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

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 <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
>
> ...
>
>
>