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84 lines
3.1 KiB
TeX
84 lines
3.1 KiB
TeX
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If you are making a game or a demo for the old Apple II, you will want
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some sound.
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Having a built-in speaker was a nice feature in 1977 when the Apple II
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first launched, but your output audio is a bit limited as it is driven
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by a flip-flop that you need to bitbang (and there are no timers or
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interrupt sources to help like there was on the IBM PC).
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(Note, we are discussing the first few generations of Apple II products here,
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the IIgs which came out in 1987(?) had a built-in advanced sound chip
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but that is another story for another day).
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Despite that you can do some interesting sound, including sampled audio
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(that would take up a good fraction of a flopp disk, like in the game
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X) or even two-channel ``Electric Duet'' (which used up most of the CPU
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power leaving not much time for anything else).
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The Apple II did have 7 expansion slots, and so many sound boards were
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made for it. None became a standard though, so only a small fraction
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of games released for the platform.
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One of the more common was the Mockingboard.
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A Mockingboard card has two AY-3-8910 sound chips on it, and optionally
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had speech chips (type). You will often find empty sockets there as they
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are xpensive these days.
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The AY-3-8910 was common at the time, made by X who went on to become
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Microchip (of PIC fame).
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The chip was found in many arcade cabinets, as well as
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the Atari ST and some models of ZX Spectrum.
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It can make three channels of square waves, with noise applied, and
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one hadrware evenlope.
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It can make for some nice sound in skilled hands, although the Commodore
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readers are probably already scoffing in comparison to their SID chips
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(and we won't even start with the Amiga fans).
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Programming the AY-3-8910 is fairly straightforward, the diagram is in Figure
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X. There are 16 registers, but the top two are only used for GPIO lines
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on the earlier 40-pin versions of the chip.
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You have registers for the audio period of channel A, B, and C, 12-bits.
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Then a 4-bit amplitude setting for each channel (logarithmic).
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Then a noise channel, a mixer to pick what noise/audio channels are playing.
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Then the preiod and type for the hardware envelope.
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Writing the registers is straightforward, using three wires with a protocol
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easy enough to do in small hacking projects. Typically with some shift
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registers and SPI.
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The Mockingboard has two AY-3-8910 chips (allowing 6-channels, split
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left/right stereo). It uses two 6522 I/O chips to interface with the
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Apple II bus, and they also provide a timer interrupt source (something
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a stock Apple II lacks).
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One complication is frequency. The Apple II drives it at 1MHz, but for
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example the ZX runs it at 1.77MHz, so the frequencies will be different
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unless you translate before hand.
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TODO: diagram of chip
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Related Work/Earlier
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French-touch
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guy playing on Apple II with z80
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YM5 files, limitation
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Vortex Tracker format
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By a Russian, russian docs, Delphi/Pascal.
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Research
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First to C on Raspberry Pi
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Validation against AY_emul
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Then to 6502 assembly
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Decode to screen using COUT, re-direct to "printer" on emulator,
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take text output and diff against proper output with Linux tool.
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Final Output
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