On Mar 2, 2009, at 10:01 AM, Iain MacDonnell - N6ML wrote:
> Hmmm. I suppose one benefit could be that it shows some (short)
> history.
> If someone tried calling off-frequency, and you didn't get to RIT
> quickly enough (maybe you were working someone on the other radio),
> you
> can estimate how far off he was by looking at the traces in the
> waterfall.
On top of that, the eye is extremely good at correlating and picking
up "regular" structures in successive scan lines so you can see
signals way down into the noise.
I had also mentioned to the original poster by private email that if
done right, a waterfall can have a much larger "resolution" than a
spectrum. A spectrum is typically 100 pixels to 800 pixels tall.
False color together with 200 levels of intensity scale in a waterfall
can give you a much larger range of differentiation. But you do have
to choose the color transition carefully to take care of the RGB luma
variation that the eye perceives. Good waterfall color design is
obviously not a haphazard picking of colors, and unfortunately, it
also may not match people with color deficiencies. I have
occasionally been tempted to provide a waterfall preference for the op
to choose waterfall colors that are better for red-green or blue-
yellow type of color blindness.
If your program provides a way to replay data back, a waterfall is
indispensable. With a spectrum, not only wouldn't you know where to
click for the signal, you also cannot tell the program how much of the
past to replay back. With a waterfall type spectrogram display, the
horizontal axis is the frequency offset and the vertical axis is the
time offset into the past.
Spectrums are very useful, nevertheless, especially if you want
quantitative numbers. If you want to see how wide an RTTY signal is
spreading, you need a spectrum. With a waterfall, it is hard to tell
how many dB a keying sideband is down by, even if you do know the
color coding that was used. But for rapid (rabid?) tuning, I always
prefer the waterfall.
So to anyone implementing software modems -- provide both! :-)
73
Chen, W7AY
P.S. the first place I saw a waterfall use effectively in ham radio
was in Skip KH6TY's DigiPan program. That single program converted
PSK31 from a quaint mode into arguably the most popular digital mode
today. If you still have to tune PSK31 with a VFO knob, PSK31 would
never be were it is today. Can you even imagine tuning in an MFSK16
signal without the aid of a waterfall?
(The first place I encountered the use of a spectrogram (waterfall)
for weak signal detection is in the conference papers of NASA's
Project Cyclops back in 1971 or thereabouts. I am almost certain
though, that the technique was used much earlier for radar and sonar
work.)
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