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Envelope and sweep
http://forums.nesdev.com/viewtopic.php?f=6&t=14426
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Author:  FaTony [ Mon Jun 20, 2016 8:08 am ]
Post subject:  Envelope and sweep

So I'm making a chiptune plugin for GNU/Linux and I want to use NES APU as a reference. So far, I have implemented Pulse, Triangle and Noise and now want to implement envelope and sweep. How do they work? Does envelope change linearly? What about sweep? Does it oscillate like triangle?

Author:  tepples [ Mon Jun 20, 2016 8:11 am ]
Post subject:  Re: Envelope and sweep

Did you find anything in APU Envelope and APU Sweep unclear?

Author:  FaTony [ Mon Jun 20, 2016 8:26 am ]
Post subject:  Re: Envelope and sweep

Pretty much everything. I barely understand hardware side of things.

Author:  tepples [ Mon Jun 20, 2016 8:46 am ]
Post subject:  Re: Envelope and sweep

If you're just making a plug-in, and you don't expect compatibility with NES ROMs or NSF files, you can simplify the model by always using the constant volume mode. Let the instrument define the channel's output from 0 to 15 and change it 60 times a second.

Likewise you can mostly ignore the sweep units. Let controller messages (portamento, pitch bend) and your modulation LFO change the period every frame.

Author:  FaTony [ Mon Jun 20, 2016 9:03 am ]
Post subject:  Re: Envelope and sweep

tepples wrote:
If you're just making a plug-in, and you don't expect compatibility with NES ROMs or NSF files, you can simplify the model by always using the constant volume mode. Let the instrument define the channel's output from 0 to 15 and change it 60 times a second.

Likewise you can mostly ignore the sweep units. Let controller messages (portamento, pitch bend) and your modulation LFO change the period every frame.


Yes, the end result will probably be like so but I want to at least at this point make it somewhat like NES on a conceptual level. So, envelope - linear downward slope? So I can set the maximum amount and just set it to max volume at Note On MIDI message and then decrease linearly?

Sweep - triangle? So I just set the amount of vibrato and frequency and change pitch accordingly?

Author:  tepples [ Mon Jun 20, 2016 9:29 am ]
Post subject:  Re: Envelope and sweep

Sweep is not a triangle. Sweep is linear, more like portamento than like vibrato. Think of Mario's jump sound.

Author:  FaTony [ Mon Jun 20, 2016 9:34 am ]
Post subject:  Re: Envelope and sweep

Ah, I get it now. So what about envelope?

Author:  tepples [ Mon Jun 20, 2016 9:59 am ]
Post subject:  Re: Envelope and sweep

It's a decay, linear (saw) in amplitude space but quantized to 16 levels, from maximum amplitude to silence. The time of the decay can be controlled between 1/15 and 16/15 seconds in 1/15 second units. Whether or not the volume resets to maximum after expiring can also be controlled.

Very few games past 1983-1984 actually used the envelope outside constant volume mode.

Author:  FaTony [ Mon Jun 20, 2016 10:04 am ]
Post subject:  Re: Envelope and sweep

tepples wrote:
It's a decay, linear (saw) in amplitude space but quantized to 16 levels, from maximum amplitude to silence. The time of the decay can be controlled between 1/15 and 16/15 seconds in 1/15 second units. Whether or not the volume resets to maximum after expiring can also be controlled.

Very few games past 1983-1984 actually used the envelope outside constant volume mode.


Thanks. Now I get everything.

Author:  dougeff [ Mon Jun 20, 2016 12:35 pm ]
Post subject:  Re: Envelope and sweep

Note, triangle has no volume control. It either on or off. And my perception is that triangle is slightly quieter than the other channels (and of course, an octave lower).

Maybe you know that already. IDK.

Maybe for a chiptune tool, you can ignore all that.

Author:  FaTony [ Mon Jun 20, 2016 1:21 pm ]
Post subject:  Re: Envelope and sweep

dougeff wrote:
Note, triangle has no volume control. It either on or off. And my perception is that triangle is slightly quieter than the other channels (and of course, an octave lower).

Maybe you know that already. IDK.

Maybe for a chiptune tool, you can ignore all that.


Yes, I do all 128 velocity levels provided by MIDI standard. The code is here: https://gitlab.com/ftz/nes-synth
But I decided not to limit myself to NES and do a general chiptune plugin which means the project will be renamed after I do one and only "NES" release.

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