PSA: Phase modulation and frequency modulation are not effectively the same!
BotB Academy Bulletins
 
 
91868
Level 13 Chipist
post #91868 :: 2017.11.05 11:33am :: edit 2017.11.05 11:35am
  
  ViLXDRYAD and MiDoRi liēkd this
A lot of people seem to think they produce the same effects, they do not! Frequency modulation can go forwards only, phase modulation can go backwards with enough modulator amplitude!

The effect is that phase modulation can much, much more faithfully recreate sounds like the bass guitar or the trumpet than frequency modulation can! Any time you look at writing for an FM synth, look up whether it is frequency modulation or phase modulation, and keep this in mind if working with phase modulation! It can help you create FAR more accurate instruments with FAR less efforts!

PSA over.

EDIT: just realized I should've rattled this posturd while FM synth was still going on entries, oh well.
 
 
91871
Level 18 Pixelist
post #91871 :: 2017.11.05 11:49am :: edit 2017.11.05 11:50am
  
  ViLXDRYAD and kinkinkijkin liēkd this
Wait, how is it suppo~
Nvm, my mind processed it with some sluggishness, but i think i get it :)
(Cuz yeah, i'm better at learning things when accompanied by some visual diagrams)
Nice insightful little post, kink
 
 
91927
Level 10 Criticist
post #91927 :: 2017.11.06 11:53am
  
  MiDoRi, kinkinkijkin, RazerBlue6 and pedipanol liēkd this
All Yamaha FM things are using Phase Modulation BTW, only FM is the LFO.
 
 
91941
Level 18 Criticist
Xyz
post #91941 :: 2017.11.06 5:13pm
  
  kinkinkijkin liēkd this
FM can go backwards too by going through negative frequencies. Look up "Thru-Zero" FM.
 
 
91978
Level 13 Mixist
post #91978 :: 2017.11.08 6:42am :: edit 2017.11.08 1:24pm
  
  MiDoRi and irrlicht project liēkd this
  
  Jangler, pedipanol and kinkinkijkin hæitd this
If you want accurate instruments you should probably use (((waveguides))) :D

I don't get the debate on whether FM or PM "sound" better. Both sound like someone is farting into a tin bucket most of the time ^^ It's the usability that matters to me. If I can get the sounds I like from it without going through a lake of fire, that's good. Doesn't need to be realistic at all.

(Usability 3 anyone? Describe in song how you use your favourite synth)

€dit: U-uh, hey guys, I don't actually haeit FM or PM ! ! Actually I am quite fond of them! But honestly I do love me some karplus-strong and waveguides c:
 
 
92004
Level 22 Chipist
post #92004 :: 2017.11.09 1:43pm
Yeah, KS = Kool Shit :D
Btw, do you know of any resources that explain waveguide synthesis implementations without resorting to convoluted scientific hokus-pocus language, ie something that a mathematical inept person like me can understand?
 
 
92020
Level 7 Chipist
mad
post #92020 :: 2017.11.10 8:24am
Do you know calculus?
FM is effectively PM with the integral of the modulation input.
The integral of a sine is also a sine, so aside from phase differences they are the same if you have only 2ops and no feedback and sines only.

As for Karplus Strong, you have to add a lot of stuff to make it sound good... In particular, strings beat and shimmer against themselves because every harmonic has a slightly different frequency in the horizontal, vertical and compression axis, and the effects of this are quite audible.
 
 
92024
Level 22 Chipist
post #92024 :: 2017.11.10 10:32am :: edit 2017.11.10 10:36am
Might have learned some calculus in school, but if I did, I certainly and profoundly have forgotten all about it :D

I understand how FM/PM works and I can implement it in software. Same for KS. What I have trouble understanding is the more generalized waveguide approach.

I should mention that I'm mainly interested in implementing this stuff on slow 8bit hardware. So I'm not so much interested in how to get good quality sound, but rather in how to reduce/simulate/fake things as much as possible. Like, simple KS is super cheap, just read some sufficiently pseudo-random data from somewhere (ROM) into a buffer and keep iterating over it while continuously applying el cheapo low-pass (current_sample = (current_sample + previous_sample) / 2) :D
 
 
92066
Level 25 Chipist
post #92066 :: 2017.11.11 5:03pm
  
  funute, Xyz, kinkinkijkin, MiDoRi, irrlicht project, Cessor Safari and pedipanol liēkd this
"Frequency modulation" as a term usually means phase modulation. Yamaha called all their phase modulating synthesizers "FM".

There is a difference between Chowning's original "frequency modulation" idea, and phase modulation, but PM for sound synthesis is very practical to implement and actual frequency modulation is not. (It's practical for other purposes, like radio, though.)

So, fundamentally I must disagree without opening statement. "Frequency modulation" has TWO meanings in different contexts, and one of those is actually "phase modulation". It is correct that one of these two meanings does make a distinction between them, but in the world of sound synthesis FM is PM.

As mad was mentioning, at least for some ideal cases you can provably produce exactly the same waveform result with both. In practical implementations for sound synthesis, though, it is very important to have control over the relative phase of both operators continuously, and this just isn't easy to do with true modulation of frequency. As soon as the phases slip between the two, the sound changes drastically. Phase modulation, on the other hand, puts it directly under control, very simple, and very stable.

This is the same reason that "frequency modulation" on the FDS is hard to use for that purpose. It's actually modulating the frequency, because it was intended for use only as vibrato, not as a waveform synthesis technique. When you put it up to high frequencies, there is not enough numerical stability, and the phase between your modulator and carrier is constantly slipping. Like you can sorta do FM sounds with it if you can live with them also having a constant "phaser" effect on top, and some detuning.
 
 
92131
Level 12 Mixist
post #92131 :: 2017.11.14 6:12am
  
  MiDoRi and kinkinkijkin liēkd this
FM8 is phase modulation too. I'm not sure I know an actual FM vst synth. Does anyone have any example?
 
 
92133
Level 22 Mixist
post #92133 :: 2017.11.14 10:26am :: edit 2017.11.14 11:12am
  
  kinkinkijkin liēkd this
Synthmaster is the only VST I know that lets you choose between phase modulation and frequency modulation. I'll try to make a comparison clip. Download the demo and find out for yourself. They seemed almost identical at first but once you start combining more modulators the differences become pretty obvious.

Phase modulation is more predictable/controllable/musical. True FM seems better for sound design and more wacky sounds.
 
 
92142
Level 13 Chipist
post #92142 :: 2017.11.14 6:23pm
  
  MiDoRi liēkd this
thank you guys for explaining the real differences better than me, I am just an pleb!
 
 

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