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physics of fat?

pete psingpy

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Talking about sound here, and maybe I should say "phatt" instead, anyway.


I'm pretty weak on understanding the physics of sound in general, but, what exactly is different in a technical sense with a sound that's "fat"?


For example - Everyone can probably relate to one of those "fat" moogish lead types of sound. You play a sawtooth wave in at least a few detuned oscillators. It seems the detuning is the key to getting fatness...and the number of osc's.


But a saw wave has a discrete distribution of frequencies, or harmonics, at integer levels over the fundamental (I think in theory at least). How is the "fattened" saw wave different?


I'm guessing that the detuned waves somehow add more density or "fatness" to this distribution of harmonics. Any idea?

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Well, firstly it depends on your interpretation of 'fat'. :)


In terms of the harmonic spectrum, there's a definite difference between the theoretical spectrum of a sawtooth wave and the real world spectrum. That's a small part of it.


Secondly, when you have multiple oscillators, especially in a true analog system, set to generate the same waveshape, the resultant discrepancies between oscillators plays a large part in the 'fatness' of the sound.


Then there's the filtering; eliminating the high frequence content is a huge part of creating 'warmth' to the sound (which may or may not be a part of your definition of 'fat' ;) ).


Of course, when you're using a ROMpler or other digital synth to create these sounds, you need to 'add' the imperfections in order to help achieve the result you want.


Wish I had more time to discuss, but I'm off to a gig shortly. I'm sure lots of others have input on the subject, and I look forward to reading them when I get back. :thu:

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Some random ideas I've gathered ...


- When you detune, you introducing beating (fluctuation in amplitude) whicn enlivens the sound simply becuase of movement.


- You also cancel and emphasize various frequencies, adding interest to the sound


- Since the same percentage deviation results in faster rate of beating at higher pitches, it is in your interest to detune lower notes more than high notes. Also it can be helpful to have a mellower (or LPF-ed) saw vave for the detuning, thus focusing on slower beating. Some discrete analog synths appeared to have their pitch calibrated on the high notes, thus creating this effect.


- When you are detuning multiple oscillators, some assymetry can be useful. Not all oscillators need to be at the same volume .. sometimes a quite oscillators can be detuned more without adverse effects.


- When you overdrive the sound slightly, you reinforce and add harmonics refulting in a fatter sound. In general even order harmonics are said to be more pleasing but at low levels any harmonics are fun, even white noise.


- White noise filtered to emphasize the 1-2 KhZ range seems to add girth at low volumes


- Drift (the slow pitch movement of oscillators) seems to add attractive variation to the detuning process)


- Jitter (relativel high frequency instability in pitch) appears to be there in microscopic quauntities on some desirable oscillators. Controlling jitter in musical amounts at high frequencies is tricky. Handle with care.


- If every note-on is slightly different the resulting sound seems more alive. So free-running oscillators are a good thing. Alternatively some slight pitch randomness on keystrike can help.


- Real analog saw waves are often rounder than perfect digital ones. If you have variable waveshaping on a digital synth (Ion, AN1X) there is room to play here.


- There is something magical about certain analog filters in adding girth to a sound even at zero resonance. Some digital filters add some girth. Others don't. This is in the magic department.


Hmmh ... I seem to have run out. Hope this helps someone. My$0.02



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I am making this post so that others might approach this topic from other angles and make this a really phatt thread. Being a blues and classic rock kinda guy, I am always looking for FAT PIANO, RHODES and ORGAN sounds, as opposed to "thin" or "cheesy". My quest has lead me currently to a FantomXR w/srx12 and Voce V5+. I find TUBE PREAMP and tweaking EQUALIZATION optimizes PHATNESS. Thanks for the topic! I learn sooo much here...
Never try to play anything live.
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The thing is that "fat" is often the wrong term to think of. What you really want is "depth" or "richness" of sound which really comes from a lot of factors such as "movement "rather than actual "fatness." Also, the sound is "reactive" to the input. (I suppose in an analog polysynth, this would extend to crosstalk between voices.)


Another point is that a good sound is often more like a jewel in the middle of a room. Sounds that take up too much space leave little for anything else, and when you listen to a recording that sounds as though the sounds are "fat," what yoiu're hearing is the "combined fatness." Secondly, the impression of size is achieve through blank space... think of a building with a big space in front so that you get can get a "vista".


So, large doses of simple static detuning often just leads to clumsy and ugly sounds. It is also tempting to build huge layers but this often leads to a mess as well. And it's highly desirable for many sounds to be as clean and "thin" as possible.


Finally, there are limits to what programming can achieve. If, after exhaustive efforts you can't get the sound you want... it's probably time to get a better synth. Indeed, many excellent synths have extremely attractive and "fat"... or rather "rich"... presets right out of the box... while "plug-in of the week" sounds bad no matter what.


Originally posted by Tusker:

Some random ideas I've gathered ...

Great advice! :)
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Suppose hypothetically that you looked at the frequency spectrum for a single perfect saw wave. I believe you'd see the fundamental freq plus harmonics of decreasing amplitude going up indefinitely. They would be perfect spikes I think, again in theory.


Now suppose you added another saw wave detuned slightly to make a "fatter" sound. What would the distribution look like? I'm guessing you'd see the same harmonics but that maybe they would be blurred - denser or wider?


Does a chorus effect do something similar? I was just thinking that a chorus seems to add the same kind of effect.

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The frequency spectrum will looks similar, but the waveform will be varying widely with time. With chorusing you are detuning and varying the amount of detune with an LFO.


It might help you to check out some of these:











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There was a video of a single Moog oscillator hooked up to an oscilloscope somewhere, I think it was on the Matrixsynth blog. I think it's really instructive just to look at that and see the constant "movement."


(I couldn't find it in a search I just did, but I did find this 1967 article by Bob Moog from Electronics World magazine:


http://orac.vu/papers/Moog-ElectronicsWorldFeb67.pdf )


As for bass, you really want to keep it clean and ideally even high-pass filter off the bottom few hundred hertz from many synth parts, keeping just the bass and percussion down there.


That said... it's lots of fun to hit the lower octaves on an analog synth up loud. :D

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