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ChristopherJam
Registered: Aug 2004
Posts: 1409 |
6581 vs 8580, unfiltered drum
I'm currently building a drum sound out of an unfiltered swept triangle wave. It's sounding good on my 8580 equipped c64c, and sounds nigh identical under reSID-fp emulating the same.
However, if I switch reSID over to 6581 it sounds very different indeed - there's a twangy/swirly overtone that distracts from the punchiness.
I thought the main differences between them were related to filters and digi-playback, neither of which I'm going anywhere near; what am I missing? |
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Frantic
Registered: Mar 2003
Posts: 1648 |
Hi!
Not sure about this actually. Do you get the same strangeness for all variants of the 6581 emulation?
It would be very nice if someone would start an article on codebase that summarizes and lists all the known differences between various SID chips, in accurate technical detail. |
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SIDWAVE
Account closed
Registered: Apr 2002
Posts: 2238 |
the waveforms are a bit different, 8580 is more perfect with less flaws, and the adsr is different. if you want them to sound identical, you must find some middle way in the freq table. ? |
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Reggy
Account closed
Registered: Oct 2009
Posts: 19 |
out of curiousity... you're describing the sound as an "unfiltered swept triangle wave", but how did you manage to make it sweep, without the filter? |
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Wisdom
Registered: Dec 2001
Posts: 90 |
Perhaps you meant pulse sweep? If you can provide a .sid file, it should be easier to help.
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SIDWAVE
Account closed
Registered: Apr 2002
Posts: 2238 |
you are not missing anything. simply, if you want the sound to be same on both chips, you have to rework the sound and test. find some neutral state, and remove the overtones by tweaking the freq until you get it as you want. there is no other fix, than manual labour. |
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RaveGuru
Registered: Apr 2002
Posts: 43 |
I'm guessing in this case OP is referring to a frequency sweep. That's basically how you build membrane instruments (drums); by quickly sweeping from higher to lower frequencies.
I'm very curious to hear that the actual waveform differs between the models as I've never noticed that, except for combined waveforms.
I'm putting my bet on an ADSR 'incompatibility'. Try experimenting with different envelope settings and see if you can find a solution.
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ChristopherJam
Registered: Aug 2004
Posts: 1409 |
Thanks all.
I get the strangeness for all the reSID 6581 variants, but not for the 'fast' 6581 emulation. Think I'll have to see if I can source a real '81..
Straight triangle wave ( $19 first few cycles, $11 for most of note, $10 for last few frames), 6x player, just sweeping the frequency down as RaveGuru suspected.
Envelope's $00f0, but I'll have a play and see if it makes any difference. |
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Jammer
Registered: Nov 2002
Posts: 1336 |
ADSR is $00f0?? No wonder that it sounds different on both chips ;) It's not proper sound, just a fart because of rapid volume change thus it's probably louder on 6581 ;) Or maybe you mean hardrestart envelope? ;) |
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Frantic
Registered: Mar 2003
Posts: 1648 |
well, since the test bit is on for the first few cycles anyway, is shouldn't matter much what is in the AD register I suppose. So I don't really see how ADSR could be the explanation.
The rapid volume change may still have something to do with it though -- considering the difference in digi playing on the 6581 and the 8580 -- but I must say that I am not sure exactly how this would be the case, since I guess that would probably only affect the initial moment of the sound, and I can't see how that would result in a swirling sound.
Also, since the "removal" of the test bit starts the oscillator from the beginning, I also fail to see how that in itself would produce different results on the 6581 and the 8580. (Well, there is a once cycle difference in where the oscillator is on the two chips, after toggling the test bit, but I can't see how that would produce a hearable difference in this case).
Hmm.. I am indeed confused. If you could post some complete, but minimal, test-code that reproduce the issue, I could certainly have a look. |
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booker
Registered: Jul 2003
Posts: 334 |
Triangle wave sounds different on old and new SID. |
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