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So the idea here is to make a pedal that takes an input waveform directly from the guitar, inputs it into an FPGA for the user to do whatever they want, and then mix back in the new signal with the original with volume and mix control. For my purposes, I am not going to make this a marketable pedal with switches for bypass etc or make a metal enclosure because I am trying to save my money as a college student, however one day I hope to be able to add these things in and maybe even sell it. This project is also best for the electric guitar at its standard tuning notes as that is what I want to use it for.
First, we have to get the analog inputs and outputs out of the way. These should be AC coupled and for the input biased at some point between 0 and 3.3V. 2.5 is fine for now, we will see if 1.25V is not a big hassle to add later.
The design of this is based on minimal group delay, stability, and noise. There is an emphasis on group delay which we want to keep under 1ms overall for inaudible delay in the range of 300Hz - 1kHz, and below 10ms for frequencies under 200Hz (Same link). The computing delay required by the FPGA can be neglected for now because the analog characteristics dominate the delay created by the FPGA.
Here are the simulations of using the ADC and then communicating the values received right back to the DAC, with 4 0’s concatenated to the end since this ADC is only 12 bit and I have 16 bit output dac. I am using the ad5660 and here is its output communication sequence as well. It requires a 33ns period between writing data, and I have more than that because our DAC is sampling faster than the ADC anyways. There isn’t an issue with this because the same bit of information can keep being written to the DAC.
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