Changes

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2 ports are used to control the MIDI soundcard: &FEAC for data, and &FEAD for control and status.

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OPL3 is register compatible with OPL2, with twice the number of signal sources, 4 new operator modes, 4 new selectable waveforms, and stereo output.

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The most basic elements of an OPL FM synthesiser are:

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The OPL3 uses two look-up tables, each containing 256 entries. [https://g.co/kgs/Da1vEi4 Source]

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The original OPL only has access to the sine wave.

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FM synthesis does a great job relative to its hardware requirements at reproducing melodic instruments but isn't especially useful for harsh percussion noises like drums. Yamaha provides separate drum noises for this purpose. The programmer has to forfeit three channels of regular FM synthesis, freeing up six operators for five rhythm instruments.