Beeper Music

Overview

The Sinclair ZX Spectrum's 48K model has no dedicated sound chip. It has a single bit — bit 4 of port $FE — wired to a small internal speaker (the beeper). Setting that bit high pushes the speaker cone out; setting it low lets it spring back. Toggle it fast enough and the cone vibrates, producing a tone whose frequency is half the toggle rate.

This sounds primitive. It is. But the Spectrum's audio library proves what a creative programmer can do with one bit: monophonic melodies, two-and-three-voice harmonies via interleaving, percussion via noise bursts, even speech synthesis via PCM streaming. The Spectrum's most musically ambitious games — Manic Miner, Jet Set Willy, Stormlord, Sanxion, Match Point — drove that one bit at audio rates and produced tunes that defined the platform's sound.

How a tone works

To produce a tone at frequency f Hz, the program must toggle the speaker bit 2f times per second. For middle A (440 Hz), that's 880 toggles per second.

The Spectrum's Z80 runs at 3.5 MHz, which is 3,500,000 T-states per second. The delay between toggles for a given frequency:

  delay_t_states = 3,500,000 / (2 × frequency)

For middle A: 3,500,000 / 880 ≈ 3,977 T-states between toggles. A simple tone routine spins a delay loop for that many T-states, toggles the bit, and repeats.

The 48K ROM has a built-in tone generator at $03B5 (the BEEPER routine), called by Sinclair BASIC's BEEP command. It takes a duration and a half-period in HL/DE and produces a steady tone. It's good enough for type-in BASIC games' "ping" sound; serious music drivers write their own routines for tighter timing.

How a melody works

A monophonic melody driver maintains:

A note table — frequencies (or directly, half-periods) for each pitch in the scale.
A song — a list of (note, duration) pairs to play in sequence.
A driver loop — for each (note, duration), call the tone routine.

Most period-faithful Spectrum music drivers use a tick-based timing model: the song is in beats; each beat is N frames at 50 Hz; the driver runs from the main game's IRQ handler.

note_data:
    defb NOTE_C4,  4    ; note, duration in ticks
    defb NOTE_E4,  4
    defb NOTE_G4,  8
    defb $FF,      0    ; terminator

The driver advances through the table, plays each note for its duration, and loops back to the start.

How polyphony works

The beeper has one bit. Two voices at once should be impossible. It almost is.

The trick is interleaving. The driver maintains two (or more) virtual voices, each with its own current note and frequency. On every iteration of the toggle loop, the driver advances both voices' phase counters. Whenever a voice's phase counter overflows, the speaker bit flips. The two voices' toggle rates combine into a complex waveform that the human ear hears as two simultaneous pitches.

The cost is precision. Each voice's pitch is approximate; tight intervals (a third, a fifth) shimmer slightly. But Tim Follin, Wally Beben, and Mark Cooksey demonstrated repeatedly that 2-voice and 3-voice beeper music was musically viable, producing some of the platform's most memorable tunes.

Famous examples

Manic Miner (1983): Matthew Smith's title screen plays "In the Hall of the Mountain King" on the beeper while the game waits for the player. The first widely-heard demonstration that the Spectrum could play music, not just sound effects.
Jet Set Willy (1984): Smith again, with "If I Were a Rich Man" — a longer composition that loops in-game.
Stormlord (1989): Tim Follin pushes the beeper into 3-voice harmony with countermelody. Often cited as the most musically advanced beeper composition on the platform.
Sanxion loader (1986): Mark Cooksey's stylish beeper title music for Stavros Fasoulas's game.
Match Point (1984): The Pat Cash tennis simulator's loading screen tune by Ian Andrew.

Beeper vs AY-3-8912

The 128K Spectrum (and the +2, +3) added an AY-3-8912 sound chip with three full voices, noise generator, and envelope control. From 1986 onwards, most commercial titles supported both: 48K builds used beeper music; 128K builds used AY. Many games shipped with both versions on the same tape, and the loader detected which model was present.

The beeper-only experience continued throughout the platform's lifetime — every commercial Spectrum game had a 48K-compatible audio track, often the original score that the AY version embellished rather than replaced.

Single-channel melody on the Sinclair ZX Spectrum's one-bit speaker