General Instrument AY-3-8912

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The AY-3-8912 was a 3-voice sound chip designed by General Instrument. Yamaha also produced the design under license as the YM2149. The 8912 was a popular chip in many arcade games, and was used on, among others, the Intellivision and Vectrex video game consoles and the MSX, Atari ST, Amstrad CPC and Sinclair ZX Spectrum+ 128 home computers as well as the Mockingboard sound card for the Apple II family.

It produced very similar results to the Texas Instruments SN76489 and was on the market for a similar period.

Contents

Chip variants

The 8912 is one of three related designs: the AY-3-8910 included two general-purpose 8-bit parallel I/O ports, the 8912 had one port, and the AY-3-8913 had none. Originally intended as a "cut down" version of the 8910, the 8912 removed one port to reduce the pin count by 12, and thus cost of implementation. The 8913 reduced it only another four pins, making it not that much more attractive. While even the single port on the 8912 seems uneeded in most applications, it was nevertheless the most popular version by far.

Description

The 8912 was register-based, with a set of sixteen registers controlling the sound output, programmed using a set of eight data/address pins that were controlled by the CPU (the data/address mode controlled by a pin on the chip). Six registers controlled the tones produced in the three channels, two eight-bit registers per channel but limited to 12-bits for other reasons, for a total of 4096 different tones. Another register controlled the period of a pseudo-random noise generator, with another controlling the mixing of this noise into the three channels. Three additional registers controlled the volume of the channels, as well as turning on or off the option envelope controls on them. Finally the last three registers controlled the times of the ADSR envelope controller, by setting the lengths of time for each stage of the cycle. Unlike most systems the 8912 used fixed times for the sustain and release, and a repeatable attack and decay pattern. For instance, the system could repeat the AD cycle of the sound over and over, or alternately invert it, starting loud and reducing to the sustain level without any attack phase.

Creative uses

Although the chip wasn't designed to handle raw PCM data (digital samples), the effect could be simulated. The chip used a simple OR based mixing function for combining noise and tone on its three channels and could be persuaded to produce a level non-zero wave. By altering the volume this level wave could be shaped into a waveform. Obviously, this involved more CPU usage than chips designed for this purpose (such as the Commodore Amiga's 8364 "Paula" sound chip), but it was nevertheless a technique widely used on platforms such as the Atari ST to play sampled music, and on the Amstrad CPC to play short audio samples, in some games.

Doing the same thing less times per second (in the order of a hundred per second) can replace the limited envelope functionality (any envelope you can think of), and last but not least works with each of the three channels individually. This takes negligible cpu power (provided there is some timer interrupt or vertical blank interrupt) and can be used in games.

In turn, the now useles envelope functionality can be set to very high frequency, actually generating a waveform that is not the usual squarewave. The granularity by which high frequencies can be set however is low (one may not be usable to play music, the author hasn't tested this).

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