Re: LF: DJ8WX 8.97 kHz

["Markus Vester" <markusvester@aol.com>]

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

To: rsgb_lf_group@blacksheep.org

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

 

 

 

From: Markus Vester

Sent: Saturday, March 23, 2013 12:15 PM

To: rsgb_lf_group@blacksheep.org

Subject: LF: DJ8WX 8.97 kHz

Hi Uwe,

 

your 8970 Hz signal is widely visible across Europe

 

http://abelian.org/vlf/fbins.shtml#p=1364032800&b=070&s=sp

http://www.qsl.net/pa1sdb/

http://www.ok2bvg.cz/vlfgrabber/

http://www.iup.uni-heidelberg.de/schaefer_vlf/DK7FC_VLF_Grabber.html

http://dl.dropbox.com/u/23312120/vlfgrabber.html

 

and now also good traces here

http://www.mydarc.de/df6nm/vlf/vlfgrabber.htm

 

Best 73,

Markus (DF6NM)