Synthesizing RF with a RPI GPIO Pin !

So neat! So far all I know is that GPIO 4 is configured to output a clock signal

I believe the software claims an unused PLL and then DMA's control words into one of the config registers to frequency modulate the carrier.

There is also this github repo: https://github.com/F5OEO/librpitx which I guess is the low level implementation...

Thanks for expanding on this concept!

I saw your comment on the Tek forum and made a mental note to look into it later. (But then quickly forgot.)

Do you know if there is some underlying magic? Or are they (rpitx) bit banging the GPIO just right?

Well for FM, I believe they are just adjusting a PLL's output frequency by loading new divider values.

The output spectrum contains spurs and harmonics, so it's not a pure RF tone. So if one actually wanted to transmit into the air an appropriate bandpass filter would be needed. But for playing with test equipent or an SDR its no big deal.

How they create AM or SSB, does feel a bit like voodoo at this point. It would seem they have devised a method to go from IQ samples to a GPIO pin state, i'm not entirely sure what that involves, must be magic! 

An impressive amount of fun for just soldering a resistor to a GPIO pin!

Oh wow, that's clever!

Even more, wow, searching FT8 and Pi Zero comes up with a bunch of stuff. Like, this FT8-specific project. I had no idea people were doing anything like this with "just" the GPIO.

Gist failed?

void clkgpio::SetPhase(bool inversed)
{
	uint32_t StateBefore = clkgpio::gpioreg[GPCLK_CNTL];
	clkgpio::gpioreg[GPCLK_CNTL] = (0x5A << 24) | StateBefore | ((inversed ? 1 : 0) << 8) | 1 << 5;
	//clkgpio::gpioreg[GPCLK_CNTL_2] = (0x5A << 24) | StateBefore | ((inversed ? 1 : 0) << 8) | 1 << 5;
	clkgpio::gpioreg[GPCLK_CNTL] = (0x5A << 24) | StateBefore | ((inversed ? 1 : 0) << 8) | 0 << 5;
	//clkgpio::gpioreg[GPCLK_CNTL_2] = (0x5A << 24) | StateBefore | ((inversed ? 1 : 0) << 8) | 0 << 5;
}

Well that would take care of BPSK modulation 

Ah yes. I totally see how that is the case .......

For AM I believe you change the duty cycle of carrier, but don't quote me on that. I don't understand why they don't use the full FM bandwidth with the RPITX.
Other cool stuff that I've seen is that some have transmitted even colored NTSC signals. FPGAs are also pretty good at this because you can get very accurate pin control at very high speeds.

rpitx indeed is great, actually it is developed from pifm what was already out 10 years ago and developed by by Oliver Mattos and Oskar Weigl. I made a SSTV transmitter with pifm back in 2014:

 Raspberry Pi Slow Scan Television (SSTV) Camera 

and:

https://www.agri-vision.nl/portal/projects/7-raspberry-pi-sstv-camera

You got the gist format wrong. This was spitting out a ton of errors that we spotted in one of our tools, so I've fixed it

I also noticed the interface for the github gist with the instructions on how to use it has link that people outside of the company won't be able to access, so I've raised a ticket to make that interface easier to use and better documented

Opps, I'm sorry, I barely know how to use git. Thank you for researching the correct way, I will reference your example in the future.

PS I like how the mp3 files embedded showed up with an in page player, very cool!