midi_to_cv

parse midi messages with ALSA sequencer API, and output SPI message to
MCP4921 DAC for controlling an analog synthesizer

tested hardware:
	raspberry pi Model B or libre computer AML-S905X-CC
		(other linux boards with spidev drivers should also work, but the GPIO
		pins may be different)
	MCP4921 12-bit DAC

NOTE: you'll need to use aconnect or similar to send MIDI messages from a
	device to this program. eg)
	$ aconnect -l
	$ aconnect 128:0 64:0

useful resources:
	MIDI:
		http://fundamental-code.com/midi/
		https://www.alsa-project.org/alsa-doc/alsa-lib/group___sequencer.html
		https://soundprogramming.net/file-formats/midi-note-frequencies/

	SPI:
		https://forums.raspberrypi.com/viewtopic.php?t=304828#p1856388
		https://www.kernel.org/doc/html/latest/spi/spidev.html
		https://hub.libre.computer/t/how-to-enable-spi-on-aml-s905x-cc-le-potato/285

	GPIO:
		https://git.kernel.org/pub/scm/libs/libgpiod/libgpiod.git/tree/README
		https://git.kernel.org/pub/scm/libs/libgpiod/libgpiod.git/tree/include/gpiod.h
		https://github.com/starnight/libgpiod-example/blob/master/libgpiod-led/main.c
		https://libgpiod.readthedocs.io/en/latest/index.html

	MCP4921 datasheet:
		https://ww1.microchip.com/downloads/aemDocuments/documents/OTH/ProductDocuments/DataSheets/22248a.pdf

	AML-S905X-CC:
		https://hub.libre.computer/t/libre-computer-wiring-tool/40
		https://hub.libre.computer/t/aml-s905x-cc-wiring-tool-overlays-description/62