Two weeks ago, the entire focus was on writing our design document. This was an extension to our design presentation, and consumed most of our time that week. It’s attached below if you want to check it out!
Our Rev 1 PCB was delivered this past week. Adam and Xingran wrote up a test bring up plan and proceeded to test the boards in Ansys Hall. See Adam’s post for more details, but while some modules worked as expected, there were some set backs. However, this was expected, and was one reason our plan includes Rev 1 as a demo board.
Bring up of low software is now the focus for the group, especially getting the BM83 Bluetooth module to work, and creating a UI. Once we can get an audio signal to a speaker, that will be a huge success, and it’ll be even better if the user has basic control over parameters. As such, Sam is halting work on DSP effects and Xingran will be focusing on work with the BM83.
While we are deviating from our original schedule, the included slack time inhibits any real danger to our progress. The next few weeks will be pivotal to getting a MVP product, but once that is completed, the additional features can be added more easily.
The past two weeks, I have focused on our MVP DSP effects of chorus, delay, and EQ. I have successfully prototyped delay and EQ with user-controlled parameters that shape the sound according to our design spec. Chorus is very close, but I am running into a bug where there is a crackling sound introduced with the audio. I suspect this might be a feature induced by block processing that I have to find a way to work around. If I can’t get it working, I will reach out to Tom Sullivan or maybe Ryan for some guidance.
I provided a demo of the MVP effects to my team last Monday, and received positive feedback. During our Monday sync-up, Tom suggested wah-wah as an additional effect if we have, especially I have a mid-peak filter working. The implementation would be as simple as sweeping the center frequency of the filter.
I am behind on the STM32 implementation of effects, but I am still optimistic I can port my MATLAB code to C quickly.
For this next week I am shifting my focus from DSP effects to UI design. I will start with a wireframe and then from there write code for the menu layout, assuming I am provided high-level functions such as write_to_lcd(char[] str). This is a change in our schedule, but one we deem as more essential to getting a working product than the MVP effects.
Below I have included an audio file with our custom delay effect added with the following settings:
This week Adam gave our design presentation, and we got to see how other groups have refined their project. We met on Friday to update each other on progress, discuss Rev1 budget details, and help debug the SPI protocol that Xingran was implementing.
We faced a risk in that the intended PCB manufacturer charged more than we expected and had an estimated arrival of much later than we deemed acceptable. As a result, Adam found a new manufacture that would be cheaper and offer much quicker delivery. The downside is that we will have to hand-solder some of the components with this service. However, the savings in cost (1/2 the cost) and shipping time (a week or two at most) justify this small setback. Another decision was to buy two Rev1 boards since the overall cost of fabrication would be the same with our supplier. This incurs additional component cost, but we agreed it would be better for risk mitigation to have two boards anyways.
Adam has agreed to take a larger role in the design document next week to give Sam and Xingran more time to work on software development since both are a little behind schedule. However, we are still mostly on track, especially given our ample slack time.
Next week we will work on the design document during class time, Adam will work on the mechanical enclosure, Sam will continue MATLAB prototypes and begin working on STM32 algorithm implementation, and Xingran will continue low level software bring up.
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.
This week we reviewed Adam’s schematic, adding labels/clarifications where necessary and correcting errors. We received feedback from Ryan to include a clipping indicator, which we added to our design specification to be implemented in software. Ryan also suggested a audio-specific ADC, which we decided to use. In addition, we determined that the SPI protocol would be a better choice than the 6800 parallel interface for our LCD display.
We met up to work on the design presentation slide deck, splitting up development of the content on the slides for Adam to present.
Sam and Xingran discussed the implementation of the DSP effects, and due to the limited SRAM of the microcontroller, decided to switch from reverb to delay as an MVP effect. We adjusted the schedule to give Sam another week to work on prototyping the MVP effects in MATLAB
We worked on the overall software block diagram to supplement our hardware block diagram to prepare for the design presentation.
Adam will give a dry run of his design presentation on Sunday to Xingran and Sam to receive feedback for the actual presentation Monday/Wednesday.
We will be ready to order our Rev1 board by Wednesday!
This week I focused on researching algorithms for our MVP effects, EQ, reverb, and chorus. The standard way to implement EQ is by having a filter for each band, and applying an amount of gain to each filter, to shape the frequency response. This should be simple to implement, requiring filter coefficients, and buffers of inputs and outputs of the signal. Chorus is implemented with overlaying copies of a signal with variable delays controlled by LFO’s. In essence it should should like a “chorus” of singers or instruments playing all at the same time, with slight phase variations. This will require storing some inputs and outputs, past what we are storing in our DMA input and output buffers. I researched reverb, and after a lot of research into different implementations (mostly Schroder based reverbs such as “Freeverb”), it seems that for a versatile reverb with a large delay time, we run into issues with storing inputs and outputs. To store 1 second of inputs and outputs, required for a 1 second reverb-time, we would use just about all of our available SRAM. After some consideration, the team and I decided that going for delay would be a better option. A 1 second delay is practical, whereas a 1 second reverb time is a bit low. Delay is an IIR system that involved a feedback loop with a gain less than 1.
