Message-ID: <3D53C21E.83A9554@usa.net>
Date: Fri, 09 Aug 2002 15:22:38 +0200
From: "Alberto di Bene" <dibene@usa.net>
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To: rsgb_lf_group@blacksheep.org
Subject: LF: Re: G4JNT and SM6LKM Jason beacons
References: <17chZQ-13Oc8eC@fwd05.sul.t-online.com>	 <5.1.0.14.0.20020808182124.00b0a1b0@gemini.herts.ac.uk>	 <005701c23f26$3e24c720$162565d5@oemcomputer>	 <17cwaH-07Lv5UC@fwd11.sul.t-online.com>	 <5.1.0.14.2.20020809073006.009f6690@mail.pncl.co.uk> <5.1.0.14.0.20020809115706.00a8a048@gemini.herts.ac.uk>
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James Moritz wrote:

> [...]
> Under these conditions, the IF signal was quite
> severely clipped all the time (see the attachment). The audio sounded very
> distorted of course. Backing off the RF gain so that little or no clipping
> occurred made the signal unreadable by Jason, and almost invisible on a
> waterfall display - whilst with the "Jimi Hendrix style" gain settings, the
> waterfall was 'O', and Jason could copy perfectly - the difference was very
> striking and counter-intuitive.
> [...]
> This is the opposite to the optimum
> setting required for receiving QRSS when there are multiple signals in the
> passband; in this case the gain must be reduced until no clipping occurs,
> otherwise "blocking" affects the weaker signals.

Hmmm, maybe the explanation is that with QRSS the ultimate decoder is
the combination eye-brain, which is affected by the amount of visual noise
in the waterfall. And, in addition, when computing the waterfall brightness
from the magnitude of the received signals, the scale factor of the transformation
must be adapted to the mean magnitude, and strong signals tend to dominate,
masking weaker ones.
With Jason, everything that falls outside the decoding window
(the two yellow lines) is plainly discarded. So, if the Jimi Hendrix style of
reception has the effect of bringing up the signal, even if this causes severe
clipping of noise bursts, the bottom line should be on the positive side.
I am fairly sure that the sigma-delta ADCs of the sound cards work in
saturation mode and do not wrap around, so increasing the input level is
beneficial (up to a point, of course).
And, from a software-decoding point of view, the SNR which plays
the definitive role is that computed in a bandwidth equal to the FFT bin size.

Re Johan beacon, nothing received so far, sorry (but couldn't you turn your
dipole E-W ?  :-)

73  Alberto  I2PHD