1-Bit AVR Synthesizer
By Matt Sarnoff (msarnoff.org/1bitsynth)
October 2009-October 2010
========================================

This is an interesting little synthesizer/noisemaker that makes Atari-like
sounds. All sounds are generated directly from an ATmega48 microcontroller,
and have only 1 bit of amplitude; in other words, pulse waves and nothing else.

For more info, photos, and videos, see
http://www.msarnoff.org/1bitsynth
or
http://www.youtube.com/user/74hc595


Compiling
---------
A Makefile for avr-gcc is included. Just type "make hex", then connect your
ISP programmer to the ATmega48 and type "make fuse" and "make flash".


Hacking
-------
The code in its original state is about 3k, leaving some room for additions.
Additionally, things should work fine on an ATmega168 or ATmega328P if you
need more space. 

The MIDI note table is specially computed for a 12MHz clock frequency.
If you change the crystal, you'll have to recompute this table.

The circuit supports 9 analog inputs, but only 7 are used. Feel free to do
whatever you like with the other two (as long as the signal is between 0 and
5 volts).

You might notice the "tiasnd.c" file is not used at all.
This file contains an implementation of the actual waveforms used by the
TIA chip in the Atari 2600. (basically, a 9-bit linear shift register connected
in interesting ways.) I decided not to use it, since I wanted this synth to
make its own sounds, rather than mimic the sounds of some other chip, but it
shouldn't be too difficult to hook it back up if you like. The algorithms
are adapted from the source code of the Stella emulator.
(http://stella.sourceforge.net)