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| Subject: | LF: False Decode or Real? - Opera vs opds |
| From: | Markus Vester <[email protected]> |
| Date: | Sat, 23 May 2015 14:23:31 -0400 |
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Hi Graham, OPDS makes use of Wolfs some what excellent spectrum software as DSPSpectrum 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.txtIn 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 2Opera 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 regularlyfills 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 criteriaTo 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 falsedetections 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 tablehence 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 arecriteria , 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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