Oric's 8912 volume scale

Probably the most technical forum around. It does not have to be coding related, or assembly code only. Just talk about how you can use the AY chip to do cool sounding things :)
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meynaf
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Re: Oric's 8912 volume scale

Post by meynaf » Mon Aug 08, 2016 10:06 am

The links he gives at the very end of the video are also useful.
Next time someone pretends the oric (or whatever ym emulation program) plays the ST tunes identical i'll send him to sndhrecord.atari.org for a comparison :p

Btw. Does someone know if there is a site with oric tune recordings ? Having a good reference is always useful !

Godzil
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Re: Oric's 8912 volume scale

Post by Godzil » Mon Aug 08, 2016 7:40 pm

Dbug wrote:On the topic, there's some interesting details about sound on Gunstick's presentation: https://www.youtube.com/watch?v=yLDtiTS4JLg
Around the 34th minute there are some Audacity sample output of the three channels, and discussion of the shape on emulators vs real machine.
I have a friend that could explain why emulation need some clever math for giving the correct sound, I haven't seen this video yet, I can ask him to make a simple text to explain why if some of you are interested!

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meynaf
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Re: Oric's 8912 volume scale

Post by meynaf » Mon Aug 08, 2016 9:14 pm

Godzil wrote: I have a friend that could explain why emulation need some clever math for giving the correct sound, I haven't seen this video yet, I can ask him to make a simple text to explain why if some of you are interested!
I already know why emulation needs some clever math. What i don't know is what this clever math can be. If you friend knows, he's more than welcome to explain !

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Re: Oric's 8912 volume scale

Post by ThomH » Wed Oct 19, 2016 3:06 pm

meynaf wrote:
Godzil wrote: I have a friend that could explain why emulation need some clever math for giving the correct sound, I haven't seen this video yet, I can ask him to make a simple text to explain why if some of you are interested!
I already know why emulation needs some clever math. What i don't know is what this clever math can be. If you friend knows, he's more than welcome to explain !
I'm probably far too late and/or answering the wrong question, and I'm learning this stuff almost as I type, but as far as I can make out there are two segments of maths involved:
  • voltage decay;
  • resampling.
Despite it being about a different chip, the simplest explanation of voltage decay that I could quickly find is under "the imperfect SN76489" here (at least, prior to about a minute ago; it's gone HTTP 503 while I was writing this post but I'm sure it'll be back later; Google has it cached), specifically:
Wherever a voltage (output) is artificially held away from zero, there will be leakage and the actual output will decay towards zero at a rate proportional to the offset from zero:
dV
-- = -kV
dt
where k is a constant.
The diagrams there are artificial but note how they match the measured waves given in the Youtube video linked earlier. So output voltage is a differential equation, a function of voltage and time since the voltage was set. The AY will hit the voltage it is supposed to, then decay until the next conscious change.

Rampling is much more trite — see any of the windowing techniques, e.g. as per here. It's also not something the original hardware does, it's just how you squeeze what the original hardware does out of your speaker. So there's more leeway.

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