This Rack Extension synthesizer is inspired by a programmable sound generator chip that was used at least three generations of home computers or video game consoles. My interest is in a particular variant that was used in both an 8-bit and 16-bit systems, and a portable, all designed by the same company. It has three voices with square wave oscillators, and one pitched noise channel.
Like all of the devices in this series, I'm not going for a sample-accurate emulation. While the tones produced by this synth should be unmistakable, there are some changes behind the scenes. I'll not be adding drastic new features, the limitations inherent in an 8-bit chip are the source of a lot of its charm. Though, I will be making adjustments to the interfacing to have it feel more at home in the Reason rack.
One of the largest changes to this particular chip is in the design of the accumulator. The original drained a 10-bit accumulator at 1/16 speed of its 3.5 MHz reference clock sourced from the NTSC color carrier. This restricted the lowest reproducible tone to 109 Hz, but the highest was a staggering 112 kHz. That's fine when making music for dogs who don't like bass, but not so much for humans. Giving it a floating point accumulator makes it so the range covers the entire MIDI note range. The portable system that used this chip was capable of basic two-channel output, with each voice being able to be assigned to either the left or right, or both channels. I'm not including this, but with the per-voice direct outputs it's possible to pan them in a mixer instead_