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VE2HAR > MT63 07.03.05 16:49l 201 Lines 6532 Bytes #-7502 (0) @ WW
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Subj: Re: [MT63] 20m Olivia experiment results
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Hi mark, and all others interested,
My comments here. There were times when the audio was below the
threshold of the ear (mine for what that is worth) to be able to
distinguish from the noise. My bandwidth was 1.8 Khz for all tests. We
did discuss over the air not being able to hear the audio signals. When
Dave reduced his power level to the 1 watt level, I actually could not
hear his audio for ALL of the test, which was at 4/125 setting. There
was just vissible difference within the "crosshairs" in the waterfall. I
would say that at these low levels, and with the slow responce of the
decoder, precise receiver tuning via a schedule would be needed. Not a
DX chasing mode like PSK31 can be. My obsevation is that this would be a
fantastic tool for ARES and RACES traffic handling where 100% copy is
needed.
Steve/WM5Z
Las Cruces, NM USA
Mark Miller wrote:
> Dave,
>
> Your results are interesting.
>
> At 12:46 AM 3/7/2005, you wrote:
> >Steve and I started by establishing communications in Olivia's default
> >mode of 32
> >tones within 1,000 Hz bandwidth. We both experienced 100 percent
> print at
> >both 50
> >W and 5 W, notably after we moved up 500Hz to avoid a packet station.
> >
> >Settings of 16 tones and 500 Hz produced the same results at both power
> >settings,
> >just with slightly slower print. At 50 W, Steve reported that I was
> S7 on
> >his meter,
> >while I sent him an S6. The noise level at my QTH was sitting right
> around
> >S3, and
> >when we went QRP, Steve's signals hovered between S3 and S4, but I never
> >lost a
> >character.
> >
> >Then we went to a setting of 32 tones in 250 Hz of bandwidth. What a
> >change! The
> >text, while running between 75 to 100 percent copy, slowed to a crawl at
> >7.81 baud
> >and 0.6 characters per second. The spacing of the tones was quite narrow.
>
> This is interesting. I wonder if either station was able to reduce
> receiver bandwidth to 1000Hz, 500Hz, and 250 Hz? There is a penalty for
> increasing the number of tones, but that can be offset by reducing the
> bandwidth thus increasing the S/N ratio. The other affect with Olivia is
> increasing the interleaving depth, which seems to have had little affect
> during your test. I would guess that the QSB was not too bad (boy talk
> about relative terms). Another way of saying this is that when your fade
> margin is higher, increasing the number of tones has little constructive
> affect.
>
>
> >Next we went to 8/250 and the print was much better, near perfect at
> both
> >at 50 and
> >5 W. As a tuning aid, I was sampling the audio out of the back of the
> 706
> >and feeding
> >that to a old laptop running DigiPan, so I could use the panoramic
> >display. The
> >difference between 32 tones and 8 tones in the same bandwidth was
> visually
> >appreciable.
>
> I don't think interleaving was helping you that much, and I suspect the
> receiver bandwidth did not change, so you were paying less penalty for
> the
> number of tones.
>
> >Much wider tones made for a more robust contact. While a tad slower
> >throughput than with the default 32/1000 (2.4 cps at 32/1000 vs. 1.5 cps
> >at 8/250),
> >we were having a solid QSO and taking up one-quarter of the bandwidth,
> >which all
> >digital operators can appreciate.
>
> I think you are saying that the tone spacing was wider, as the tone
> duration is a function of baud rate.
>
> >Then Steve suggested we go for broke and set up at 4 tones and 125 Hz
> >bandwidth.
> >Surprisingly, this set-up was as good or better than 8/250. The baud
> rate
> >was the
> >same (31.25), although we lost one-third of the character rate, dropping
> >to 1.0 cps.
>
> This shows again that the interleaving depth was not a factor.
>
> >We decided to go to low power again, and at the end of Steve's
> >transmission, he
> >revealed he'd been at 200 mW, as low as the Mk V Field would go.
> Whoa! At
> >times his
> >signal faded out altogether, but on the receiving end, I only lost one
> >character.
>
> This is an example where interleaving depth and Viterbi decoding did help
>
> >The
> >software was actually copying below the noise!
>
> I doubt that it was copying below the noise. Even on an AGWN channel
> when
> the average noise and average signal are equal, 50 % of the time the
> signal
> is above the noise, and 50% of the time the signal is below the
> noise. If
> we spread out our bit errors by interleaving, and we have FEC, the
> protocol
> can "fill in the gaps" during the time that the signal falls below the
> noise. I hear this "below the noise" claim from PSKers all of the
> time. The average signal may be below the average noise, but there are
> times when that signal pops its head above the noise. Otherwise you
> would
> not copy.
>
> Very nice report Dave. It certainly gives us some things to think about.
>
> 73,
>
> Mark N5RFX
>
>
>
>
>
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