Exploring Sound Capabilities of the ZX Spectrum: Beeper and AY-3-8910 Synthesizer

ZX Spectrum Sound Hardware Overview

The ZX Spectrum series provides two primary options for sound generation. All models include a 1-bit beeper, which operates similarly to sound systems found in the Apple II or PC speaker. Later models, starting from the 128K Spectrum, also incorporate a General Instrument AY-3-8910 synthesizer chip. This chip is comparable to the one used in the Atari ST, though it runs at a different clock speed and lacks a built-in BIOS interface for communication.

Utilizing the 1-bit Beeper via BIOS

The 1-bit beeper functions by toggling a specific bit on the I/O port ($10) to control the speaker's status. Precise timing of these toggles generates different tones. While direct programming is possible, the ZX Spectrum's BASIC BEEP command simplifies this process by calling a generic routine. This routine, BEEPER, is located at memory address $03B5 and handles the timing for sound output.

BEEPER Routine and BEEP Command Mechanics

The BEEPER routine uses the HL register to specify delay times in units of 4 cycles, achieving high precision for tone generation. The DE register defines the duration of a note by specifying the number of wavelengths. The BASIC BEEP command, found at $03F8, translates user-friendly inputs (notes in half-steps from middle C and durations in seconds) into the values required by the BEEPER routine, often utilizing the system's floating-point engine for calculations.