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Physics anyone?

For fans of synths, pianos or keyboard instruments of any sort.

Physics anyone?

Postby dxvxd » Mon Jun 01, 2020 1:14 am

As I delve deeper into sound design, some questions keep coming up in my mind, questions which some may dismiss as purely theoretical, as unrelated to the nuts and bolts business of sampling sounds, and playing with oscillators and filters and so on. And yet…

If you will humor me…

Here's a little experiment you may have done:
Turn on your sequencer, and play a steady stream of 16th notes on the same note for a couple of bars and then loop it. As you play it back, increase the metronome setting of the sequencer. As the repeated note gets faster at some point another tone emerges out of the frequency of the 16th notes.

So I know that sound is a disturbance in the air which is propagated as a wave and that the temporal frequency of the wave is what gives a sound it's pitch. So if you play those 16th notes, say middle C , each middle C is an instance of the aforementioned disturbance in the air. So you're kind of using a repeated sound, the middle C, to make a new sound.
And this is what puzzles me.
Because if sounds are made of other sounds, then it seems you have a problem of infinite regress.
But that can't be right.
And this is where I get stuck because I have very little knowledge of the physics of sound, just some recreational reading.

I think this is the type of question that many people will roll their eyes at, and move on. But if any of you find this sort of thing interesting, and happen to know more than I do, i'd love to hear some responses.

Dave
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Re: Physics anyone?

Postby Eddy Deegan » Mon Jun 01, 2020 1:26 am

You have just described FM synthesis, albeit in something of an unusual manner. If you're thinking like this (and that's a good thing) then you should find the Synth Secrets series of articles in Sound On Sound an interesting and enlightening read, specifically https://www.soundonsound.com/techniques ... modulation and https://www.soundonsound.com/techniques ... modulation which dive deeply into what you seem to have discovered for yourself.

In the case you are describing, the sequencer becomes an oscillator itself once it's playing notes fast enough and this interacts with the tones produced by each individual note in interesting ways.

dxvxd wrote:So you're kind of using a repeated sound, the middle C, to make a new sound.
And this is what puzzles me.
Because if sounds are made of other sounds, then it seems you have a problem of infinite regress.
But that can't be right.

It is right, up to a (non-infinite) point. Any possible sound can be reconstructed by layering pure sine waves of varying frequencies on top of each other (the Synth Secrets series touches on this in the very first installment here: https://www.soundonsound.com/techniques/whats-sound and revisits it here https://www.soundonsound.com/techniques ... -synthesis). As a corollary any possible sound can be deconstructed into its sine wave components. This is done using Fourier Transformations.

It's a really good thing to discover or otherwise wonder about something that's already been independently discovered. I remember when I was 8 or 9 asking my piano teacher why it was when I played an octave in the left hand that I could clearly hear a couple of additional intervals in addition (that I wasn't playing). Thus I discovered harmonics.

That you have stumbled across this stuff of your own volition shows you're thinking in the right way. Fortunately there is a lot of material to read up on and Synth Secrets is a fantastic place to start. I recommend you read the whole series :thumbup:
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Re: Physics anyone?

Postby dxvxd » Mon Jun 01, 2020 2:33 am

Thank you for that insightful response. I will most certainly check out the links you provided. It's interesting that you should mention FM synthesis. Back in the day, by which I mean the 80s, I had a DX 100, the DX7’s four operator little brother. I remember that if you played a sufficiently low note, it would transform into series of clicks. That may be the origin of this question, for me.
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Re: Physics anyone?

Postby Eddy Deegan » Mon Jun 01, 2020 3:23 am

dxvxd wrote:Thank you for that insightful response. I will most certainly check out the links you provided. It's interesting that you should mention FM synthesis. Back in the day, by which I mean the 80s, I had a DX 100, the DX7’s four operator little brother. I remember that if you played a sufficiently low note, it would transform into series of clicks. That may be the origin of this question, for me.

The output of any synthesizer, whether FM or not, if asked to play a low enough note will be perceived as a series of clicks.

When I bought a Moog Mother-32 analogue monosynth some years ago I was trying something out and only heard a series of clicks instead of an expected tone. After asking for advice on why this might be (thinking there might be something wrong with the synth) I was advised by Hugh Robjohns (SOS Technical Editor) that I was probably inadvertently patched into an LFO (Low Frequency) output instead of a VCO (normal audible frequency) output.

He didn't own a Mother-32 but he was exactly right and my problem was solved. This was, and is, a great example of how knowing the theory behind synthesis can be useful :-)
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Re: Physics anyone?

Postby DC-Choppah » Mon Jun 01, 2020 3:56 am

A middle C note played for say 1/2 second, then released for 1/2 second, then that pattern repeated is a 'modulation' of a sine wave (220 Hz) by a lower frequency (1 Hz) square pulse wave.

Modulation means that the waveforms are multiplied together.

When you modulate waveforms, you don't get the SUM of the two, you get the product of the two. That product contains lots of new frequencies that were not present in either of the two originals.
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Re: Physics anyone?

