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Re: LF: False Decode or Real? - Opera vs opds

To: [email protected]
Subject: Re: LF: False Decode or Real? - Opera vs opds
From: Steinar Aanesland <[email protected]>
Date: Sat, 23 May 2015 23:06:06 +0200
In-reply-to: <[email protected]>
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Markus, he is "religious". Don't bother. It is not worth it.

LA5VNA S


On 23.05.2015 20:23, Markus Vester wrote:
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 <[email protected]>
An: rsgb_lf_group <[email protected]>
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

...




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