The Beeper and the Border-Sound Heritage

Synopsis

Bit 4 of port $FE toggles a single speaker line. That’s it. That is the entire ZX Spectrum sound hardware — and a generation of programmers extracted multi-channel music, percussion, and even sampled speech out of it. The Next still has it, faithfully reproduced, and it’s still a great way to make noise without learning a single AY register.

Topics:

• The hardware. Port $FE bit 4 — the speaker line. Bit 3 — the MIC line, historically used to drive the cassette interface but audible on real hardware. The Next mixes the beeper with everything else through the standard audio path.
• Square waves the easy way. A delay loop, a toggle, a delay loop, a toggle. The frequency-vs-loop-count formula.
• PWM. Modulating the duty cycle to get a “louder” or “softer” tone — gives you crude volume control on a one-bit speaker.
• Multi-channel beeper. The classic trick: pulse-position modulation so that two (or more) virtual square waves share the same speaker line. Reference algorithms.
• CTC-paced beeper. Driving the speaker from a CTC-2 interrupt instead of a busy loop, so the CPU stays responsive. Needs the CTC chapter.

What you should know first

• I/O and NextRegs.
• The CTC — for the interrupt-paced exercise.

Planned exercises

• Square-wave melody. A short tune driven by a busy loop.
• Two-channel beeper. Two simultaneous tones using the pulse-position-modulation trick.
• CTC tone. The same square wave, but driven by a CTC interrupt — the CPU is now free to do something else while the tone plays.