Inspired by the works of Oliver Mattos and Oskar Weigl the creators of pifm and Evariste Okcestbon the creator of rpitx, I would like to push the boundaries of radio-transmission on the rpi platform. My main interesst lies in the transmission of multiple audio channels at the same time, this requires a decent amount of bandwidth and some dynamic range to function.
In my eyes the beauty of pifm and rpitx is that basic radio-transmission functionality is implemented using existing rpi-hardware peripherals in unorthodox ways together with clever programming. Only an output filter is recommended for operation. So the question arises what would theoretically be possible by adding just a little more hardware to this equation - namely an external mixer IC (SA602). We still would need a digital-to-analog converter in order to generate the required baseband which would be upconverted by the mixer to the desired band. Building an ~4MSPS/8Bit DAC can be done using the PWM-peripheral (in serial output mode) and a software SigmaDelta-modulator, this reduces the required external hardware to some filtering and the external mixer.
The project can be split up in the following subtasks:
* (arm-)optimized SigmaDelta function
* DMA enabled driver to feed the SigmaDelta-data to the PWM
* Low-Pass Filtering for the SigmaDelta-Output
* Mixer circuit
* Low or Bandpass-Filter of the Mixer-Ouput
Software is licensed under GPL2
Hardware is licensed under CC-BY-SA 3.0
Warning: Always check that you have the legal requirements to transmit and use appropriate filtering to not generate unwanted noise in the spectrum.