Chris Anderson @chrisanderson1973@librem.one

It's nice to be creating stuff again! The day job is fine, but there is not much room to reinvent the "framework" at the moment. So, here I am in C, inventing a way to pass along settings from the Qt app to this effects box.

Ha - well, that went well! It looks like alien wah works very reasonably within the STM32 realm! A couple of videos here!

This is a big test of the platform, I guess. I am attempting to port an effect from the iPad/iOS synth I released in 2014 and 2019. It's called "alien wah" and it was a hit with users on the App Store. I have around 512Kbytes of memory to work with, so I think the effect can fit in an effects chain. I may have to limit the number of instances - but it may only make sense to use it once in a chain, anyway. This gets tricky if the platform goes stereo, and a memory expansion is needed..

First bit of DSP in effect! This is a simple notch filter to get things going. It works well, except that I need to get rid of any blips between switching filters/effects on and off.

Yeah, now I have the serial ack/nack verified and working. The initial setup doesn’t quite work the way it should, but I can look into that part later. I did manage to test the audio working alongside transmit/receive, and it seems pretty decent.

Well, I completely cracked the composite USB driver deal! I now have a serial USB interface transmitting/receiving while an audio interface works as an audio input to macOS / Logic. Very happy, indeed! There's a little work to be done on the packet code/decode, but not much, really. This opens up the ability to configure/reconfigure DSP modules on the fly which is insanely cool. You might say I on the way to creating a software-defined DSP system.

Well, I have this much figured out, with some still to go. I have managed to place the audio portion of the USB driver, successfully, underneath an overriding composite USB class. Logic Pro appears to even register the audio in as a live signal, which is a great sign! More to come.....combining this with the CDC part.

A little more progress on building the USB stack, after getting a rough composite and then audio stack going. The USB descriptor is the tricky part.

Part of the audio out / serial interface completed today! I had an issue where the firmware would freeze, but I found that it was simply device config pin conflicts. Once I removed the unused devices from the "main" function, all was good. On the to-do list is to set up the Mac oscilloscope....

Starting on the integration of the composite USB project with the microphone USB project. I worked through one problem this morning, and I want to complete the integration today and then move on.

Saturday morning work, at the office.

A good night working through parts of the USB audio stack. I am now looking at the audio data transfers to USB, as.well as the ADC sampling (which is the easy part!). Fun stuff!

The “kitted” DSP platform is altogether now, such that it is. It’s not the cleanest-looking mock-up in the world, but it will get the job done. I believe I may hold off on working with it much until I arrive back later this week. It might be a good idea to clean up the lab area since I finally have a sensible hardware platform for prototyping.

Kitting partially there!

Today’s work is all about the power supply (9V battery to 5V regulator - for now). I also need to place the input amplifier and STM32 Micro system in a kit/container for transport.

The “new” amplified I prototyped probably isn’t required, and it is not quite right, in terms of design, due to the reliance on a double-ended power supply and amplifier configuration. The single supply amplifier design from the other week was tweaked to amplify at 10x, rather than only 2x, and the result is a maximum Vpp of 1V, which is the current target.

These oscilloscope pics show the original and then output signals, along with the iPhone playing the constant synth sound!

More work today - getting the input amplifier integrated and looking at the initial DSP chain.

Updated schedule.

Finished prototype of the input amplifier.

Finished the pre-amp prototype this morning. Ready for testing this evening and further integration.