> You may say "But nyquist says you can't transmit or receive frequencies at more than 1/2 the sample rate (40 MHz in this case). To a degree that is true. Some people thought it may be overtones, but what happens in reality something stranger happens. Everything you transmit is actually mirrored around 1/2 the sample rate (40 MHz).
For anyone interested this is called aliasing.
If you've ever noticed wheels going backwards in racing or train footage, this is exactly what's happening: the wheel's angular velocity is above half the sampling rate (i.e. the camera's FPS), producing a mirrored version which in turn appears to rotate backwards.
Neat. Note that this technique will work on any system that has a high-speed I2S bus that can be operated by DMA, which these days is quite a lot of microcontrollers.
It's a kind of reverse "TEMPEST": rather than detecting a computer's accidentally radiated emissions, deliberately radiate emissions in a particular pattern to produce a radio signal.
I see the value of having micropython but not of 3Dtv. Not taking anything away from 3Dtv. It should not be used as a reason to increase fundraising in an already complicated project. This is a comment on managing people's (backers) expectations and not on the organizers abilities or ethics.
A whole lot of testing and functionality. I've used micropython on a few of my esp boards, and while it works, it is exceedingly buggy. I write most of my apps for esp boards in the Arduino IDE in C, which is totally fine, but it is easier to prototype in Python, so I backed it.
Would be cool to see an spectrum plot of the output signal.. the explanation of why the signal is picked up seems a bit too vague for me.. Since the transmitted signal will be a square wave I imagine, I would expect to see some side-bands that are pretty significant above and below the 80MHz.. And then the 61MHz signal that the TV would normally receive should be actually received using some local oscillator that I would guess is down around like 10MHz or something.. so would love to get a more detailed explanation on this stuff if anybody knows what actually is happening beyond "some people think its blah, but really its blah!! huzzah!" type of explanation..
First, a 80MHz square wave does not have side bands that are significant below 80MHz, only above! And if it's configured for alternating 0/1 bits, a serial port at 80MBaud creates a square wave of exactly 40 MHz.
Furthermore cnlohr doesn't use a fixed square-wave but a variable sequence of pulses. And a single pulse of (1/80Mhz)=12.5ns will have a sinc spectrum. sinc f=(sin f)/f. For an infinitely sharp-edged pulse, this sinc-spectrum basically extends to infinity, for real-world pulses you can expect to see a few harmonics.
cnlohr basically uses the synchronous serial i2s interface, running at 80Mbits/s as a 1bit DAC with a zero-order hold (stretching out a constant voltage over the bit time), so this DAC will recreate anything between 0..40MHz correctly (within the capabilities of its pretty poor quantisation noise and within the first lobe of the sinc-spectrum).
But, any signal you recreate will, because it's made up of the single pulses with the sinc spectrum, not only have energy between 0...+Fs/2 but also between N-1/2 and N+1/2 times the sampling frequency. There's a obvious trick involving the fact that convolution in the time domain corresponds to multiplication in the frequency domain, but unfortunately I can't really recall it correctly now... so you have to just take my word for it ;)
And that's why, if this "DAC" outputs 1MHz, you also get 79 and 81.
Conversely, when you try to output a sharp pulse on your soundcard, e.g. an audiofile consisting of {zeroes, one sample of 32000, zeroes...}, the output to your headphones will be something like a sinc pulse. The Fourier transform of this since pulse in the time domain will be a box function from 0..half the sample rate in the frequency domain, and hence no higher harmonics will be output.
Ah ok, this is great thanks. Yea so outputting a string of 11001100.. would produce a 20MHz waveform, so, then the 3rd harmonic of that is 60MHz, right, so that's what is being picked up? Yea you are right about the harmonics of the ideal square wave, but I was thinking along the lines of a distorted version of the perfect square wave that you get from a typical RF type signal that I've seen on spectrum analyzers many times -- typically you don't have this perfect delta function sitting at your carrier frequency -- it has some extra stuff going on next to it above and below (other than the modulated signal -- not nearly as far away as the F*2 or F/2 type range), but hey I'm not an RF engineer, so I'll just stop speculating in that area. I'm an EE, so I get what you are talking about but thanks for the details--I'm sure plenty of people will benefit from it. Still would love to see a PSD spectrum plot of the output signal from 10MHz to 160MHz or something like that kind of range.
For anyone interested this is called aliasing.
If you've ever noticed wheels going backwards in racing or train footage, this is exactly what's happening: the wheel's angular velocity is above half the sampling rate (i.e. the camera's FPS), producing a mirrored version which in turn appears to rotate backwards.