Wizball, That Oldskool Beat and unsupported gate 2 mode 1 observations. \ VOGONS

Wizball, That Oldskool Beat and unsupported gate 2 mode 1 observations.

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First post, by jmk

Posted on 2008-04-15, 23:40

jmk Offline

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Posts: 34
Joined: 2008-04-13, 10:50
Location: Paisley, UK.

Wizball should have a bit of PC speaker sampled audio in it, according to this:
http://www.oldskool.org/sound/pc

Here is the routine which might be trying to do it. It is using a PIT mode which has not been added into the emulation - the Hardware Retriggerable One Shot.

10F63:4463 "routine". DI = 09 (usually) SI = Source? CX = Length?
24463 cli
34464 in al,61 or al,2 out 61,al (Speaker data enable).
4446A in al,62 or al,20 out 62,al ("timer 2 channel out").
54470 in al,61 and al,FE out 61,al (Speaker data disable).
64476 mov al,92 out 43,al (PIT mode port: counter 2, counter latch, mode 1 (one shot), binary.
7447A mov al,FF out 42,al (PIT counter 2, cassette & speaker).
8447E mov dl,0
94480 push cx
104481 in al,61 or al,01 out 61,al ("timer 2 gate to speaker enable").
114487 and al,FE out 61,al ("timer 2 gate to speaker disable").
12448B mov al,[si] out 42,al -> big table of "sample timing data" (usually 1A-2E).
13448F mov cx,di
144491 loop 4491
154493 pop cx
164494 inc si
174495 loop 4480
184497 sti
194498 ret

I'm no expert on the timers and their interaction with the internal speaker... I think it may be trying some sort of pulse-width modulation?

Anyway, if someone who knows better would like to investigate, then I'd be grateful. 😉

Reply 1 of 2, by Qbix

Posted on 2008-04-16, 12:49

Qbix Offline

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Posts: 11324
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Location: Fryslan

well there is some support for mode 1 in our code. In both the PIT and the PCSpeaker.
Not sure if our emulation is compatible with this game though.

Water flows down the stream
How to ask questions the smart way!

Reply 2 of 2, by reenigne

Posted on 2008-04-16, 16:15

reenigne Offline

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Location: Cornwall, UK

I tried to implement something similar for MESS a while back, but it turns out to be quite difficult. While it's possible to use trickery to get good sound quality in games like Digger (which modulate pulse widths to control the volume of rectangular waves) and also possible to use different trickery to reproduce sampled sound if you know what the sample rate is, it's hard to know which of these techniques to use when.

A technique that would always work and give great sound would be to sample the speaker bit at very high rates (like 1.193MHz - the PIT frequency) and then downsample that to a rate suitable for output on a normal soundcard. This could be done with Fourier techniques or by convolving with a suitable filter kernel, but I never got around to writing this code. It also might be rather processor intensive.

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