I did not implement any MVP effects in MATLAB, so we updated the schedule to give me another week to actually implement the effects. Prototyping the effects properly will save time in the end as we try to port the effects over to the STM32.
While researching the MVP effects, I looked into the memory usage of the effects, and talked with Xingran about implementation decisions- we ultimately decided on double input and output buffers served by DMA. We expect a processing latency of about 12ms.
Since we changed the schedule, my focus will be on implementing our MVP effects in MATLAB and providing the team a demo on Friday.
This week we delivered our proposal presentation and finalized the interfaces between systems, specifically between modules and the microcontroller module. This helped Adam with the schematic for the PCB. Also, we selected components and finalized a budget, which is crucial so that we can order our STM32 discovery board and get working on software and firmware development.
We need to review the schematic, both individually for errors, and also with Tom and Ryan to get some expert opinions. It is imperative to get Rev1 finished asap so that we can test it and make changes as needed for the final PCB.
We did not change any requirements, since we fleshed those out before the proposal presentation. However, we made final choices for a lot of components, which helps to firm up our direction.
This has been a huge week for tuning our system specification and documentation. We’re looking forward to the schematic review and to play with the discovery board!
This week was proposal presentation week! We got to see some cool projects, many of which are audio or signal processing related. We received positive feedback from other groups!
This week I finished up the UI trade, outlining our user interface module’s requirements and looking at different options for components. This required research into different kinds of displays (such as TFT LCD pros and cons), and I learned about how LCD screens work in the process. One constraint we had is being able to power it using a 3.3V supply, which limited our options. From there, I prioritized cost effectiveness and size. We want to make sure the UI is easy to navigate and able to enable a user to quickly adjust parameters. I also selected a rotary encoder with a button, but we ultimately ended up getting a new model of the same one for cheaper!
I also worked on the interface between the UI module and the microcontroller. I got my hands dirty looking at the LCD screen’s data sheet and also how utilize the FMSC feature of the microprocessor in order to efficiently connect the LCD. I did some research on the pinout for the rotary encoder and switch, and found some GPIO pins to connect it to. Finally, I researched different footswitches, since we want to include a toggle switch for quick effect-toggling during a performance. I learned about single, double, and triple pole switches and also single, double, and triple throw switches. While a TPDT switch is standard for a lot of guitar pedals, we can have the microcontroller process the toggle, so we really only need a SPST switch. The other thing I looked for was a clicky/tactile feel when you toggle the switch since I know I personally like that feedback when I am using an effects processor.
I also worked with Adam and Xingran to finalize a budget for our project, selecting our components.
I looked into implementing digital effects, and have started a game plan for each (EQ, reverb, chorus). Next week I will begin to write some MATLAB code to actually test my architected designs. I’ll use the sample guitar input I provided Adam to test the analog effect and hear how the effects shape the output. The UI to microcontroller interface took longer than expected, so I did not work on this as much as I had hoped this week. Progress should be a lot better now that I can focus on it this week!
This week I did some research on a good choice for our analog effect. I concluded that overdrive/distortion would be the most valuable analog effect due to the difficulty in effectively modeling transistor clipping in digital software. From there, I looked at available reference schematics to see what options would be quick to implement, without the need to re-invent the wheel. I was also hoping to find something relatively unique but fully practical. I found a vintage 70’s analog overdrive effect called the Colorsound Overdriver and loved the sound after watching some YouTube demos of the pedal. Not only did it have a great sounding overdrive, but if you boosted the gain high you could get a nice distortion, and even a fuzz effect at the highest gain. It’s a super versatile effect, and it would be a cool bonus to add to BARI given that clones of the effect cost $100+ and the original can sell for over $1000 (according to Reverb).
I narrowed down our MVP digital effects to be EQ, reverb, and chorus. EQ will be useful for guitarists who want to adjust the tone of their Bluetooth speaker, since unlike traditional guitar amps, the speaker won’t have bass/treble dials. Reverb will be great for singers or even instruments who want a fuller sound, and it is a staple effect among musicians. Chorus is a modulation effect that gives the impression you have a “chorus” of guitars playing together, and makes for a good combination with the other effects. The team helped me narrow down some metrics to test the quality of the effects, such as control resolution for adjusting parameters for the effects. We are thinking of using MATLAB to compare spectrograms of the MATLAB simulations of the output of each effect to BARI’s output to determine if we successfully implement the effects on BARI.
I also worked on some miscellaneous tasks such as setting up the website and providing information for the use cases slide for the project proposal. I need to finish writing about the System UI in our system spec. I am behind on this task, but will get it down early next week. Next week I will provide Adam with a file representing voltages from a guitar input so he can simulate the output of his Colorsound Overdriver circuit. I’ll also be available to help with the effect if needed. I will begin writing MATLAB scripts to simulate our digital MVP effects in software, which will be my focus for the next couple of weeks.
