5 ms·

This is probably how it worked: https://www.rtl-sdr.com/transmitting-fm-am-ssb-sstv-and-fsq-.. https://www.rtl-sdr.com/transmitting-fm-am-ssb-sstv-and-fsq-....

This is probably how it worked: https://www.rtl-sdr.com/transmitting-fm-am-ssb-sstv-and-fsq-.. https://www.rtl-sdr.com/transmitting-fm-am-ssb-sstv-and-fsq-.... and how it also had unintended consequences. Note how many comments there are from people who don't seem to have understood the disclaimer. I'd bet our rogue dongle-jammer was in much the same boat :)

sigmar 8y ago
You're probably right that they used a guide like that. It seems irresponsible to publish that blog post. 99% of Pi tinkerers will read that disclaimer in the third paragraph and either not understand that life critical devices _do_ can be interfered with or think their use-case is special/low-powered and won't do any harm.
- riskable 8y ago
  The irresponsible part is that he says you "must use a filter" if you attach an antenna but does not describe what that would entail. There's not even a link anywhere that would describe how to make a filter or even what he's talking about ("I have a filter... There's no bad power going into this thing!") When you, "make something dangerous easy" you have a responsibility to be very specific in how to use your invention safely. Otherwise disaster seems inevitable.
  - maxerickson 8y ago
    You can buy razor blades almost everywhere for like a dollar. (Reviewing this comment later I see it can be read in sort of a nasty way; my meaning was that we don't really assign blame to providers of dangerous tools, the low price of razor blades reflects this)
argd678 8y ago
I see, so it was constantly broadcasting and then he’d encode some message in the signal to indicate if someone was up stairs, instead of only broadcasting when there was a message to send. That certainly would explain it.
- jjoonathan 8y ago
  That certainly exacerbated it, but arguably it's not even the worst RF sin being committed here. The other is spurious emissions. Toggling a GPIO at frequency f doesn't just result in frequency f, it also results in 3f, 5f, 7f, and so on because the GPIO is outputting a square wave not a sine wave. That's the best case. Unreliable timing could turn that from a frequency "comb" into a forest of spurs across the RF spectrum, and whichever spurs make it out of an antenna are going to break the law (best case) or interfere with important RF devices (worst case).
ourlordcaffeine 8y ago
So they are generating a signal by toggling the IO pin on and off. Not only will there be a huge amount of harmonics, wouldn't processor interrupts wreak havoc on the output signal and cause even more interference?
- deleted 8y ago
  [deleted]
- myself248 8y ago
  Yes and no; if they're doing it right, they're using the DMA or PLL engine which runs independent of the main CPU/ALU. See https://github.com/F5OEO/PiFmRds https://github.com/F5OEO/PiFmRds "This version modulates the PLL instead of the clock divider for superior signal purity. The harmonics are unaffected, so the legal warning still applies." Filtering this stuff isn't hard, but I'm realizing there aren't any good stone-simple tutorials for it. Might have to write one.
- tknkx 8y ago
  Can someone explain to me why generating a square wave creates lots of harmonics, as opposed to other kinds of waves?
  - davidgay 8y ago
    A square wave IS the base frequency plus ALL[1] the odd harmonics (https://en.wikipedia.org/wiki/Square_wave#Fourier_analysis https://en.wikipedia.org/wiki/Square_wave#Fourier_analysis explains it better than I could). [1]: Yes, so in reality you neither get an infinite set of harmonics, nor a perfect square wave.
  - gmueckl 8y ago
    The spectrum of a signal is its Fourier transform. Only sine and cosine signals have a "pure" signal with a single frequency. Anything else decomposes into a whole host of spectral contributions. Square waves are especially bad because they require contributions from infinitely many harmonics to produce the steep flanks. That is also why a clean square wave cannot exist: some kind of band limit is always present, resulting in ringing around the flanks. See also Fourier Analysis on the Wikipedia article: https://en.m.wikipedia.org/wiki/Square_wave https://en.m.wikipedia.org/wiki/Square_wave

sigmar 8y ago

You're probably right that they used a guide like that. It seems irresponsible to publish that blog post. 99% of Pi tinkerers will read that disclaimer in the third paragraph and either not understand that life critical devices _do_ can be interfered with or think their use-case is special/low-powered and won't do any harm.

riskable 8y ago
The irresponsible part is that he says you "must use a filter" if you attach an antenna but does not describe what that would entail. There's not even a link anywhere that would describe how to make a filter or even what he's talking about ("I have a filter... There's no bad power going into this thing!") When you, "make something dangerous easy" you have a responsibility to be very specific in how to use your invention safely. Otherwise disaster seems inevitable.
- maxerickson 8y ago
  You can buy razor blades almost everywhere for like a dollar. (Reviewing this comment later I see it can be read in sort of a nasty way; my meaning was that we don't really assign blame to providers of dangerous tools, the low price of razor blades reflects this)

argd678 8y ago

I see, so it was constantly broadcasting and then he’d encode some message in the signal to indicate if someone was up stairs, instead of only broadcasting when there was a message to send. That certainly would explain it.

jjoonathan 8y ago
That certainly exacerbated it, but arguably it's not even the worst RF sin being committed here. The other is spurious emissions. Toggling a GPIO at frequency f doesn't just result in frequency f, it also results in 3f, 5f, 7f, and so on because the GPIO is outputting a square wave not a sine wave. That's the best case. Unreliable timing could turn that from a frequency "comb" into a forest of spurs across the RF spectrum, and whichever spurs make it out of an antenna are going to break the law (best case) or interfere with important RF devices (worst case).

ourlordcaffeine 8y ago

So they are generating a signal by toggling the IO pin on and off. Not only will there be a huge amount of harmonics, wouldn't processor interrupts wreak havoc on the output signal and cause even more interference?

deleted 8y ago
[deleted]
myself248 8y ago
Yes and no; if they're doing it right, they're using the DMA or PLL engine which runs independent of the main CPU/ALU. See https://github.com/F5OEO/PiFmRds https://github.com/F5OEO/PiFmRds "This version modulates the PLL instead of the clock divider for superior signal purity. The harmonics are unaffected, so the legal warning still applies." Filtering this stuff isn't hard, but I'm realizing there aren't any good stone-simple tutorials for it. Might have to write one.
tknkx 8y ago
Can someone explain to me why generating a square wave creates lots of harmonics, as opposed to other kinds of waves?
- davidgay 8y ago
  A square wave IS the base frequency plus ALL[1] the odd harmonics (https://en.wikipedia.org/wiki/Square_wave#Fourier_analysis https://en.wikipedia.org/wiki/Square_wave#Fourier_analysis explains it better than I could). [1]: Yes, so in reality you neither get an infinite set of harmonics, nor a perfect square wave.
- gmueckl 8y ago
  The spectrum of a signal is its Fourier transform. Only sine and cosine signals have a "pure" signal with a single frequency. Anything else decomposes into a whole host of spectral contributions. Square waves are especially bad because they require contributions from infinitely many harmonics to produce the steep flanks. That is also why a clean square wave cannot exist: some kind of band limit is always present, resulting in ringing around the flanks. See also Fourier Analysis on the Wikipedia article: https://en.m.wikipedia.org/wiki/Square_wave https://en.m.wikipedia.org/wiki/Square_wave