Waterfall Display

[K5KNT]

I'm looking for resources on how to read/use a waterfall display. Google search hasn't been very helpful or I'm just not using the correct terms. Does anyone here have some advise or care to point me to some references? Preferably not too technical.

Thanks.

[Token]

A waterfall display in what application?  Are we talking an audio based waterfall of the detected signal or are we talking something wider banded, such as on the IF of a traditional receiver or as is commonly found with many hobby SDRs?

Assuming the waterfall flows down the page (shouldn’t all waterfalls?  But, all do not, some are sideways or even flow up the page) then down the page is a history.  The newest data/information/signals are at the top of the display and the oldest are at the bottom.  How old depends on the flow rate.

Most commonly lower frequencies are to the left and higher to the right.

Intensity or signal strength is often in the Z axis.

Assuming the waterfall flows down, the lower freqs are to the left, and the intensity is in the Z-axis:

At a glance then you can tell several things about any signal on the display.  First you can tell when it happened, several seconds ago, now, etc.   This is seen in the vertical axis.  You can also tell if signals across the bandwidth displayed are synchronized or related in time.

Next you can tell frequency of the signal (or many signals at one time), because of the left to right (horizontal) relationship of the bandwidth displayed.  Remaining in the horizontal axis you can also tell the bandwidth of the signal.

And finally you can tell the strength of the signal.  Quite often “brighter” is stronger, but the color palette can be set up almost any way you want, you could have dark be strong if you so desire.

With relatively little exposure you can tell signal mode at a glance also.  AM signals can be easily differentiated form SSB.  USB and LSB can each be visually identified.  CW signals are clearly identifiable.  More complex signals, such as various digital modes, can often be visually identified to specifics of the signal, such as the shift of an FSK signal.  On much more complex signals, like radars, the modulation techniques used and basic performance capabilities can sometimes be quickly approximated by the visual representation.

Waterfalls are a completely different way to deal with signals.  If you have never used one it is like opening your eyes for the first time (in my opinion) and if you have used one going to a situation without a waterfall is like losing a dimension or a sense.

Then again, I have been using waterfalls for over 35 years, so I might be a little biased or jaded.  It is, and has been for a long time, second nature now.

T!

[skeezix]

I find the waterfalls invaluable. Only been using them for a year and a half or so, but provides so much information as Token mentioned. And using it became second nature for me after about a week or two (same with the spectrum display).

If you're looking for a weak station, can pick that out even if you can't hear it. Then can sit & wait to see if propagation improves and then can listen to it. Otherwise, may never have noticed it and skipped right by it.

I used it the other night specifically to check for a station. The Mighty KBC was testing on 5955 kHz at 0900-1000Z. I knew there was a very, very slim chance that I would hear it, but since I happened to be up anyway (  ), gave it a try. Went to the SDR and looked at the waterfall for about 20 seconds and didn't see even a hint of a carrier. That was enough confirmation that no way was 5955 coming over here at that hour, so went back to bed (after confirming on the Twente WebSDR that they were transmitting, which they were). 

Listen to what you hear and compare with what you see. With Token's suggestions and using it, you'll figure it out in no time. 

Some applications also have a live spectrum display. I use one that has both visible at the same time and trying to find signals with the two displays is exactly like shooting fish in a barrel, especially ones that aren't constantly on.

[K5KNT]

skeezlx and Token: Thank you for the replies they were very helpful.
Token, I have answers to your questions and some more of my own interspaced below.

A waterfall display in what application?  Are we talking an audio based waterfall of the detected signal or are we talking something wider banded, such as on the IF of a traditional receiver or as is commonly found with many hobby SDRs?

I was thinking generally, but more specifically I am trying to get Gqrx running on my iMac and on an Eeepc with Ubuntu, so Gqrx for specifics.

Intensity or signal strength is often in the Z axis.

What exactly is the Z axis?

With relatively little exposure you can tell signal mode at a glance also.  AM signals can be easily differentiated form SSB.  USB and LSB can each be visually identified.  CW signals are clearly identifiable.  More complex signals, such as various digital modes, can often be visually identified to specifics of the signal, such as the shift of an FSK signal.  On much more complex signals, like radars, the modulation techniques used and basic performance capabilities can sometimes be quickly approximated by the visual representation.

This, I think, is getting to what I was really trying to ask. Exactly how does one tell the difference and are there images of them somewhere on the web?  With what little I've experimented, I haven't been able to tell the difference visually. For example I have seen some solid lines and when clicking on them and going though the various modes (AM, FM, USB, LSB) I will finally hear a tone, usually on either of the side bands but I couldn't tell the difference just by looking.

Since I am currently limited to one of the VHF/UHF USB dongles and the supplied antenna haven't been able to locate AM signals. I would like to see what some of the data signals look like. I know that some of the local hams are using APRS, so that shouldn't be too hard to find.

Waterfalls are a completely different way to deal with signals.  If you have never used one it is like opening your eyes for the first time (in my opinion) and if you have used one going to a situation without a waterfall is like losing a dimension or a sense.

