Sinclair ZX SpectrumAudioIntermediate

AY Envelopes and Noise

Let the AY shape a note's volume in hardware and add its noise channel — a decaying pluck from the envelope generator, an explosion from noise, each fired with a single register write.

aypsgenvelopenoiseaudio

Overview

A flat tone at a fixed volume is a starting point; a sound moves. The AY has two things to move it: an envelope generator that sweeps a channel’s volume through a chosen shape, and a noise generator of pseudo-random hiss for anything that isn’t a pitch — drums, explosions, wind. Point a channel’s volume at the envelope, pick a shape, and the chip fades or repeats it for you; enable noise in the mixer and a channel plays hiss instead of, or alongside, its tone. Each effect is a handful of register writes and then the chip runs it.

Code

; =============================================================================
; AY ENVELOPES AND NOISE - ZX SPECTRUM 128K
; A plucked tone (envelope-shaped) and an explosion (noise + envelope)
; =============================================================================

R_NOISE  equ 6            ; noise period (5 bits) — timbre of the hiss
R_MIXER  equ 7            ; tone / noise enables (ACTIVE LOW)
R_A_VOL  equ 8            ; channel A volume  (bit 4 = follow envelope)
R_C_VOL  equ 10           ; channel C volume
R_ENV_LO equ 11           ; envelope period, low byte
R_ENV_HI equ 12           ; envelope period, high byte
R_ENV_SH equ 13           ; envelope SHAPE — writing it retriggers the envelope

; A plucked note: channel A's volume follows a one-shot decay
ay_pluck:
    ld   a,R_ENV_LO
    ld   e,$00
    call ay_out
    ld   a,R_ENV_HI
    ld   e,$10             ; envelope period -> how long the decay lasts
    call ay_out

    ld   a,R_A_VOL
    ld   e,%00010000       ; bit 4 set: volume tracks the envelope, not a number
    call ay_out

    ld   a,R_ENV_SH
    ld   e,$00             ; shape %0000 = \___  (fall once, hold silent)
    call ay_out            ; the write itself fires the envelope
    ret

; An explosion: channel C plays noise, shaped by the same decay
ay_explode:
    ld   a,R_NOISE
    ld   e,$10             ; noise period — lower = deeper rumble
    call ay_out

    ld   a,R_MIXER
    ld   e,%00011111       ; noise C ON (bit 5 = 0); all tones off
    call ay_out

    ld   a,R_C_VOL
    ld   e,%00010000       ; channel C volume follows the envelope
    call ay_out

    ld   a,R_ENV_SH
    ld   e,$00             ; one-shot decay again
    call ay_out
    ret

; A = register, E = value
ay_out:
    ld   bc,$fffd
    out  (c),a
    ld   b,$bf
    out  (c),e
    ret

Trade-offs

Aspect Cost
CPU None after the trigger — the chip runs the shape
Memory A short routine per effect
Limitation One envelope generator, shared by all three channels

When to use: Plucked and percussive sounds — anything that fades, and any drum or explosion.

When to avoid: A sustained held tone; that’s just a fixed volume, no envelope needed.

The envelope shape

Register 13 is a 4-bit shape, and writing it restarts the envelope from the top. The four bits — Continue, Attack, Alternate, Hold — combine into the classic set:

Value Shape Sounds like
$00–$03 \___ a pluck — fall once, stay silent
$08 ||| a repeating saw-down — a buzz or drone
$0A /\/\ a rising-falling wobble
$0C /|/| a repeating ramp-up
$0D /‾‾‾ swell up, then hold loud

The envelope period (registers 11–12) sets how long a shape takes — a short period for a snappy pluck, a long one for a slow swell or an audible LFO-style wobble on the repeating shapes.

One envelope, three channels

The catch every AY musician hits: there is a single envelope generator, and every channel that sets its volume bit 4 follows the same shape at the same time. You cannot pluck channel A while channel B swells — not in hardware. Drivers work around it two ways: reserve the envelope for one channel (often the bass or the drums) and give the others fixed volumes, or drop the hardware envelope entirely and write each channel’s volume by hand every frame — a software envelope, which the music player sets up next.

Noise, and mixing it in

The noise generator is a single pseudo-random source shared by all channels; register 6’s 5-bit period sets how fast it runs — low for a deep rumble, high for a bright hiss. In the mixer, a channel can take tone, noise, or both: clear a channel’s tone bit and its noise bit and you get a pitched note with a rough edge — the standard recipe for a snare or a hi-hat.

Patterns: AY Tone

Vault: AY-3-8912 | ZX Spectrum