Waterfall and Spectrum display questions

[KC8QVOParticipant]

Hi All,

I just started looking in to SDR technology and have some questions on waterfall and spectrum displays.

First off – I am assuming all SDR type receivers can act as panadapter interfaces with traditional transceivers (provided that they either have an IF output, or pass-thru IF out/in, or modification is done to tap a board-level IF line)?

Second – and this is a bit more detailed, but hopefully you can follow along – what I want to accomplish is to get a high amount of flexibility in “tuning” a waterfall display that covers much wider bandwidth than an AF frequency range – so I can resolve even FM signal traces across a band – but allows for long-term recording and recall/analysis. For example – I am using WSJT-X software and the wide screen waterfall lets me tune the noise averaging of the waterfall and the smoothing affect of the spectrum display (if the display mode is “linear average” only). Specific to the waterfall display – it does a nice job of averaging “noise” at particular frequencies to where even inaudible, to the ear, signals show as a slight brightening of the trace. That is all well and good on a 50-200hz wide “signal” – but can that be done with a 25KHz wideband FM (normal ham FM) signal with a much wider overall spectrum width?

It looks like the RSP’s can display 10MHz of bandwidth so a 25KHz signal, or more importantly – many 25KHz signals across a narrower band (say, 2m from 146-148MHz = 2MHz “bandwidth” [or if you want to throw in the SSB/CW portion of the bottom – 144-148MHz = 4MHz, still well under the 10MHz spec?]). So from a hardware standpoint what I am after can be done. I think my main question here, then, is specific to software – the interface making use of the signals that the hardware is providing.

It is one thing to have high-speed, high-resolution spectral displays, but on the opposite side of the coin – is there software that will allow more flexibility in the processing and display of the spectrum and waterfall displays?

Just an example of what I’d like to do is watch over, say, 160 meters to pictorially show the drop in band noise as the sun sets and then the rise in band noise as the sun rises the next day – plus the band activity in between. I have no idea what kind of computing power it would take to do that, but I presume the “computing power” would be in processing the data to make each “row” of the waterfall display. Once it is created it is just a picture, correct? So then I guess how big of a file, say, 10 hours of “monitoring” would create may be a question. In the same 160m example – what I want to do is be able to set the recording in the evening then get up the next day, open the file, and just like scrolling through the list of topics in here, or any other forum – slide the bar down the page from the top down and see the band activity over the recording period. I presume the higher amount of noise averaging the shorter the file size would be, but maybe the more processor-intensive the ability to do it becomes? Say, if you watch 2MHz of bandwidth and the averaging algorithms run every 10 seconds vs the same bandwidth averaging every 1 second? It is the same amount of data over time, but the 10 second batch is 10 times the data in that session.

Will the software also allow for segmenting a waterfall file, as in add time gradients across the display at every 5 minutes or 10 minutes? Same for frequency across horizontally? Is there a mouse cursor display of frequency (x plane) and time (y plane)?

I’ve seen some 3D renderings on a small scale of RF-related things also. I suppose that it would be conceivable to create a 3D rendering of my above waterfall example on 160m – frequency = x, time = y, dB/strength/RSSI = z? Though, I’d think trying to display 10+ hours of data this way would take a ton of computing power unless there is a way to vastly compress the data?

Last question – is it possible to record RF spectrum? For example – if there are, in theory, 25 frequencies on FM that are in-use on 2m (simplex and repeaters) can that be recorded and then demodulated/converted to audio (or digital) later? I suppose that answer is yes because the heart of SDR technology, I believe, is an immediate A/D conversion so everything is digital. So I guess that means the limit to this is available resources (processing power, RAM, storage space to put it all)?

Anyway, I think it is interesting what can be seen with higher quality spectrum displays (waterfall showing historical spectrum or a more simple instant/average spectrum graph) and would like to experiment with some things. I may be asking a lot, or maybe my questions are actually very elementary given the power of SDR technology? In any event, just looking to learn here and add a new dynamic to my radio hobby.

Steve, KC8QVO

[G8BQRParticipant]

Hi Steve,

The bit below caught my eye, as I also use wsjtx.

“It is one thing to have high-speed, high-resolution spectral displays, but on the opposite side of the coin – is there software that will allow more flexibility in the processing and display of the spectrum and waterfall displays?

Just an example of what I’d like to do is watch over, say, 160 meters to pictorially show the drop in band noise as the sun sets and then the rise in band noise as the sun rises the next day – plus the band activity in between.”

The waterfall is a series of single scans of the spectrum placed one after the other. The rate and frequency of these lines affect the visual averaging of noise – a slow rate gives a more visible carrier, which doesn’t disappear from the screen so quickly. My display for 2 min long WSPR signals is just over an hour from top to bottom, and even -28 dB signals are just noticeable using high contrast. The downside is that short signals appear as spots, like random noise, even if a lot stronger.

SDRUno has a x100 Time scale which slows down the waterfall a lot, and time bars can be added across the screen at intervals, I use 15 sec, for FT8, (but the audio waterfall has only 10 sec & 20 sec bars – one has been missed) I forget where I found them, I blame age 🙁

As an initial experiment it may be worth seeing if the waterfall can be slowed down to a rate which gives about 25 hrs on the screen – possibly stack a couple of monitors one above the other 🙂 I’d guess a recording would be the size on disc of a 25 hr film ?

Hope this helps to go in the direction you want 🙂 Regards, Ed G8BQR

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