.moetronix.com/ae4jy/pathsim.htm
 
The levels may  not be  inside the  Opera  decode  range , I think  -50 was the  best  over  short range tests  , but  those tests  also  showed that  the  signal  still  left  a  trace  on the  qrss plot  and its possible , using  you  correlation  process  , detection  at  lower  levels may be possible as  well ? 
 
 One thing that  did  seem to  arise from the  early  pie fight's   ( eating , not 3.142)  was  testing  over  a  real path  and a  static simulation did  provide differing  results , assumedly  based on the  'decode  profile'  , a lot of tests  may be  in conflict  with  design parameters , rather  than providing  an evaluation , say like comparing   slick and  treaded tyres in the  rain , especially  in   free  running  data  mode's , where , for  example , interlacing would be  based on  qsb  profile  (hf)  and  on MF are found to  fail  due to  much longer  qsb -
 
I see  a  follow  up from Uwe , may try  other  modes  in a week  or  so  , if  so  , may be the  Op monitors  using  mini-whips  for  136  could take a  look  for  his  signal  as  well  for  a  day/night ? , if  there  is to  be a  test , can post on the  user group looking  for  monitors, in case any one needs  to  re arrange things , tnx . 
 
73 -G.
 
 
 
 

Sent: Saturday, March 23, 2013 6:14 PM
Subject: Re: LF: DJ8WX 8.97 kHz

Hi Graham,
 
yes this is an interesting option.
 
The correlation detector looks for spectral peaks in a long narrow FFT, which integrates across the total duration of the Opera sequence (about 2000s for Op 32). For a reliable detection, a phase-coherent central peak needs to be at about 15 dB SNR in the corresponding bin width (0.5 mHz). Taking a 100% duty cycle carrier for reference, at 50% duty cycle we have 25% (-6 dB) energy in the central peak, and another 25% in both AM sidebands together. Thus for Op-32, the threshold is around 21 dB carrier-SNR in 0.5 mHz, scaling to -46 dB in 2.5 kHz (or -50 "dBOp" on the Opera scale). The modulation energy will ideally also bring up a 15 dB correlation peak, which is normally sufficient to ensure unambiguous callsign identification. Extending the duration from 32 minutes to ~ 4 hours would gain another 9 dB, arriving at a threshold of -55 dB in 2.5 kHz (-59 dBOp).
 
On my rather noisy VLF grabber, the carrier from DJ8WX reached about 9 dB SNR today. Scaling the bandwidth from 1.5*0.48 mHz to 2.5 kHz, we get -56.5 dB SNR (or -60.5 dBOp) - just 1.5 dB bit short of an Op-4H detection. Of course other online VLF receivers (namely Paul Nicholson and OK2BVG) do have significantly better receive conditions so Op-4H correlation should be quite feasible there.
 
For the standard Opera software decoder, Rik's measurements had indicated a decoding threshold of -23 dB in 2.5 kHz for two-minute Opera-2. This scales to -35 dB for Op-32 and -44 dB for Op-4H (-39 and -48 dBOp respectively). Thus for Op-4H you'd want to see a carrier SNR of 21.5 dB in a "qrss 6000" grabber window, which seems a fair challenge at the distances we're interested in.
 
For comparison, Stefan's famous LF reception at VK2DDI showed about 15 dB SNR in an 11 mHz spectrogram, equivalent to -37 dB in 2.5 kHz (-41 dBOp). Thus an "easy target" for Op-32 or even Op-8 correlation, but still slightly below the edge of the classic Op-32 decoder.
 
Best 73,
Markus (DF6NM)
 
From: Graham
Sent: Saturday, March 23, 2013 1:39 PM
Subject: Re: LF: DJ8WX 8.97 kHz

Marcu's / Uwe
 
This would  make a  good  test  for the   Op4H  ( 4 hour)  mode  and  the  correlation   detector ?
 
To date  there  have been  no  'dx' test , with the  addition  of  the  correlation  ,then  that  should give  three   possible detections  ?
 
OP4H decode ,  qrss plot  and  Correlation    ( and  50%  saving on power hihi)
 
OP4H has AFC  as the  shorter  modes and  is  designed for  normal sound card  use. with  sound-card  audio or  CW-comport Tx keying , rx  audio is  direct  in from  sound  card  , in short range  tests  -50 db s/n  has been reached , but  that  is a data  decode , would the  correlation , take the  level  below the  qrss  plot ?
 
73-Graham
G0NBD
 
 
 

Markus (DF6NM)
 
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