Jan 22nd 2013

6 comments

This is a simple MIDI file player that runs directly on the Raspberry Pi hardware, ie. it doesn't run Linux or any OS (although I did end up implementing a bunch of sketchy POSIX functions). This means it's essentially an embedded project, and you can see from the video that it boots up more or less instantly.

I've learned a bit about doing baremetal hardware programming in C. It feels quite rewarding, and despite being primarily intended as a Linux computer the Pi is quite usable as an embedded device. If you're interested in that sort of thing, this is a good place to start looking.

Unless anybody knows different, this project breaks new ground in two ways:

It contains the first baremetal audio code for the pi. Unfortunately I was only partly able to figure out how this should be done. My code uses the 'oscillator' clock source for the PWM, which leads to fairly dreadful audio quality. If anybody knows how to make the PWM work with the 'PLLA' source used in Linux, I'd love to hear about it.

It's the first embedded implementation of an LV2 plugin host. It's a rather loose interpretation of the spec as there's no working filesystem yet, so dynamic linkage and runtime reading of .ttl files aren't possible.

The source and binaries are available here. It includes three fairly bad sounding synth plugins, which you can hear demoed in the video.

Next step is to hook it up to the MIDI keyboard you can briefly see in the video.

Thanks to David Welch for his baremetal pi tutorial on github, to Remo Dentato for his midi file code, to Dave Robillard for his LV2 work, and to Wayne Stallwood for lending me a Pi to work on. You are the wind beneath my wings.

Update: I got featured on hackaday.com! Cool.