POKEY makes sound, but that is only part of its job. In the Atari 8-bit computers it also serves keyboard, paddle and serial-interface functions. Doug Neubauer, who worked on its design, told ANALOG Computing that the original plan included keyboard, paddle control and sound like the Atari 2600’s. Finding room on the chip allowed him to add serial communication and improve the audio. (Lee H. Pappas, interview, October 1986, pp. 89–90.)
That participant account explains the combination better than treating POKEY as an isolated synthesiser. Its registers belong to a device with several responsibilities, and software must account for the way those responsibilities share the hardware.
Four channels, with choices
Each of the four sound channels has a frequency register and an audio-control register. The frequency value controls a divider. The control register selects volume and the tone or noise behaviour. Pseudo-random sequences supply different noise characters, useful for effects as well as notes. (De Re Atari, chapter 7.)
The divider’s resolution matters. At small divider values, changing the number by one makes a relatively large change in frequency. A pitch table can only choose from the results the hardware can produce; it cannot invent a value between two adjacent counter settings.
Finer pitch spends another counter
POKEY can join channels in pairs to make 16-bit frequency dividers. The wider value gives a finer range of choices, especially useful where an eight-bit divider’s steps are too coarse. It also means using two counters for one resulting channel. Four independent eight-bit channels and joined-channel operation are alternative arrangements of the same resources. (De Re Atari, “16-Bit Frequency Options”.)
The clock selection is another part of the sound. The programmed divider and its input clock together determine the result. Quoting only a frequency-register byte without the control settings leaves out part of the instrument’s setup.
A volume-only setting also lets software change the output level directly. Producing a waveform by repeated writes consumes processor time, unlike leaving a hardware divider to run. The book shows that cost with a BASIC loop that spends its time changing volume yet produces only a low buzz.
The machine still matters
Neubauer’s interview describes a serial port initially intended for a cassette recorder and the loss of a proposed parallel interface. That history concerns the computer’s complete I/O design. It does not mean sound instructions alone explain the speed of every peripheral.
POKEY also appears in the Atari 5200. A shared chip gives the machines related facilities, but controller wiring, memory maps and system software still differ. The ANTIC and GTIA entry covers the separate hardware responsible for the Atari computer’s display; POKEY does not draw its pixels.
Sources
- Atari, De Re Atari, 1982, chapter 7: sound hardware and programming trade-offs. Original book transcription and scan.
- Lee H. Pappas, “An interview with Doug Neubauer”, ANALOG Computing 47, October 1986, pp. 89–90: a participant’s account of POKEY’s design. Original issue scan.