Postby Folderol » Mon Jun 01, 2020 7:52 am

FM synthesis can produce some quite dramatic results. Counter intuitively the simplest basic shapes can produce the most dramatic change - e.g. a sine wave modulated by another sine wave!
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Re: Physics anyone?

Postby Arpangel » Mon Jun 01, 2020 7:59 am

One of the most attractive things for me regarding FM synthesis is high frequency aliasing, it’s always good to play FM sounds on an 8 octave keyboard, the top octaves make the sounds really break up into something completely different, using the pitch-bend wheel and changing the modulator frequency at the same time produces really interesting sounds.
I always wanted to route Midi controller data to individual operators in an FM system, is this at all possible now?
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Re: Physics anyone?

Postby Folderol » Mon Jun 01, 2020 8:06 am

Arpangel wrote:I always wanted to route Midi controller data to individual operators in an FM system, is this at all possible now?
It is in Yoshimi :bouncy:
MIDI-learn is your friend :wave:
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Re: Physics anyone?

Postby Arpangel » Mon Jun 01, 2020 8:37 am

Folderol wrote:
Arpangel wrote:I always wanted to route Midi controller data to individual operators in an FM system, is this at all possible now?
It is in Yoshimi :bouncy:
MIDI-learn is your friend :wave:

I didn’t even know Yoshimi had FM!
I’m not keen on it, as it looks so complicated, but that’s me being lazy.

:crazy:
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Re: Physics anyone?

Postby innerchord » Mon Jun 01, 2020 10:00 pm

Arpangel wrote:I always wanted to route Midi controller data to individual operators in an FM system, is this at all possible now?

You mean like NI's FM7 offered almost 20 years ago? Where have you been? :)
I'm sure dozens of other FM synths can do the same now.
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Re: Physics anyone?

Postby Moroccomoose » Mon Jun 01, 2020 11:40 pm

Geek alert! If anyone is interested, I created a spreadsheet that plots wave forms for up to four tones and the sum of the tones. Its quite nice to see how the wave forms interact

You can set each tone to be Sine, Cosine, uphill saw, down hill saw, Tan (doesn't work very well) or step.

You can either input a frequency or determine the frequency musically with note, octave and cents

You can alter the phase and amplitude of each tone and the DC offset (Makes it easy to split the different forms out on the graph)

The things you do for fun! Feel free to have a play!

https://docs.google.com/spreadsheets/d/1sTx5v1V29J-yIHGzbilmntUVNmmowg8UBgqNa2iTBAQ/edit?usp=sharing

The original is excel and on my local drive, so no damage can be done!



Stu. :beamup:
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Re: Physics anyone?

Postby Eddy Deegan » Tue Jun 02, 2020 1:31 am

Moroccomoose wrote:The things you do for fun! Feel free to have a play!

https://docs.google.com/spreadsheets/d/1sTx5v1V29J-yIHGzbilmntUVNmmowg8UBgqNa2iTBAQ/edit?usp=sharing

The original is excel and on my local drive, so no damage can be done!

Wow, that's really quite something. I had to make a copy of it into my own google drive to access the editing controls but it's definitely worth the effort and a lot of fun to play with!

:clap:
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Re: Physics anyone?

Postby Martin Walker » Tue Jun 02, 2020 1:31 am

Arpangel wrote:One of the most attractive things for me regarding FM synthesis is high frequency aliasing, it’s always good to play FM sounds on an 8 octave keyboard, the top octaves make the sounds really break up into something completely different, using the pitch-bend wheel and changing the modulator frequency at the same time produces really interesting sounds.

As often happens, Arpangel manages to subvert the nature of the SOS Forums by managing to transform the bane of digital audio into something creative and desirable.

Bravo that man! :clap:


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Re: Physics anyone?

Postby Arpangel » Tue Jun 02, 2020 6:58 am

Martin Walker wrote:
Arpangel wrote:One of the most attractive things for me regarding FM synthesis is high frequency aliasing, it’s always good to play FM sounds on an 8 octave keyboard, the top octaves make the sounds really break up into something completely different, using the pitch-bend wheel and changing the modulator frequency at the same time produces really interesting sounds.

As often happens, Arpangel manages to subvert the nature of the SOS Forums by managing to transform the bane of digital audio into something creative and desirable.

Bravo that man! :clap:


Martin

Well, I it's like an annoying microscopic insect, it ceases to be annoying or uninteresting when you look at it through a microscope, suddenly it’s an immensely complicated piece of biological engineering full of beautiful atoms and cells, that’s how I focus in on some things, musically, they aren’t a problem, in fact, exactly the opposite, they become a vast untapped resource.
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Re: Physics anyone?

Postby blinddrew » Tue Jun 02, 2020 10:28 am

Eddy Deegan wrote:Any possible sound can be reconstructed by layering pure sine waves of varying frequencies on top of each other
Pedantry alert: my understanding was that this is only true of tonal sounds and that some sounds are based on noise rather than tones?
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