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The AY-3-8910 is a 3-voice programmable sound generator (PSG) designed by General Instrument, initially for use with their 16-bit CP1610 or one of the PIC1650 series of 8-bit microcomputers. The AY-3-8910 and its variants became popular chips in many arcade games, and were used on, among others, the Intellivision and Vectrex video game consoles, Amstrad CPC, Oric 1, Colour Genie, Elektor TV Games Computer and Sinclair ZX Spectrum 128/+2/+3 home computers as well as the Mockingboard and Cricket sound cards for the Apple II family. It was also produced under license by Yamaha (with minor modifications, i.e. a selectable clock divider pin, and a double-resolution but double-rate volume envelope table) as the YM2149F; the Atari ST uses this version. It produced very similar results to the Texas Instruments SN76489 and was on the market for a similar period. After General Instrument's spinoff of Microchip Technology in 1987, the chip was produced for a few years under the Microchip Technology brand instead. The chips are no longer made, but a declining stock is still obtainable for servicing vintage machines. A VHDL equivalent description has been written, for use in FPGA recreations of arcade machines and others like those mentioned above. The VHDL source code is available on the Internet, and compiles to fill about 10% of a Xilinx XC2S300 FPGA. == Description == The AY-3-8910 was essentially a state machine, with the state being set up in a series of sixteen 8-bit registers. These were programmed over an 8-bit bus that was used both for addressing and data by toggling one of the external pins. For instance, a typical setup cycle would put the bus into "address mode" to select a register, and then switch to "data mode" to set the contents of that register. This bus was implemented natively on GI's own CPUs, but it had to be recreated in glue logic or with the help of an additional interface adapter such as the MOS Technology 6522 when the chip was used with the much more common MOS Technology 6502 or Zilog Z80 CPUs. Six registers controlled the pitches produced in the three primary channels. The wavelength to generate was held in two eight-bit registers dedicated to each channel, but the value was limited to 12-bits for other reasons, for a total of 4095 (the register value is used as the frequency divider and 0 is treated as 1) different pitches. Another register controlled the period of a pseudo-random noise generator, while another controlled the mixing of this noise into the three primary channels. Three additional registers controlled the volume of the channels, as well as turning on or off the optional envelope controls on them. Finally the last three registers controlled the times of the envelope controller, by setting the envelope type and envelope cycle time. Envelope types include sawtooth shape or triangle shape, starting on either maximum or minimum. The shape can also be set to repeat for a cycling effect. As there was only one envelope shared between all three channels, many programmers ignored it and programmed their own envelope controllers in software (controlling volume directly). A well known trick was to run the hardware envelope at cycle times above 20Hz to produce sawtooth or pulse-wave like bass sounds. Although there are only 16 registers, the four MSB bits of the 8-bit bus must be set to the factory default '0000' value when selecting a register. Incorrectly setting the MSB bits causes the chip to ignore the register change. General Instruments did take orders for customized MSB bits (factory set to other than '0000'). The chips made with customize-set MSB register bits allow the same processor to control more than one AY chip on the same bus (e.g. Mockingboard sound card). There are many new-old-stock (NOS) chips on the secondary market with MSB bits factory set to a non-'0000' value. The non-0000 value can cause significant developmental troubles for designers and repair technicians. Software must be written to identify the correct value of the MSB bits on any given chip. Also, software must be changed or hardware added to allow these factory set MSB chips to be used in place of the default '0000' chips. The AY-3-8910 generates tones with base frequencies of up to 125 kHz (4 MHz input clock, or 8 MHz with the YM2149F), well beyond human perception and into the ultrasonic range. This is not as wasteful as it may first appear, however, as it offers a finer frequency resolution than would otherwise be available under the frequency-divider scheme whilst only sacrificing a few of the total 4096 values; dividers of 6 thru 9 imply frequencies from a borderline-ultrasonic 20.8 kHz down to a decidedly audible 13.9 kHz. Frequencies equivalent to the top octave of a piano keyboard can be defined with reasonable accuracy versus the accepted note values for even-tempered scale, to nearly 1 Hz precision in the A440 range, and even more finely at lower pitches. Despite the high maximum frequency, the ability to divide that figure by 4096 means the lowest directly definable output frequency, is 30.6 Hz, roughly equal to B0, the third lowest note on a normal 88-key piano, and as good as subsonic with everyday speaker systems. In essence, the chip is able produce decently musical output at all reasonable pitches found in most compositions. By contrast, the SN76489 only has 10 bits of precision for its frequency dividers. Having the same base frequency of 125 kHz, it should in theory lack the two lowest octaves of the PSG. To get around this, the SN76489 plays its tone generators one octave lower than their calculated frequency, giving it one octave less in the bass and one octave less in the top compared to the PSG. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「General Instrument AY-3-8910」の詳細全文を読む スポンサード リンク
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