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Author Topic: Decoding Digital Signals - How do you ID the Signal Type?  (Read 416 times)

Offline ultravista

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Decoding Digital Signals - How do you ID the Signal Type?
« on: January 07, 2019, 0420 UTC »
What can I use to identify digital radio signals to determine the signal type (FSK, MFSK, PSK, PSK31, STANAG 4***), then the Mode (LSB/USB), and Bandwidth to determine whether or not it is encrypted?

Knowing the details of the signal is the first step in decoding.

The question is for signals found outside of the amateur radio bands.

Offline Josh

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Re: Decoding Digital Signals - How do you ID the Signal Type?
« Reply #1 on: January 07, 2019, 2218 UTC »
The first means to id a sig is the ear/brain. After you've heard them a few times you begin to know what mode is what and understand the main differences between the most commonly found types. These are fsk, mfsk, psk, ofdm, fax, etc.

You can also look at the logs submitted by members of groups like udxf, hfunderpants, etc to see what, if anything, has been noted previously on the frequency in question.

If you have a lot of money (or are familiar with torrents, cough) you might obtain software decoders such as wcode, k500, hoka, etc, and these often have a routine for determining signal type, rate, bw, etc. You can help these routines by pointing them in the right direction, such as using an fsk analyser on fsk sigs, a psk analyser on a psk signal etc. For example, wcode and hoka have routines for determining rate, sense, and language of a given S4285 signal, and will automatically start decoding once the parameters have been determined - that is, for a non encrypted signal. I suppose most people start out today with sorcerer, multipsk, and sigmira, and advance from there.

You can use an am demodulator to do some signal id work;
find a psk signal, form a bandpass filter around it to exclude noise, and look for a peak on the audio frequency scale, this will be the bitrate, ie 2400 in the case of a S4285 signal.

There are online resources to help, one is udxf, another is the sigidwiki, and there are others. This is one of my favs but be warned it can be math heavy;
http://i56578-swl.blogspot.com/

There are all sorts of books online and at libraries on signals analysis and so on and so forth, employ your google fu.

http://www.kd0cq.com/2013/07/sorcerer-decoder-download/
http://f6cte.free.fr/index_anglais.htm
http://www.saharlow.com/technology/sigmira/
« Last Edit: January 07, 2019, 2222 UTC by Josh »
Conveniently located near Vincennes Indiana.

Offline ultravista

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Re: Decoding Digital Signals - How do you ID the Signal Type?
« Reply #2 on: January 08, 2019, 0251 UTC »
Josh, thanks for the thorough reply.

With my SDR and active mini-whip antenna here the SouthWest, I routinely and consistently can find STANAG and what I call FSK like signals with the 'twin towers' or left and right peaks. On any given day, I can find at least 20-30 similar signals and I would like to at least start somewhere.

With FLDIGI, MultiPSK, or Sorcer, I don't know where to start.

Decoding FAX and ACARS has been easy. The rest, not so much.

If you don't mind my asking ... which software do you use for decoding?
« Last Edit: January 08, 2019, 0403 UTC by ultravista »

Offline Strange Beacons

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Re: Decoding Digital Signals - How do you ID the Signal Type?
« Reply #3 on: January 08, 2019, 0430 UTC »
Josh mentions using the ear/brain, but with modern SDR technology, you can also use your eyes.

I've been monitoring digital signals now for the past ten years. I've gotten to where I can identify signals just by hearing what they sound like. But seeing them on a waterfall is also a great help.

I use FLDIGI and sometimes it is a bit of work determining what mode a signal is, and then just trying different settings on FLDIGI until you start to get a decode (which is only possible if the signal is not encrypted, I should add).

Offline Josh

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Re: Decoding Digital Signals - How do you ID the Signal Type?
« Reply #4 on: January 08, 2019, 1521 UTC »
Seeing a fft of a given signal can tell you a lot, as well as a waterfall image. Recently someone here posted about a unid signal and suggested it was hfdl, but hfdl has a doppler tone only at the beginning, not at the end, and this sig had a tone at each end. Also, the image kinda clearly depicted mfsk rather than psk as the waveform. So yes the eyes can also help nail down what signal we're trying to id.

As to what decoders are in use here, K500, Sorcerer, WCODE, Centurion, and a mess of HAM digital signal apps. I almost always tell people to start out with Sorcerer for ease of use and performance compared to other popular decoders, but use whatever you want/can get with no hard feelings from me.

It was a lot harder back in the days when only the most 1337 (elite) had a panadaptor that could give you a fft image of a given sig, let alone a waterfall display. Back when I had to walk uphill to school, both ways, I only had a lowly Icom R70, and infotec M600, and a monochrome crt, you had to tune around a sig a bit to get the leds on the ends of the tuning bar of the M600 to synch with the signal, then try various baud rates, framing, and character sets and hope for the best. Things were much less automated than things are today. Going from a Hallicrafters 110 that drifted if the ac or furnace came on directly to a r70 was pretty much a revelation, the only thing to impress me more was the advent of sdr rigs and of course software decoders.

:D


https://www.universal-radio.com/CATALOG/commrxvr/r70.html
https://www.universal-radio.com/used/sold057.html
« Last Edit: January 08, 2019, 1536 UTC by Josh »
Conveniently located near Vincennes Indiana.