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Digital Audio from a Yamaha OPL2/3 chip

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First post, by Cloudschatze

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I posted this on the QuestStudios forum, but thought it might interest some of the VOGONS crowd as well.

Well, this took me by surprise.

Yesterday evening I was doing some comparisons between the SoundBlaster Pro 2 and Pro AudioStudio, and happened to load a game given me by my brother-in-law, which I thought had SB support (the box has an "AdLib-Supported" sticker) -

Capstone's "Bill and Ted's Excellent Adventure."

Radical, huh?

...

Anyway, the game begins with a looping piece, which I initially thought was a combination of an FM melody with PCM drum samples. Desiring to test this theory, I muted the FM volume - nothing played. Restoring the FM volume, I then muted the SB DAC - everything played.

I performed the same test on the PAS16 card in a separate PC. With the SB-compatible hardware completely disabled, it produced the same result - PCM samples (including vocal snippets, sfx) are being played through the OPL chip.

I've done some Google searching, and have found very little supportive evidence to back this up, though I did find two claims that "Space Ace" does the same.

Just thought I'd share. If there are any other games which play digital audio through the OPL, I would love to hear about them.

Additionally, I would be very interested to know if the CSM (Composite Sinusoidal Modeling) mode of the OPL2 chip has ever been utilized.

Reply 2 of 3, by Lord Nightmare

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The games in the pinball dreams series are also capable of playing digital audio via the opl2/opl3.
All in all, its actually a fairly elaborate hack:
You reset the OPL chip, then use the timer to count out exactly the length of time it takes for the sine envelope to reach the peak of the wave (1/4 of the way through the full waveform)
Once there, set the frequency to exactly 0, and the clock for the waveform will freeze with its output stuck at the maximum.
Finally, send a stream of audio data to the main volume control register for the channel being played, hence using it as a "poor-man's-DAC" of sorts.
Theres at least one textfile floating around explaining how to do all this.

Lord Nightmare
lord(underscore)nightmare @t users d0t sf d0t net

Reply 3 of 3, by 5u3

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Cool. I have always wondered how they pulled off this little trick. Now I can imagine how it works.

Another game that uses PCM on AdLib: Lemmings.
I'm not entirely sure about this, but in the "special levels" (Fun22, Tricky14, Taxing15, Mayhem22) the music sounds very different, almost like the typical sound of a simple LPT-DAC or samples played via the PC speaker.

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