21 July 2024, 13:42 | #1 |
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Advanced Paula Features
Paula was mainly used to play samples and IIRC very occasionally wavetables.
However, it was capable of more than just sample playback. It should also be capable of AM and FM synthesis. There is an order you can set the channels to modulate each other with modulation being AM, FM or both. The FM modulation is 14 bit but being read as a pair of bytes means it'll only run up to 14KHz. The AM modulation is the same but it's only 6 bit out of the 16 bits read. You can chain the 4 channels together so it should be able to do something like 4 operator FM synthesis (or AM or AM/FM). Did any Amiga software ever do this? |
21 July 2024, 13:51 | #2 |
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FMsynth? (it's on aminet)....there was (at least) one other, Synthia...
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21 July 2024, 13:55 | #3 |
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This thread might be an interesting read (and watch): https://eab.abime.net/showthread.php?t=114590
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21 July 2024, 14:29 | #4 |
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Technically it would be Period Modulation, not Frequency Modulation. It's an important difference because the period value is inversely proportional. What people usually think of with FM, i.e. Yamaha OPL etc. is also a different mechanism again, Phase Modulation. This is a numerically (and computationally) better behaved approach.
The Period Modulation is not especially useful musically but you could presumably get some interesting clangs out of it |
21 July 2024, 15:26 | #5 |
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there is good soft synths on the amiga, if you want the blip bloop blop feeling. Aegis, octamed, ahx, etc...
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22 July 2024, 10:15 | #6 |
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22 July 2024, 13:17 | #7 | |
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23 July 2024, 01:54 | #8 | |
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23 July 2024, 02:12 | #9 | |
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So, it does appear to have been used, but mostly as a ringmod like effect as far as I can see. I'm wondering has it ever been used for synthesis, as in FM synthesis? |
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23 July 2024, 02:41 | #10 | |
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As all the various FM softsynths just seem to render samples, it doesn’t appear that this has ever really been used for synthesis. I find the “FM” very unpredictable, even with two lots of the same waveform, and I feel that very careful cycle accurate data would need to be used to get that working. Da Jormas struggled with this too. The AM is far more usable on any pair of waveforms, of course. Fun news though, I have requested that this gets added to OctaMED Soundstudio and turned on per step (I hope it can be done!) so hopefully this feature will inspire a whole new breed of Amiga music. |
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23 July 2024, 14:48 | #11 | |
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23 July 2024, 15:41 | #12 | |
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Same way as distance is not speed. That's not just semantics but physics. |
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23 July 2024, 15:50 | #13 | |
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When the input phase is increasing linearly, the effect on the carrier is as if it's running at a higher frequency. If the converse is true, it's as if it's running at a lower frequency. Thus phase modulation can achieve true FM, but it's not what makes it useful as a means of creating complex spectra. |
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25 July 2024, 12:41 | #14 |
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Using multipart loop points for carefully created samples via the CPU would allow for something like virtual analogue synthesis and give you something similar to ADSR.
Not much you can do about filter deficiencies without using professional equipment to do the 'Prodigy style' filter effects on the samples created first but it's the same multipoint sample loops per sample principle to get around it. 7mhz 68000 isn't going to be able to do real-time modular synth style filter calcs IMO But the basic bus architecture and DAC quality is enough to do this IMO. |
25 July 2024, 15:04 | #15 |
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So it's FM achieved by period modulation. So you'd need to invert the waveform.
And probably shape the modulator wave so it does linear FM. From reading the Amiga Hardware Reference Manual, it says the modulator replaces the equivalent registers. It's a bit vague so I'm taking that to mean it's just writing into the period register directly. This could get complicated because this means the centre frequency of the modulator will determine the pitch of the carrier. There's no way to change the pitch of the carrier independently of the modulation. As soon as you play a different note, I'm not sure what will happen but it'll probably go horribly out of tune. To actually play different notes, you'll need different waveforms for the modulator with different centre points. I'm beginning to understand why nobody did a synth around this, it'd be a complete pain! |
25 July 2024, 19:57 | #16 |
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I don't know if the intention for it was to be musically useful but I did wonder if it could be used to encode data in a manner akin to how the zx spectrum did it. You'd have your carrier playing a fixed square wave output at one of two different period values, controlled by the modulator, which is DMA fetching the values from a buffer. That buffer would be filled by inserting the appropriate period value depending on the state of the next bit to be encoded.
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25 July 2024, 20:00 | #17 |
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FWIW, integer arithmetic is not ideal for phase modulation. The OPL hardware does it using a form of floating point, IIRC. It uses, among other things, logarithmic encoding too, so that multiplication of values (which is very common) can be achieved by adding.
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26 July 2024, 15:15 | #18 |
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OK. I learn something new every day. (though I never thought duration or wavelength was the same as frequency, nor did I think distance was the same as speed)
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26 July 2024, 15:33 | #19 |
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Just wait until you start looking at how FM feedback modulation works.
There's actually quite a good intuition for it - all rising edges get steeper and all falling edges get shallower. No matter what you start with, it gets driven towards a sawtooth. https://github.com/0xABADCAFE/random.../doc/Shaper.md |
26 July 2024, 17:19 | #20 | |
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If you set the period to less than 123/124 it stops reading from memory and just repeats the 2 bytes in the buffer. You just set one as 0 and one as max and set volume to max and modulate it. This works right up to ~1.7 MHz So, Paula is also an FM radio encoder, who knew? |
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