LF: False Decode or Real? - Opera vs opds

[Markus Vester <markusvester@aol.com>]

Hi Graham,

> OPDS makes use of  Wolfs   some what excellent spectrum  software as 
DSP

Spectrum Lab is used as the frontend for opds, the only signal 
processing done there is a straight high-resolution FFT before 
exporting the data. Opds then searches for spectral peaks, and 
transforms a 1024 bin channel around each peak back to time domain (0.5 
Hz wide, 4x oversampled). For coherent signals, the central carrier is 
extracted (including possible drift and fading), and synchroneous 
demodulation is performed. Then the resulting real waveform is Fourier 
transformed again for a CPU-efficient crosscorrelation to each of the 
callsign templates.
> both  systems  reliably  produce   false detections  when  subjected 
to  noise ..

Opds-32 has been running on LF continuously, typically analyzing about 
20 QRM peaks every 10 minutes in a 115 Hz wide band. The searchlist has 
currently around 50 entries. With a correlation threshold of 15 dB, 28 
false detections have occured in 20 weeks since year 2015:
https://dl.dropboxusercontent.com/u/26404526/opds32.txt
In a pdf file distributed with Opera 1.5.7 software, the author EA5HVK 
mentions that he tested opds and got 11 false detections in only 4 
hours - an observation which is obviously not corroberated by my 
statistics.
> hence  ultra stable  TX and  RX can give  advantage
By using synchroneous demodulation rather than power detection, opds 
can detect coherent and stable signals that about 4 dB weaker. Attached 
are two plots, showing a side-by-side comparison of detection 
probability and SNR output from the Opera 1.5.6 decoder and the 
opds2h5c detector.
SpecLab's digimode terminal was used to generated coherent and 
perfectly timed Opera signals, and white noise from the test signal 
generator was added with variable power density (dB/Hz). The very same 
output was analyzed wihin SpecLab to feed opds, and played to the Opera 
software usingh VAC. To speed up the experiments, all testing was done 
at Op-05 speed (30 seconds), and SNR values were then scaled down by 24 
dB to Opera-32. During the test, no false detections were observed in 
the output from either program.
In the attached splot uccessrate.png, the solid lines with squares show 
detection probability (0 to 100%) against average SNR in 2.5 kHz. The 
classic Opera decoder (red) achieved 50 % detections at -40 dB. Opds 
correlation (blue) goes down to -49 dB, showing a 4 dB advantage for 
these ideal signals. The blue crosses indicate correlation dB output 
from opds - note that only hits above the standard 15 dB threshold 
(dashed line) were counted as successful detections.
SNR_output.png shows indicated SNR values versus actual SNR from both 
programs. Opera 1.5.6 seemed to consistently read 1 dB high, whereas 
opds reads approximately 1 or 2 dB low, with a larger scatter. Part of 
this negative offset is because I had originally assumed the "dBOp" 
scale to be referenced 4 dB (instead of 3 dB) below PEP.
I have not yet tested the dynamic deep search in Opera 1.5.7, but the 
claimed -45 dB threshold (ie. 5 dB better than the decoder) seems quite 
plausible. I believe opds is also around 4 dB less sensitive for 
non-coherent signals, which would then put both programs in same 
ballpark.
> Now mines a  pint  or  are we onto  shorts  now ?
Please explain...

All the best,
Markus (DF6NM)

-----Ursprüngliche Mitteilung-----
Von: Graham <g8fzk@g8fzk.fsnet.co.uk>
An: rsgb_lf_group <rsgb_lf_group@blacksheep.org>
Verschickt: Sa, 23 Mai 2015 4:48 pm
Betreff: LF: MF 630m: False Decode or Real?

 intelligent  life in other Galaxy's ?  Like the  Bar ? 11:14 . Suns 
not
over the  yard yet .. Tad early  Eddie ?

Armature radio V  Armature  hour , take 2

Opera  MF and LF  is a BOGOFF  mode , Buy  one  and  get one  free ,  
just
that  its free  to  uses and the first  is a data  mode  the   second  
mode
is  a correlation  system , dynamically  engaged ..  page 70  seems  to
miss  this  rather  important  fact along with  the  design  solutions
embodied  therein , I'm waiting  for  page 71 ..

The  test of a good design  is that no one  notices , it just works .

In  that  24  hour  window ,  Opera  LF produced no  false  data  
detections
or  false  dynamic  detections , where as  the  wspr  system regularly
fills the LF map , a simple  test of design , preventing  false  
correlation
detections  is more  difficult  than  false data .  Opera  LF is  -40 
dB
and  -45 dB  ... that  well  cool as  J C would (of) said


Now the  tacky bit

As Im sure  Markus  will  tell,  OPDS  and   Dynamic   share the  same
design criteria
To  drag  low  signals  out  of noise , by pattern  matching , OPDS 
makes
use of  Wolfs   some what excellent spectrum  software as DSP  , whilst 
Mr.
Ros  uses his  own  designs .  both  systems  reliably  produce   false
detections  when  subjected to  noise .. not all  the  time ,  just 
depends
on the  detection monkeys sense of humour on the  day.

The  design  solutions'  branch ,   OPDS  makes  accurate  frequency
measurement and  Bandwidth , along  with allowing  parameters to  be
adjusted by  the  user ,as  well  as  maintaining  the  look up  table
hence  ultra stable  TX and  RX can give  advantage .. And as  pointed  
out
, presents  the  user  with a  set of  parameters, which may be  used 
as
validation

Opera Dynamic retains the  Plug and  Play  house  style , yes these are
criteria , but  are evaluated by the  system  , load it  and  it dose 
the
rest ,  Opera  data  runs  as  normal ,  Opera  dynamic  is  engaged 
should
the  decoder  fail , the  sever handles the  validation  and  maintains 
the
list.

Both  systems OPDS and  DYNAMIC   produce  false  real  hits ,  Dynamic 
take
things one step on, where wspr  uses the  internet as  part of  the  
DSP
sync . Opera Dynamic  uses  the  internet  to  pool  ' recovered time'  
data
and  validate the  spots by  identity and  coincidence of  time ..

Now mines a  pint  or  are we onto  shorts  now ?

73-G

...

Success_rate.png
Description: PNG image

SNR_output.png
Description: PNG image