The AY-3-8912 Family: Three Chips, Nine Channels

Synopsis

The AY-3-8912 is the chip that gave the 128K Spectrum (and the Amstrad CPC, and the MSX, and a small army of arcade machines) their distinctive sound. The Next has three of them, addressable through the same legacy ports — pick which chip you’re talking to via NextReg $06. Three channels per chip, nine channels total, plus envelopes, plus a noise generator per chip, plus stereo positioning.

Topics:

• The classic AY register file. Tone period, mixer, amplitude, envelope shape, envelope period, I/O ports — all 16 registers, what each does.
• The two ports. $FFFD (register select), $BFFD (register data). Identical to the original 128K Spectrum; legacy AY tracker code runs unchanged.
• The three-chip selector. NextReg $06 bits — pick which AY responds to the next port write. Combined with the standard ports, this gives you all nine channels.
• Stereo modes. ABC, ACB, mono — NextReg $08 bits 4-3.
• Envelopes. The eight envelope shapes, the period registers, and how to create vibrato/tremolo by writing fresh envelopes mid-note.
• Tracker formats. A tour of the common AY music formats (PT3, AY, STC, ASC) and how their players unpack into the same register writes.

What you should know first

• I/O and NextRegs.
• Interrupts — most music players run from the frame ISR.

Planned exercises

• Three-chip chord. Play the same note on all three AY chips simultaneously — hear the difference between mono, ABC, and ACB modes.
• Envelope showcase. Step through all eight envelope shapes on a single channel, displayed as a label on screen.
• Tracker player. Load a small PT3 tune from the example pack and play it back from the frame interrupt.