Message-ID: <3D4257FC.CDC26C4D@usa.net>
Date: Sat, 27 Jul 2002 10:21:16 +0200
From: "Alberto di Bene" <dibene@usa.net>
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To: rsgb_lf_group@blacksheep.org
Subject: LF: Re: Jason 0.94 tests
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jannsen wrote:

> Hi Jim, Alberto es all
>
> my report see attachment. conspicuous the spurii during the switchovers from
> tone to tone. has Alberto an explanation ?

Hello Uwe and the group,

  yes, there is an explanation.
The waterfall window has a strong visual AGC applied, so to put into greater
evidence the tone received. The AGC gain is computed only taking into
consideration what falls inside the two yellow lines.

Now picture into your mind the FFT buffer as a long glass tube, where coloured
water enters from one side, flows all along the tube and exits from the other
side. The water can have different colours, which correspond to the frequency
of the incoming signals. Suppose a single frequency is arriving, corresponding to
a red water colour. This is the predominant colour, your eye sees only it, unconsciously
discarding spurious strikes of other colours. Now the tone changes. While the
red water still flows towards the exit end of the tube, from the other end blue
water is coming. Until the tube is full of only blue water, there will be a changing
mixture of both colours in the tube, red slowly decreasing and blue increasing.
In this condition, your eye is no more dominated by a single chromatic sensation,
and you see the red, the blue and the other strikes. This also explains the
blurring that can be seen, lets' say, in Spectran when receiving a DFCW signal
and the tone changes.
In this analogy your eye is the equivalent of the FFT processing, which is presented
with the input buffer (the tube).

It's not a perfect example, many things could be criticized, but I do hope it can
explain a little what's happening.

73  Alberto  I2PHD