Figure 28 - The analog to digital converter is an R2R ladder
To make an 8 bit digital port output an analog sound signal, an analog to digital converter is needed. Much like the 4 bit R2R ladder that generates the luminance signal, the sound generator will also use an R2R ladder DAC, but this time having an 8 bit resolution. The resulting sound signal is an analog voltage with 256 steps closely matching the line input level of most VCRs and televisions sets. The output is then filtered a bit using a .1uF capacitor to ground in order to reduce some of the harsh edges on the 8 bit analog data. If I have any free pins on the sound generator when it is completed, I plan to allow full control over this filter by having it connected to an output pin. Back in the day, the Amiga 1000 used this same filter technique and called it a low pass filter.
Stay tuned... this project is a current work in progress (October 14 2010).
Once this breadboard prototype is completed, a hand wired perforated board will be made to fit inside a console unit. Full schematics and source codes for the sync generator, sound generator as well as the games and demos I plan to write will also be available. The final version of Lazarus-64 will be available as a small PCB that will fit into a solderless breadboard or into a cabinet to create a TV top game console. This version will have a Xilinx XC2C256 CPLD to replace most of the logic, although operation will be cycle exact. The CPLD version is actually completed already, but I wanted to document the entire build process to show how Lazarus progressed from breadboard to console.