Downtown chicago's most unusual AFSK POCSAG frequency -> 454.475 MHz

[YalekW]

So Ive been scanning for a really long time, and ive encountered every single type of telemetry signal I can find, from v.22 modems to many forms AFSK, custom-made pager signals in europe (on the spyservers), QAM glenayres (including one right next to this one >> 454.425 with KNKH915 << not listed in the frequencies of that license, thanks FCC). So techincally you could call me a radiomodem phreak now.

But theres one that a I REFUSE to understand, and its in downtown chicago >> and we have on the sigidwiki named as "PSK paging link" (also known as PURC but thats not the right title).

The actual tones this thing uses use a distorted idle thats supposively not even idle, V.27ter modulations (the data is used in all the modes).

I have tried so hard to get a proper Phase plane out of this and the page is supposively describing 8PSK but I think its trying to use MSK. I need further confirmation though. It does use MDC1200 as its beginning and ending sequences.

It also does a Bell 202/AFSK PL every so often >> and thats decodable by hand. There is no morse ID, and there is instances where this transmitter does its MDC1200 sequences in a secret fashion, and then glitches the main Bell202/AFSK PL and "glitches" the main signal before ending. Data comes back when it has completed its "secret id".

Does ANYBODY from this area know what modulations its using, as it seems to be putting the POCSAG pulses as phases instead. I also had somebody put up a live stream of it on Icecast a year back.

You have to be in downtown chicago to be able to receive this though, sometimes no data transmits, or sometimes (maybe on new years like this date), theres nothing but absolutely non-stop POCSAG pages going through it >> THROUGH V.27ter sub-modes.

 

[YalekW]

i guess i missed something. LOL. But can anybody PLEASE tell me what this thing is modulating? I know it is POCSAG but under various modes but Im trying to get a further analysis about it. People tell me left and right "signal is distored" or other things. It uses all 3 bit speeds tho.

 

[mikegilbert]

Please post a recording so the group can solve the mystery!

 

[YalekW]

I have 2 but the first one is 2 minutes long. Here:

454475_1.mp3

drive.google.com

 

[RBMTS]

I don’t believe that is paging. It sounds like a radio data telemetry transmission. There is a MDC burst at the end of each TX.

 

[K9ID]

That sounds like Motorola DRC, used on radio links to control paging transmitters. What sounds like MDC are control commands to various remote sites, all listening to a distribution link. You can find an explanation here: TONE REMOTE CONTROL

 

[kf8yk]

454.425 with KNKH915 << not listed in the frequencies of that license, thanks FCC)

Here's the current license: WQMT747. Part 22 paging is exempt from ID requirements so sending an old or incorrect callsign is not a violation.

Sure sounds like a typical Motorola PURC paging link.

 

[K9ID]

When I worked for Rogers (Ward Rogers, Chicago, not the Canadian Rogers) in the '70s, 454.425 was a mobile phone channel, which we were using almost exclusively for paging.

 

[YalekW]

Here's the current license: WQMT747. Part 22 paging is exempt from ID requirements so sending an old or incorrect callsign is not a violation.

Sure sounds like a typical Motorola PURC paging link.

interesting...So its the the purc/PSK stuff? My question is why isnt there other transmitters doing the same thing? Lol.

 

[K9ID]

The other transmitters would not sound the same. This is a link, sending modem tones as audio to sites which will take the modem's digital output and digitally modulate the transmitter, which is on a different frequency. That transmitter will sound like any other paging system.

 

[YalekW]

Well, acutally: I just discovered something even stranger with this:
I simulated this with manual modulation methods in audacity (as in, it took some DSP steps to form these out), For their "other waveforms", they are not using PSK and FSK standalone. They combined them together to fool *me* and possibly all the others who live there and listen to that frequency daily.
What did I do to do make this waveform discovery a thing now?

For your *idles*:
1. make a 37.5 hz square wave stereo track at the max amp, and make two stereo tracks with it.
2. Use a 1650 hz center audio carrier, modulate a 330 hz frequency deviation to the first audio track. (this will make that ringing noise).
3. For the second audio track, you will need a 900 hz PSK modulated waveform, (this is easily done by using the nyquist prompt to multiply an oscillator to a 900 hz audio wave, no frequency shifts)..
4. While still keeping these at max, combine them together. You will get distortion (thats normal and ok). Amplify it down -10dB. and add a 600 hz Notch filter (and this applies to all waveforms EXCEPT the bell 202 they use).
5. Add a Low pass filter of 3000 hz with a rolloff range of -48dB.
Theres your idle. Their second (or "ending") idle used 901 Hz. Which is why it has a slight wobble to it.

for *the data*:
You will need to use 3 distinct audio carriers separately for each data mode, both via PSK and AFSK modes as you combine them together in the same fashion as above:
1650 hz, 1980 hz, and 2250 hz. Be careful with 2250 because that may require a wider span of AFSK modulation (perhaps with a 900 hz deviation). If you want a similar but not as higher frequency center carrier, use 2160 hz. That is fine too. I would use 648 Hz deviation for the AFSK. The big difference when you go to modulate the PSK sections, for 1650 Hz, USE 1650 Hz PSK center!! The same also applies to the other center audio carriers. You will also need to use the same 600 hz notch filter and the smae 3000 hz LPF cutoff with a -48db cutoff range in amp.
Here are the results in this zipped folder (audio clips) I was able to carry out when I did this: (I was going to use 1950 as that wouldve been a good copy for the actual frequency here...but decided to use 1980 as an example here

 

[mikegilbert]

Have you tried scanning the 900mhz band to see if any pages are coming out that coincide with the UHF frequency?