The 1981 arcade classic Defender by Williams was a groundbreaking game in many ways. It had some of the most complex controls for any game at the time, with its two-way joystick, and 5 buttons. It was more difficult to learn than most arcade games, but once mastered, it was a very fluid and dynamic experience. It scrolled with variable speed in two directions - at its fastest very tricky to control, and it had a mini-map with enemy and human locations, a key feature for playing the game well.
The game had a unique sound design, brutal digital synthesis emulating laser fire, explosions, and strange alien voices. Only one voice at a time, but every sound was as intense as it could possibly be. The sound design and hardware came straight from Williams' pinball machines, where the tradition was to be as loud and noticeable as possible.
The hardware of Defender
This article is about the hardware of the Defender arcade machine from 1981, with emphasis on the sound board, which was originally designed by Eugene Jarvis for a pinball machine.
The chips that make up the sound hardware and how they work together is described, and
finally, there is a short analysis of the theoretical limits of the sound hardware in terms of sample rate and waveform duration, and how these constraints motivate custom sound algorithms over sample playback.
MC6802 die detail (photo by Vintage Teardown)
Defender Sound ROM Disassembly
In this article, we will look into the audio software of Defender, 2 KB of MC6800 machine code located in a ROM chip. This code generates all the different sounds heard in the game. We will disassemble the sound ROM so we can inspect it in assembly code form. The disassembled code with some annotation is available for download. After disassembling the ROM, we will reassemble it and check if it is exactly the same as the original, verifying the correctness of our disassembly.
Defender sound ROM