FWIW I tried an alternate FM demod approach - 4 PLLs one at each tone -
but the tones turned out to be not spaced far enough apart for that to
work well. I'd didn't bother with the 4 bandpass filter, non-coherent
approach that Marcus mentioned, since 4 coherent PLL's didn't work out.
Stick with either the FM detector or Quad Demod.
Thanks! I was going to look into giving that a try.
Not crazy; EMI is a real thing in UHF.
The source can also be PC DRAM clocks, PC monitors, etc.
Funny side note: my directional TV antenna gets lots of noise at least
once a day when it points at the galactic disc. Apparently the Milky
Way is a large source of UHF noise.
Well, there was definitely a lot of noise! and I am pretty sure the source was my computer. I got a couple of USB extension cables so I could get away from the computer and the USB hub and that helped a lot. I was able to noise floor by about 20db.
Almost every transmission is coming in clear now. Oddly enough though when I do a debug recording, it still doesn’t give me the 4 distinct levels. Any thoughts on what could be up?
Here is an example of the new and improved reception:
Here is the audio:
I should have added at the start that this is digital audio coming from a Moto SmartNet system. It is supposed to be using APCO P25 for digital, but this is Moto and they could have addd some weird proprietary stuff to it.
It was fun. Down where I live they've just upgraded to APCO P25, but
its all new and all Phase II, so no C4FM signals for me. :(
Ironically, the same thing will probably happen for me too. This system is SmartNet and will be upgraded in the next couple of years. I just hope to get reliable, clean decodes before it goes offline…. :)
- Luke
