Unfortunately a large part of my week was spent dealing with a Quartus issue; for some reason, I lost the ability to program the FPGA itself, and had to invest a large amount of time finding a workaround. As a result, I focused my effort on setting up a testing pipeline in simulation in order to look at filter / delay outputs and ensuring their stability. Once I got that to work, I debugged delay, and it looks like it works without having to clamp the delay feedback coefficient. Additionally, I used the IIR components to construct a single-pole and biquad low pass filters, and created a cascade filter using multiple single-pole IIR filters. I seem to have finally resolved the Quartus issue today, so I hope to spend tomorrow verifying the filters on the FPGA and qualitatively comparing their outputs, and to spend a large portion of next week ensuring that FM is working properly.
In terms of system testing, I hope to use the pipeline I’ve already set up for the delay output. Primarily, the testbench I’ve designed allows me to optionally add in parts of the pipeline (such as FM, ADSR, delay, etc.), and check how components interact with one another and gather data on output waveform. From this, basic visual inspection can help ensure output stability and that the output is approximately what would be expected. Checking the collected data against MATLAB scripts can allow for further verification, especially on components like a low-pass filter, where the expected output is not immediately obvious. A final check can be performed on the actual FPGA, something I am able to do now that Quartus has been configured properly. Since delay, FIR, and IIR filtering all go in the portion of the pipeline, the testing process is exactly as described above – configure the audio pipeline so we can isolate the specific component in simulation, and then perform progress checks on the output. For FM modulation, the process is slightly different, namely that I intend to directly compare against MATLAB scripts, rather than trying to visually determine if the waveform “looks” correct. I intend to remove all downstream portions of the audio processing pipeline, looking directly as how the wavetable index / waveform output behave.