The 7800 Minnie sound chip
Introduction
Minnie (GCC 1730) was a soundchip designed by GCC as an in-cart audio solution for the 7800 console.
The original plan for the 7800 console was to include an onboard sound solution (in addition to TIA audio) but the Maria die space set aside for audio was needed for other core functionality. So GCC added an audio line to the 7800 cartridge port, and began adapting their previous work on the Gumby soundchip (a soundchip design previously developed for their Spring project) into a new Mini-Gumby soundchip, the "Minnie".
Although the Minnie design was completed - prototype chips were produced, and a "Minnie Rag" demo sang out within the walls at GCC - the chip ultimately never made it to production, due to the Warner sale of Atari to Jack Tramiel, which marked an end to GCC's relationship with Atari.
Capability Overview
The Minnie design is quite unique compared to other contemporary sound chips. The synthesis technique employed is Digital Wave Synthesis, where sample data is used to represent one cycle of the sound. There were two rows of sample data and 3 predefined waveform rows, so the chip could also be driven in a Wavetable Synthesis fashion.
Adjustable sound parameters include very fine-grained volume and frequency controls, and a noise parameter. The noise parameter could be used to adjust the wave phase of any of the voices in varying levels, achieving modulation effects ranging from slight frequency instability through to pitched noise.
Features:
- 3 voices
- 16-bit frequency resolution
- 28kHz sample rate with dynamic range of 10 bits
- 2 arbitrary 64-byte waveforms stored in on-chip ROM, which could differ per-game
- 3 standard waveforms: triangle, square, sawtooth
The Minnie and TIA audio was summed together, which added another 2 voices to the system, suitable for sound effects and percussion effects.
Minnie relied on the CPU to adjust volume for achieving sound envelopes. This would have commonly occurred once per frame, along with any other sound parameter adjustments desired.
The GCC Minnie Documentation
The Minnie Subsystem Top-Level Specification.
MiniGumby Sound Synthesis System, Analog Output Section Specification.
Thanks
Many thanks to Steve Golson, who provided the GCC technical documentation this article is based on. The documentation was found by Steve, in the personal files of Gary Boone.
If you're at all interested in the GCC side of Atari history, or the 7800 at a technical level, you owe it to yourself to view Steve's talks and papers at: https://trilobyte.com/papers/
