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Appendix 5 Sound System

This is Appendix 5 of the SD-8516 Programmer's Reference Guide.

The SD-8516 is paired with the SD-450 sound subsystem; named for featuring 4 independent voices with 5 waveforms available, each with a programmable ADSR envelope.

Voice Architecture

Each voice occupies 16 bytes of memory in Bank 1:

Offset	Register	Description
+$00	FREQ_LO	Frequency low byte
+$01	FREQ_HI	Frequency high byte
+$02	GATE	Waveform/gate control
+$03	VOLUME	Volume (0-255)
+$04	ATTACK	Attack time
+$05	DECAY	Decay time
+$06	SUSTAIN	Sustain level
+$07	RELEASE	Release time
+$08	DATA1	Pulse width / noise type
+$09-$0F	Reserved	Future expansion

Voice base addresses:

Voice 0: $1ED00
Voice 1: $1ED10
Voice 2: $1ED20
Voice 3: $1ED30

Waveforms

Gate register values:

0: Silent (gate off)
1: Square wave
2: Triangle wave
3: Sawtooth wave
4: Sine wave
5: Pulse wave (variable width via DATA1)
6: White/pink/brown noise (type via DATA1)

ADSR Envelope

The Attack-Decay-Sustain-Release envelope shapes each note:

Attack: Time to reach peak volume (0-255 × 10ms)
Decay: Time to decay to sustain level (0-255 × 10ms)
Sustain: Held volume level (0.0-1.0 of peak)
Release: Time to fade to silence after gate off (0-255 × 10ms)

Example:

; Play middle C on voice 0
    LDA $112B           ; C4 frequency (262 Hz / 0.0596)
    STA [$1ED00]         ; FREQ_LO/HI
    LDAL $01            ; Square wave
    STAL [$1ED02]        ; GATE
    LDAL $4D            ; ~30% volume
    STAL [$1ED03]        ; VOLUME

Sound System Memory Map

Also see Appendix 3 Memory Map.

Sound System ($01EF80–$01EFBF) – 64 bytes total

Address Range	Size	Symbol / Register	Description
$01EF80–$01EF8F	16 bytes	SOUND0_BASE	Voice 0
$01EF80	1 byte	SOUND0_FREQ_LO	Frequency low byte (bits 7–0)
$01EF81	1 byte	SOUND0_FREQ_HI	Frequency high byte (bits 15–8)
$01EF82	1 byte	SOUND0_GATE	Gate / Waveform select (gate bit + waveform type: noise, pulse, saw, triangle, etc.)
$01EF83	1 byte	SOUND0_VOLUME	Master volume for voice 0 (usually 0–15, may include global volume in some implementations)
$01EF84	1 byte	SOUND0_ATTACK	Attack rate (0–15)
$01EF85	1 byte	SOUND0_DECAY	Decay rate (0–15)
$01EF86	1 byte	SOUND0_SUSTAIN	Sustain level (0–15)
$01EF87	1 byte	SOUND0_RELEASE	Release rate (0–15)
$01EF88	1 byte	SOUND0_DATA1	Voice-specific control / extra parameter 1 (e.g. pulse width low, filter routing, etc.)
$01EF89	1 byte	SOUND0_DATA2	Voice-specific control / extra parameter 2 (e.g. pulse width high, ring/mod flags, etc.)
$01EF8A–$01EF8F	6 bytes	—	Reserved / unused / future expansion for Voice 0

Address Range	Size	Symbol / Register	Description
$01EF90–$01EF9F	16 bytes	SOUND1_BASE	Voice 1
$01EF90	1 byte	SOUND1_FREQ_LO	Frequency low byte
$01EF91	1 byte	SOUND1_FREQ_HI	Frequency high byte
$01EF92	1 byte	SOUND1_GATE	Gate / Waveform select
$01EF93	1 byte	SOUND1_VOLUME	Volume for voice 1
$01EF94	1 byte	SOUND1_ATTACK	Attack rate
$01EF95	1 byte	SOUND1_DECAY	Decay rate
$01EF96	1 byte	SOUND1_SUSTAIN	Sustain level
$01EF97	1 byte	SOUND1_RELEASE	Release rate
$01EF98	1 byte	SOUND1_DATA1	Extra control 1
$01EF99	1 byte	SOUND1_DATA2	Extra control 2
$01EF9A–$01EF9F	6 bytes	—	Reserved / unused / future expansion for Voice 1

Address Range	Size	Symbol / Register	Description
$01EFA0–$01EFAF	16 bytes	SOUND2_BASE	Voice 2
$01EFA0	1 byte	SOUND2_FREQ_LO	Frequency low byte
$01EFA1	1 byte	SOUND2_FREQ_HI	Frequency high byte
$01EFA2	1 byte	SOUND2_GATE	Gate / Waveform select
$01EFA3	1 byte	SOUND2_VOLUME	Volume for voice 2
$01EFA4	1 byte	SOUND2_ATTACK	Attack rate
$01EFA5	1 byte	SOUND2_DECAY	Decay rate
$01EFA6	1 byte	SOUND2_SUSTAIN	Sustain level
$01EFA7	1 byte	SOUND2_RELEASE	Release rate
$01EFA8	1 byte	SOUND2_DATA1	Extra control 1
$01EFA9	1 byte	SOUND2_DATA2	Extra control 2
$01EFAA–$01EFAF	6 bytes	—	Reserved / unused / future expansion for Voice 2

Address Range	Size	Symbol / Register	Description
$01EFB0–$01EFBF	16 bytes	SOUND3_BASE	Voice 3
$01EFB0	1 byte	SOUND3_FREQ_LO	Frequency low byte
$01EFB1	1 byte	SOUND3_FREQ_HI	Frequency high byte
$01EFB2	1 byte	SOUND3_GATE	Gate / Waveform select
$01EFB3	1 byte	SOUND3_VOLUME	Volume for voice 3
$01EFB4	1 byte	SOUND3_ATTACK	Attack rate
$01EFB5	1 byte	SOUND3_DECAY	Decay rate
$01EFB6	1 byte	SOUND3_SUSTAIN	Sustain level
$01EFB7	1 byte	SOUND3_RELEASE	Release rate
$01EFB8	1 byte	SOUND3_DATA1	Extra control 1
$01EFB9	1 byte	SOUND3_DATA2	Extra control 2
$01EFBA–$01EFBF	6 bytes	—	Reserved / unused / future expansion for Voice 3

Appledog

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