I would agree and I'm looking forward to learning more and getting an opportunity to use them more.

Thanks again,

Kent

[cmradio]

What exactly is the Z axis?

Intensity.

Take a look at Chris' image here:
http://www.hfunderground.com/board/index.php/topic,10382

Blue is lowest intensity. Ramping up to red as highest intensity (strength).

However, most programs offer other display options. For extremely complex signal study, often an inverted B&W display is preferred (as in studying buzzsaw RTTY patterns).

Peace!

[K5KNT]

Thanks. Things are beginning to make some sense now. Using the same image, would the line to the left of 6840 be a carrier signal?

[skeezix]

Thanks. Things are beginning to make some sense now. Using the same image, would the line to the left of 6840 be a carrier signal?

Looks like that could be a carrier.

A much better example- If you look to that mass of color to the right, 6875 which is WWCR, you can see the carrier in the middle. LSB to the left and USB to the right.

The yellow mess just to the left of 6900 is some digital mode.

At 6885, what looks like a lone carrier is Galei Zahal from Israel. They come in many nights (if you're lucky enough to hear them, I can only see a carrier, but hear no audio being in MN.  Those on the east coast can hear them). Can just barely see some audio in the USB (to the right).

This is not a great waterfall image as there's interference from something (HAARP?).

[skeezix]

skeezlx and Token: Thank you for the replies they were very helpful.

Since I am currently limited to one of the VHF/UHF USB dongles and the supplied antenna haven't been able to locate AM signals. I would like to see what some of the data signals look like. I know that some of the local hams are using APRS, so that shouldn't be too hard to find.

On VHF/UHF/800, its usually FM or digital. You can find AM on 118-137 MHz for aircraft (and 108-118, but those are navigation aids and unless you're near one of them, probably not worth worrying about).

With the amateurs, it can be a grab bag of FM, AM, SSB, digital, depending on your location and their whimsy.


The Z axis is the third axis, that is in/out of your screen. Since that's not practical, its represented in different colors and/or intensities.

[Token]

OK, I grabbed some basic examples.  I have been meaning to add some to my pictures online anyway, so this is a good time.  Will start with the basics and maybe move on from there.

Keep in mind that your specific settings of waterfall rate, displayed bandwidth, palette selection, intensity and amplitude scale, etc, will all change the look of the signal.  This is just a general example of how the signal looks, but with some setting you can make it look very different.

First will be AM (Amplitude Modulation), full double sideband plus carrier, this is the most common type of signal called "AM" but that term can actually be applied to several different common modes.  In this case the first picture is an AM broadcast station.  The second picture is WWV on 10000 kHz.

With AM the first thing you see is the carrier in the middle of the data.  The next thing to notice is that the data is presented twice actually.  On the left and right of the carrier is the data/information/audio, and they will be mirror images of each other.  These are called the sidebands of the signal (not to be confused with SSB, Single Side Band).  And yes, they each should contain the exact same data.





T!

[Token]

This next example will be SSB, Single Sideband, an image for each, USB (upper side band) and LSB (lower side band).

With SSB you notice that there is no carrier present.  SSB is realistically an AM signal with an unwanted sideband and the carrier stripped away.  In the case of USB you strip away the carrier and the lower sideband, for LSB you strip away the carrier and upper sideband.

The first picture is USB.  Note that the data (voice) seems to bunch up at the left side of the signal with a more even line to the left and a more ragged line to the right.  Left in this case is the lower frequency side.  If your waterfall reverses this, low frequency to the right, then the data would appear reversed.

The second image is LSB.  Now the data is reversed.  The information seems more bunched on the right side of the signal.  The right side is more “even” and the left side more ragged.

This right/left smoothing ONLY works for voice, and female voices can be less clear on the waterfall, as they often have a lot of high frequency components.  When digital signals are sent via SSB it is sometimes difficult to tell, visually, which mode they use.  If the mode is symmetrical, having no dominant data tones or lines, it might be impossible to tell visually which mode is being used.

USB signal:



LSB signal:


T!

[Token]

The last example for this evening will be CW, Morse Code.  More correctly this is actually ICW, Interrupted Carrier Wave, as pure CW would carry no data.  In the Hobby Morse on a keyed carrier is just called CW by most people.

Notice that there is just the carrier, and it is interrupted, turned on and off.  This makes the Dihs and Dahs of the Morse code.  Depending on the flow rate of your waterfall and the speed of the CW sent you might be able to make out each character as it is sent.

In the case of this picture there are 3 CW signals near the center of the waterfall.  Theya re unrelated.  Using the frequency scale across the top you can tell these 3 CW transmissions fall on 7042 kHz, 7045 kHz, and 7047.5 kHz.



I will grab some other examples and post them later if I remember, maybe some digital signals, FSK, FAX, radars, etc.

T!

[K5KNT]

skeezix and Tokin, thank you again for more great information. Maybe this could be put on the wiki?