Re: Cinematronics/Sega universal sound board...

On Thu, 23 Apr 1998 13:41:22 -0700, Clay Cowgill <ClayC@diamondmm.com> wrote:

>I had a revelation yesterday...
>
>There's an off-the-shelf part that would fit 99% of our requirements for
>making a universal sound board for Cinematronics and Sega G80
>multigames.
>
>It's already been used on tons of arcade game boards-- the OKI 6295.
>
>Very neat little gizmo. It's an ADPCM playback device that supports up
>to 127 "phrases" stored in an external ROM. 4 bit ADPCM with rates up
>to 32KHz. Just over 16 seconds of playback at that speed from an
>external 27C020. It's about what we're "wishing" for-- four channels,
>low parts count (ROM + 6295 + little micro to bridge interfaces),
>compression, automatic playback...
>
>So here's the cool part. You write a two 8 bit words to the device.
>This tells it what "phrase" to play, on what channel, and how loud (16
>steps). You can have up to 127 phrases per ROM, and can bank-select
>ROMs for more. (All sounds played simultaneously would have to be in
>the same bank.)
>
>The first 0x3ff bytes of the ROM are a phrase table (8 bytes per
>phrase). If you tell the chip to play "Phrase 3" it goes to address
>0x010 and grabs 8 bytes-- the first three are the start address of the
>sound, the next three are the ending address, the other two bytes do
>something I don't recall off the top of my head.

Looping maybe?

>Then you're done. The
>chip does the rest-- fetches samples, decodes, plays the sound.
>
>For multivoice polyphony you just send it more "Phrase" addresses
>assigned to different channels.
>
>If you wanted more than 4 voices, just use 2 of 'em.
>
>We'd just put a little uController (like a 2051 or Z8 or Pic or
>whatever) in front of the 6295 to receive the sound commands from the
>game hardware and have it do the decode to trigger playback of the
>samples. Pretty clean solution.
>
>Some trap code with the emulators should be able to give us an un-biased
>assessment of how many simultaneous sounds are active at any given time.
>Long-term repeating sounds are possible (but not optimal for the
>technique), but the rest should be a walk-in-the-park.

Sounds nice, most everything in one package! ADPCM also works well with music
and sound effects (as opposed to some of the talking chips specialized for voice
that use LPC, GSM, etc).

They only thing that I would see as a pain would be for sounds that ramp up/down
in frequency. Can the different channels have there sample rates individually
controlled? Maybe that's one of the control words? Something like this will be
needed to emulated some of the frequency changes needed in things like the
background throb of Star Castle.

Many sounds are just square waves that could possibly be controlled directly be
the processor. But it would be nicer to have them play through the OKI chip for
volume control. Once again if music will need to be played (4096 tones for
Boxing Bugs) we'll need to be able to control the playback sample rate.

-Zonn

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Received on Thu Apr 23 14:54:07 1998