A 16-voice Homebrew Polyphonic Synth

Homebrew synths – generating a waveform in a microcontroller, adding a MIDI interface, and sending everything out to a speaker – are great projects that will teach you a ton about how much you can do with a tiny, low power uC. [Mark] created what is probably the most powerful homebrew synth we’ve seen, all while using a relatively low-power microcontroller.

The hardware for this project is an LPC1311 ARM Cortex M3 running at 72 MHz. Turning digital audio into something a speaker can understand is handled by a Wolfson WM8762, a stereo 24-bit DAC. Both of these chips can be bought for under one pound in quantity one, something you can’t say about the chips used in olde-tyme synths.

The front panel, shown below, uses 22 pots and two switches to control the waveform, ADSR, filter, volume, and pan. To save pins on the microcontroller, [Mark] used a few analog multiplexers. As far as circuitry goes, it’s a fairly simple setup, with the only truly weird component being the optocoupler for the MIDI input.

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The software for the synth is written mostly in assembly. In a previous version where most of the code was written in C, everything was a factor of two slower. Doing all the voice generation in assembly allowed for twice as many simultaneous voices.

It’s a great project, and compared to some of the other synth builds we’ve seen before, [Mark]’s project is at the top of its class. A quick search of the archives says this is probably the most polyphonic homebrew synth we’ve seen, and listening to the sound sample on the project page, it sounds pretty good, to boot.

Making An ARM Powered MIDI Synthesizer

What you see in the picture above is a hand-made 4-oscillator synthesizer with MIDI input, multi-mode filter and a handful of modulation options. It was built by [Matt], an AVR accustomed electronics enthusiast who made an exception to his habits for this project. The core of the platform is a DIP packaged 32-bit Cortex-M0 ARM processor (LPC1114), stuffed with ‘hand’ written assembly code and compiled C functions. With a 50MHz clock speed, the microcontroller can output samples at 250kHz on the 12bit DAC while being powered by 3 AA batteries.

Reading [Matt]’s write-up, we discover that the firmware he created uses 4 oscillators (sawtooth or pulse shape) together with a low frequency oscillator (triangle, ramp, square, random shapes). It also includes a 2-pole state-variable filter and the ability to adjust the attack-release envelopes (among others). The system takes MIDI commands from a connected device. We embedded videos of his creation in action after the break.

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Breadboarding with a ARM microcontroller

NXP’s LPC1114 ARM microcontroller is in a class all of it’s own. ARM microcontrollers are a dime a dozen, but this fabulous chip is the only one that’s housed in a hacker and breadboard friendly PDIP package. However, breadboard setups usually won’t have the luxuries of a true development platform such as flashing the part, single stepping through the code, and examining memory. [Steve] found an interesting solution to this problem that involves a Dremel and hacking up even more hardware.

[Steve] found a few LPC1769 dev boards that include a debugger and a way to program these chips. Simply by hacking off the programmer and debugger portion of this dev board with a Dremel tool, [Steve] had an easy to use interface for his breadboardable ARM.

After connecting the power rails to his breadboarded chip, [Steve] connected his programmer up and set up a gcc toolchain. For about $25, he has a breadboard friendly ARM microcontroller with full debugging capabilities.

This isn’t the first time we’ve seen a few people play with this DIP28 ARM chip; someone even milled this 600 mil chip down to 300 mils for even easier prototyping. Still, this is the best and cheapest way we’ve seen yet to turn this ARM into a proper prototyping platform.