This week I focused on prototyping our MVP effects (delay, chorus, 3-band EQ) in MATLAB. In particular, I tried to write the algorithms such that they can be ported to the microcontroller environment easily. This includes strategies such as mocking block processing by using input buffers that go through for loops.
I made good progress with chorus, setting up a superimposition of delayed copies of an input signal with delays set by a “LFO.” In its current development this LFO is more of a triangular wave than a smooth sin wave since I have not implemented interpolation between discrete points. I have not settled on how many copies of the signal to overlay. Currently the effect is audible, but it’s not very strong.
I did more research on 3-band EQ’s and instead of typical LPFs and HPFs, low shelf and high shelf filters are used. I have not implemented my 3 filters yet, but I am thinking of starting my using the specs in this report. This includes a low shelf with a center frequency of 200Hz, a band pass filter with Q = 1 and a center frequency of 1kHz, and a high shelf with a center frequency of 5kHz.
I will model the delay algorithm using this reference from Analog Devices. Their algorithm is in line with what I was thinking to do, and it’s good to see an industry standard reference.
Unfortunately I am still behind on prototyping the MVP effects in MATLAB. I think that writing them in a way such that they can be written on the STM32 environment quickly will help save time during the actual implementation, so I am not too worried. While I was hoping to give a demo of the MVP effects last Friday, I will have to push off the demo to some time this week.
I worked with Adam to finalize our purchases for Rev1 and created lists for Amazon and DigiKey, filling out the form for the Amazon order.
Next week I am hoping to complete the MATLAB prototyping and begin writing some framework code (buffers, looping structures, etc.) for implementing the effects on the STM32. In addition, I will work with Xingran to develop the outline for the user interface menus.
Check out the audio file to hear a chorus effect in action! This is an excerpt from a recording of the guitar part to Sondheim’s Assassins musical that I did for the school of drama back in January.