An Analog I/O Interface Board for Audio Arduino Open Sound Card System

AudioArduino [1] is a system consisting of an ALSA (Advanced Linux Sound Architecture) audio driver and corresponding microcontroller code; that can demonstrate full-duplex, mono, 8-bit, 44.1 kHz soundcard behavior on an FTDI based Arduino. While the basic operation as a soundcard can be demonstrated with nothing more than a pair of headphones and a couple of capacitors - modern PC soundcards typically make use of multiple signal standards; and correspondingly, multiple connectors.
The usual distinction that typical off-the-shelf stereo soundcards make, is between line-level signals (line-in/line-out) - and those not conforming to this standard (such as microphone input/speaker output). To provide a physical illustration of these issues in soundcard design, this project outlines an open design for a simple single-sided PCB, intended for experimentation (via interconnection of basic circuits on board). The contribution of this project is in providing a basic introductory overview of some of the problems (PWM output in particular) in analog I/O design and implementation for soundcards through a real world example, which - while incapable of delivering professional grade quality - could still be useful, primarily in an educational scope.