Joined: Tue Aug 01, 2023 7:40 pm
Posts: 50
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For possibly the very first and only time since ive been hunting signals in the scanner/radio/SDR related of things, something odd appeared on that journey in Chicago, IL on 454.475 MHz.
On top of Oglive transportation center, we have a one of a kind site to behold, and that is...
PSK modulated POCSAG, multi-center audio carrier frequencies, in one signal. This means constant switching on and off at busy times during the day.
Currently, there are 3 different carriers, feeding into the same signal, 1625, 1950 and 2200 Hz.
Another interesting thing to note here, the preambles are halved in frequency to give a look as if the listener was to receive a legit 1200 bits per second signal (which they are, in *data terms*, after the preamble was processed), when the actual preamble was halved in frequency. Why is that?
Upon further analysis with using the same exact program, audacity, I managed to recreate and solve the amount of confusion coming from this one of a kind signal.
With the POCSAG encoder, you will need to just keep the bitstream, not the preambles.
For a 1200 bits per second signal, you will need a 300 hz carrier.
For a 2400 bits per second carrier, you will need a 600 hz carrier.
I dont know why, but you will need this.
Next, apply a center carrier throughout your whole data stream. The example I did here was with 1625 hz center carrier.
HOWEVER:
The most important things to add here is a 3000 hz low pass filter of at least -64 dB. + a 600 hz notch filter with 0.9q. Why a notch filter? This gives the resulting distinction of a v.27ter waveform. And although there is some chunk of spectrum left over from 0 -600 hz, it must be kept to keep it original and on-par.
I would also amplify it down -1 to -3 dB in the amplify tool.
This can also be achieved with 1950 and 2200. 2200 is a bit high, so you will need to mess around with the lpf a bit more than usual, and the notch as well. But the most important thing for the notch, keep it at 600 hz.
Here is an example I once again generated with aduacity to 16 Khz IQ to VB audio cable: I also added a bell 202 AFSK modulation for conivience because that signal also uses it too. I used 1625 Hz center for the "PSK" here.
  and a sound file to match this: https://drive.google.com/file/d/1lYVO6JCZYEiwtFtgTUFdVA2O_51mhWqe/view?usp=sharing
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