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Date: Wed, 01 Feb 2012 09:14:55 +0000
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Subject: Re: LF: Re: Opera v qrs evaluation
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Hi Jim

Was -31dB the limit of decoding for OPERA, ie could you take QRS3 to 
below copy ability yet get decodes from OPERA even if not 100% of the 
time. I am still searching for the reason for never a report on my QRS3 
on 500kHz or even a QRZ to the LF Group.

I couldn't accept increasing dot length to 4s FANT0M takes nearly 4 
minutes and represent the same bits as some call signs, however OPERA 
can of course send any possible call sign YQ0YQY would take  3sec short 
of 5mins so using QRS3 is stretching it already.

I see you are about, RX'ing, did you want to do more tests, would you 
like the long RX pause shortened for testing?

73 Eddie

On 31/01/2012 22:51, James Moritz wrote:
> Dear Eddie, LF Group,
>
> I did a rough and ready comparative test on the "sensitivity" of QRSS3 
> and Op4 using your back-to-back transmissions. For 500kHz reception, 
> broadband noise from the broadcast stations just east of here is being 
> nulled out using a loop oriented N-S. Rotating the loop out of the 
> null position gives a convenient way of adjusting the SNR on Eddie's 
> signal. So I increased the noise level until I judged Eddie's QRSS was 
> just fully readable (using 0.3Hz FFT resolution), then left everything 
> in the same position for 4 transmissions, during which signal and 
> noise levels stayed nearly constant (see the attachment). Opera 
> reported an SNR of -31dB on Eddie's Op4 signal for all the transmissions.
>
> So, from what Graham said, Op4 may have a small margin in SNR with 
> these conditions. You could argue about what constitutes "readable" 
> QRSS, but there can't be more than a few dB difference between this 
> signal and something indecipherable without prior knowledge. It takes 
> 4 minutes to send a callsign using Op4; you could increase the dot 
> length perhaps to 4s  and transmit most callsigns in 4 minutes, which 
> would gain you about 1.2dB. But for practical purposes I think, in 
> this test anyway, the two modes are approximately equivalent in their 
> efficiency in sending callsigns.
>
> Cheers, Jim Moritz
> 73 de M0BMU