Namco 3-channel Waveform Sound Generator

The sound chip used in Pacman is a 3-channel Waveform Sound Generator (WSG) custom-made by Namco. It allows up to three simultaneous voices each with independent control over its volume, frequency and waveform. All of the chip functions are controlled by the following 4-bit registers:

 Register  Description
 00h-04h  Voice #1 frequency counter
 05h  Voice #1 waveform (only 3 bits used)
 06-09h  Voice #2 frequency counter
 0Ah  Voice #2 waveform (only 3 bits used)
 0Bh-0Eh  Voice #3 frequency counter
 0Fh  Voice #3 waveform (only 3 bits used)
 10h-14h  Voice #1 frequency
 15h  Voice #1 volume
 16h-19h  Voice #2 frequency
 1Ah  Voice #2 volume
 1Bh-1Eh  Voice #3 frequency
 1Fh  Voice #3 volume

Frequencies and counters are 20-bit values stored with the least significant nibble first. Voice #2 and #3 are missing the register for the least significant nibble and it is assumed to be always zero.

These registers are usually mapped into the memory space of the CPU. In the Pacman hardware the memory locations at 5040h-505Fh map the sound registers, so for example writing a value at the address 505Ah sets the volume of voice #2.

Sound generation is based on a table that contains 8 different waveforms, where each waveform is described by 32 4-bit entries. For versatility and reuseability, this data is kept outside of the chip in a 256 byte PROM.

The chip itself is clocked at 96 KHz, which is the main CPU clock (3.072 MHz) divided by 32. At each cycle the frequency counter for each voice is incremented by the voice frequency, then the most significant 5 bits are used as an index to retrieve the current wave sample from the waveform table. The sample is then multiplied by the voice volume and sent to the amplifier for output. Note that a voice is actually muted if its volume or frequency is zero.

Copyright (c) 1997-2004 Alessandro Scotti. All rights reserved.