mirror of
https://github.com/deater/dos33fsprogs.git
synced 2024-11-09 07:09:55 +00:00
84 lines
3.1 KiB
TeX
84 lines
3.1 KiB
TeX
|
|
|
|
If you are making a game or a demo for the old Apple II, you will want
|
|
some sound.
|
|
|
|
Having a built-in speaker was a nice feature in 1977 when the Apple II
|
|
first launched, but your output audio is a bit limited as it is driven
|
|
by a flip-flop that you need to bitbang (and there are no timers or
|
|
interrupt sources to help like there was on the IBM PC).
|
|
(Note, we are discussing the first few generations of Apple II products here,
|
|
the IIgs which came out in 1987(?) had a built-in advanced sound chip
|
|
but that is another story for another day).
|
|
|
|
Despite that you can do some interesting sound, including sampled audio
|
|
(that would take up a good fraction of a flopp disk, like in the game
|
|
X) or even two-channel ``Electric Duet'' (which used up most of the CPU
|
|
power leaving not much time for anything else).
|
|
|
|
The Apple II did have 7 expansion slots, and so many sound boards were
|
|
made for it. None became a standard though, so only a small fraction
|
|
of games released for the platform.
|
|
One of the more common was the Mockingboard.
|
|
|
|
A Mockingboard card has two AY-3-8910 sound chips on it, and optionally
|
|
had speech chips (type). You will often find empty sockets there as they
|
|
are xpensive these days.
|
|
|
|
The AY-3-8910 was common at the time, made by X who went on to become
|
|
Microchip (of PIC fame).
|
|
The chip was found in many arcade cabinets, as well as
|
|
the Atari ST and some models of ZX Spectrum.
|
|
It can make three channels of square waves, with noise applied, and
|
|
one hadrware evenlope.
|
|
It can make for some nice sound in skilled hands, although the Commodore
|
|
readers are probably already scoffing in comparison to their SID chips
|
|
(and we won't even start with the Amiga fans).
|
|
|
|
Programming the AY-3-8910 is fairly straightforward, the diagram is in Figure
|
|
X. There are 16 registers, but the top two are only used for GPIO lines
|
|
on the earlier 40-pin versions of the chip.
|
|
|
|
You have registers for the audio period of channel A, B, and C, 12-bits.
|
|
Then a 4-bit amplitude setting for each channel (logarithmic).
|
|
Then a noise channel, a mixer to pick what noise/audio channels are playing.
|
|
Then the preiod and type for the hardware envelope.
|
|
|
|
Writing the registers is straightforward, using three wires with a protocol
|
|
easy enough to do in small hacking projects. Typically with some shift
|
|
registers and SPI.
|
|
The Mockingboard has two AY-3-8910 chips (allowing 6-channels, split
|
|
left/right stereo). It uses two 6522 I/O chips to interface with the
|
|
Apple II bus, and they also provide a timer interrupt source (something
|
|
a stock Apple II lacks).
|
|
|
|
One complication is frequency. The Apple II drives it at 1MHz, but for
|
|
example the ZX runs it at 1.77MHz, so the frequencies will be different
|
|
unless you translate before hand.
|
|
|
|
TODO: diagram of chip
|
|
|
|
|
|
Related Work/Earlier
|
|
|
|
French-touch
|
|
|
|
guy playing on Apple II with z80
|
|
|
|
YM5 files, limitation
|
|
|
|
Vortex Tracker format
|
|
|
|
By a Russian, russian docs, Delphi/Pascal.
|
|
|
|
|
|
Research
|
|
First to C on Raspberry Pi
|
|
Validation against AY_emul
|
|
Then to 6502 assembly
|
|
Decode to screen using COUT, re-direct to "printer" on emulator,
|
|
take text output and diff against proper output with Linux tool.
|
|
|
|
Final Output
|
|
|