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NAUG Ch21 — Sound / Speech

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NAUG Ch21 — Sound and Speech

Holmes & Dickens, The New Advanced User Guide, pp.379-386. The BBC’s TI SN76489 sound generator (3 tone channels + 1 noise) sits on the slow peripheral bus, driven by the System VIA’s Port A + addressable-latch line 0. Speech (Model B / B+) is a separate optional chip.

Sound system overview

  • 76489 sound chip: 3 tone generators + 1 white-noise generator, mixed on-chip. Volume per channel (0-15, 0 = max, 15 = off).
  • MOS provides a high-level SOUND/ENVELOPE interface via OSWORDs that enqueues sound events into the 4 channel buffers (IDs 4-7, see buffers).
  • The 100 Hz IRQ (System VIA T1, interrupts) services those buffers — pulls the next byte, applies envelope, writes to the chip.
  • For maximum control / minimum overhead, write the chip directly via the slow peripheral bus. NAUG gives a worked example (§21.3 p378).

MOS interface

OSWORD &07 — SOUND

Equivalent to BASIC’s SOUND chan, amp, pitch, dur. 8-byte parameter block:

OffsetBytesField
0-12Channel
2-32Amplitude
4-52Pitch
6-72Duration

All four parameters are 16-bit little-endian, even though the underlying chip only takes 4-bit amplitudes — the high bits select special behaviour (envelope, sync-channels, etc.).

OSWORD &08 — ENVELOPE

14-byte parameter block — the 14 envelope parameters in the same order as BASIC’s ENVELOPE command.

Suppression / status OSBYTEs

OSBYTEFunctionNotes
&D2 (210)R/W sound suppressionnon-zero = sound disabled
&D1 (209)R/W speech suppression&50 = SPEAK opcode (enabled, default), &20 = NOP (disabled)
&EB (235)R/W speech-present flag0 = absent, &FF = present
&D3-&D6R/W BELL parameterschannel / amp / freq / duration (CTRL-G sound)
&18 (24)Select external sound (Electron only)—
&E8 (232)R/W external sound semaphore (Electron only)—
&74 (116)Reset Electron sound systemElectron only; no-op on B+/Master; error on Model B
&9E/&9FRead/write speech processor (Model B only)direct chip access

The SN76489 — chip-level reference

Register addressing (3-bit field within each command byte)

R2 R1 R0Register
0 0 0Tone 3 frequency
0 0 1Tone 3 volume
0 1 0Tone 2 frequency
0 1 1Tone 2 volume
1 0 0Tone 1 frequency
1 0 1Tone 1 volume
1 1 0Noise control
1 1 1Noise volume

NAUG calls the tone generators “Tone 1/2/3” and these map to BBC channels 1, 2, 3. Channel 0 in BBC numbering = noise.

Tone frequency

10-bit divider. Frequency = 4,000,000 / (32 × N) = 125000 / N Hz where N is the 10-bit value.

Examples:

  • N=1: 125 kHz (inaudible, max).
  • N=239: 523 Hz (middle C, approx).
  • N=1023: 122 Hz (low).
  • N=0 treated as N=1024 by some 76489 variants; safer to never write 0.

Volume (4 bits)

A3 A2 A1 A0Attenuation
00000 dB (max, “15”)
0001-2 dB
…(-2 dB per step)
1110-28 dB
1111OFF

BBC convention is inverted: in OSWORD &07 amplitude &FFxx to &FF00 = -15 to 0; amplitude 0 = silent. The MOS converts BBC amplitude to chip attenuation bits.

Noise generator

BitField
FB0 = periodic noise, 1 = white noise
NF1, NF0Base frequency: 00=low, 01=med, 10=high, 11 = tone 1’s frequency

Byte formats sent to chip

Latch byte (selects register + carries low data nibble):

bit:  7   6   5   4   3   2   1   0
      1   R2  R1  R0  d3  d2  d1  d0

bit 7 = 1 marks it as a latch byte (the chip uses this to distinguish from data bytes).

Data byte (extends previous frequency write with high bits):

bit:  7   6   5   4   3   2   1   0
      0   X   F9  F8  F7  F6  F5  F4

bit 7 = 0 marks it as a data byte. After writing a frequency latch byte, you may send any number of data bytes to update the high bits of the same channel’s frequency without re-latching.

Noise source byte:

bit:  7   6   5   4   3   2   1   0
      1   R2  R1  R0  X   FB  NF1 NF0

Volume update:

bit:  7   6   5   4   3   2   1   0
      1   R2  R1  R0  A3  A2  A1  A0

Direct chip write — the slow-bus dance (NAUG §21.3 p378)

The 76489 isn’t on the 6502 bus — it hangs off the System VIA’s slow peripheral bus (Port A) with /WE driven by System VIA addressable-latch line 0 (system-via).

Sequence to write one byte:

SEI                  ; MOS uses the slow bus constantly — own it
                     ; (also: keyboard scan, speech, CMOS on Master)
; 1. Set DDRA = &FF (Port A all output)
LDA #&FF : STA &FE43
; 2. Put data byte on Port A
LDA #data : STA &FE41
; 3. Pulse sound /WE low (latch line 0 = 0)
LDA #0   : STA &FE40    ; PB3 (data bit) = 0, PB0-2 (latch addr) = 000 → line 0 goes low
; 4. Hold low for ≥8 µs — the chip needs the pulse width
LDX #4
.delay  DEX : BNE delay  ; ~8 µs at 2 MHz (5c per iteration × 4 ≈ 20c = 10 µs — comfortable)
; 5. Pulse /WE high again
LDA #8   : STA &FE40    ; PB3 = 1, latch line 0 → 1 (chip /WE deasserted)
CLI

NAUG’s example uses OSBYTE &97 (write SHEILA) to do steps 1-5 — slower per write but Tube-safe.

The 8 µs hold time is a chip requirement, not a 6502 timing constraint. At 2 MHz CPU = 16 cycles. A tight DEX:BNE loop or even a few NOPs cover it.

Filed into

  • sn76489 — Chip reference: registers, frequency math, byte formats.
  • sound — MOS sound interface: OSWORDs, suppression OSBYTEs, BELL.
  • Updates: system-via slow-bus section — already documents the dance; cross-link added.
  • Updates: buffers — sound channel buffers 4-7 referenced.

Open follow-ups

  • ENVELOPE format detail — NAUG just says “14 bytes matching BASIC ENVELOPE”. The 14-parameter breakdown is in the User Guide / BASIC manual; capture if it becomes relevant for fast envelope work.
  • OSWORD &07 channel encoding — high byte of the 2-byte channel field controls sync-channel and “queue or flush” behaviour. Acorn docs the encoding (&00xx-&FFxx semantics). Worth filing if writing music engines.
  • Speech chip (TMS5220-family) — Ch21 §21.4 mentions OSBYTE &9E/&9F + the SOUND &FFxx channel encoding triggers speech. Detailed protocol is in the Speech System User Guide, not this book.
  • Master vs Model B sound timing — Master’s 65C12 BCD penalty (6502) may affect tight MOS sound-IRQ loops by a few cycles; investigate if envelope timing differs perceptibly.

This wiki is curated by Claude following the LLM-Wiki methodology — a human curates source documents, the LLM compiles structured cross-linked markdown. Content may contain errors, omissions, or stale claims. For authoritative information refer to the original source documents in the bbc-documents GitHub archive.

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