The YM2203, a.k.a. OPN (FM Operator Type-N), is a six-channel (3 FM and 3 SSG) sound chip developed by Yamaha. It was the progenitor of Yamaha's OPN family of FM synthesis chips used in many video game and computer systems throughout the 1980s and early 1990s. It was used in a variety of NEC computers, along with various arcade game machines.
The YM2203 has the following features:
- Three concurrent FM synthesis channels (voices)
- Four operators per channel
- Two interval timers
- A sine-wave low frequency oscillator (LFO)
- For channel three, operator frequencies can be set independently, making dissonant harmonics possible. (Normally, they would have a simple relation like e.g. 2× or 3× relative to a common base frequency)
- Internal implementation of Yamaha's YM2149F SSG chip
The YM2203 and the rest of the OPN synthesizer family generate sound via frequency-modulated digital sine waves. It included 12 operator "cells", each generating a 13-bit sine wave at a programmable frequency, the volume of which is controlled by a programmable ADSR envelope generator. The output of these cells could be either summed together by the mixer, or fed into the input of another cell, in 4-cell batches creating the final sound values or "channels". 4 operator cells per channel allowed a total of 8 different permutations of cell connections, known as "algorithms". The ADSR parameters, multiplier and detune settings for each operator, combined with the algorithm, make up what are known as instrument patches.
The resulting digital sound output of each channel through the mixer was then converted to analog sound via a digital-to-analog converter (DAC). The YM2203 is used with a YM3014 external DAC companion chip.
The SSG module implemented the YM2149F's three SSG channels, noise generator and dual GPIO ports.