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FM Synthesis (techniques, anecdotes)


Guest skibby

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Guest skibby

i could have worded that better. shifting the phase of the waveform does different things at different stages in the signal chain. it's basically a delay, but it gets more and more complicated depending on what youre looking for.

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I was just curious about implementing it literally using a delay line, because I've heard that mentioned elsewhere, mostly for recording engineers using analog phase shift to line up e.g. drums (which would probably be done mostly in software now).

 

Obviously a 180 degree phase shift is a piece of cake because you just invert the signal.

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Guest skibby

I was just curious about implementing it literally using a delay line, because I've heard that mentioned elsewhere, mostly for recording engineers using analog phase shift to line up e.g. drums (which would probably be done mostly in software now).

 

Obviously a 180 degree phase shift is a piece of cake because you just invert the signal.

 

i read a little about phase shifting, and it seems that in the analogue world this is accomplished using allpass filters. i'm not sure if it is merely a delay or if it somehow otherwise warps the waveform.

 

but at any rate, if you do apply a microdelay to something in your mix, it would be the same effect as a phase shift i think.

Edited by skibby
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i read a little about phase shifting, and it seems that in the analogue world this is accomplished using allpass filters. i'm not sure if it is merely a delay or if it somehow otherwise warps the waveform.

 

but at any rate, if you do apply a microdelay to something in your mix, it would be the same effect as a phase shift i think.

 

 

a lot of audio effects in software are done with allpass filters, including modulation effects and reverb. it's not exactly the same as altering phase via delay, but can have a similar end result. every kind of filter will shift the phase of a signal in some way (all analog eq and most digital eq will cause phase shift as well)

i don't know if allpass filters are used in fm synth design tho.

Edited by Ingwe
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  • 2 weeks later...

Looking at pictures of the FM8 modulation matrix, it seems you can have an operator modulate itself. Anyone know how they implemented this? Just a simple buffer between output and input of an oscillator or is there fancy math involved?

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Looking at pictures of the FM8 modulation matrix, it seems you can have an operator modulate itself. Anyone know how they implemented this? Just a simple buffer between output and input of an oscillator or is there fancy math involved?

yeah basically this, just make sure you're doing phase modulation and not frequency modulation as the feedback doesn't work as good with FM. just to clarify, most "FM" synths are actually phase modulation as it is easier to implement and has the exact same sound as FM (aside from the feedback, of course)

 

Yea by now I've read about 10 webpages on the difference between PM and FM but I still haven't figured it out lol. It's the same, but not quite, so what exactly is the difference?
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Guest skibby

That's what oversampling is for.

 

i disagree. jumping around any waveform isnt happening in PM/FM. its the _rate_ of playback that is changing rapidly, but the signal is continuous. speeding up massively, slowing down massively, as fast as samplerate will allow. even if you have a samplerate of a million samples a second, and jump nonlinearly to another point in a wavetable, there will be a digital click/pop. non-continuous signals can't be fixed by oversampling because there is missing data. it could be interpolated but since it is instantaneous, theres no way to fix it, its noise. it can be 'fixed' by a low pass filter to an extent, but the harmonics created by a non continous signal is basically a sort of pop or click which contains full spectrum noise so it will sound like a pop rather than a click.

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It doesn't make sense to be modulating at such a level that you actually get audible clicks from smooth sine waves, does it? I mean interesting stuff might happen but it would be supersonic anyway.

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whats this feedback stuff? fm is just modulating another oscillator at audio rate to get sum diff frequencies. same with Am but am retains its fundamental

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Guest skibby

i started getting familiar with a cool little FM freeware synth (forget which one atm)

 

 

makes me really want a Reface DX

 

soon there is going to be a re-release of a sytrus-like (sort of similar) except freeware FM vst.

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Guest skibby

It doesn't make sense to be modulating at such a level that you actually get audible clicks from smooth sine waves, does it? I mean interesting stuff might happen but it would be supersonic anyway.

 

if you play a sine wave and it breaks, it creates a full frequency harmonic "pop" at the worst, and otherwise a downsampley-buzz otherwise.this is because a digital leap to a new value is the same as a very very very high frequency (nyquist x infinity) that's the aliasing i was talking about.

 

if you fm around in a sine waveform by appearing at different points in that sine wave nonlinearly, it wont sound gummy and gooey and bellish, instead it will sound like a c64

 

in response to the other statements above talking about phase, phase in a static frequency wave is a shift in the time domain (left and right) which is essentially a delay of a period equal to the wavelength of a sine wave, in other words, one "circle" hence the 'degrees' term used in discussing phase.

Edited by skibby
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Guest skibby

however, changing the phase in a full frequency audio stream is kind of arbitrary, you can in theory shift the phase by seconds, or even years forward or backward in time, provided your audio has been buffered long enough.

 

what's happening with FM/PM is that the play rate of a sine wave (for xample) is being sped up and slowed down. when you slow down a wave, and if you don't want stepping or static, you need to create new samples in between the stretched ones. this is interpolation. otherwise your output will turn into DC when it's on the stretchy phase, and it sort of sounds like crap. oversampling does nothing to smooth things out really, because everything has to be downsampled again anyway, so its like working on a TIFF in photoshop and saving it as a JPEG anyway. Unless of course your DAW is running at 196, in which case its cool if the FM synth is running at that SR.

 

yamaha FM synths are wavetable based, but thats because they are digital, and all digital synths are wavetable based. even if you generate an oscillator from scratch, it still ends up as a virtual wavetable of information in order to be played by the D/A converter.

 

the really slick sounding digital delays and choruses today as well as virtual analogue synths, depend on interpolation to keep the signal continuous, and that is why they sound smooth and 'analogue' and not like a SNES or something.

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Quick question fer u guys, but what's that fm synth with all/most of the sega game music patches? I'm pretty sure it was mentioned here somewhere, but I've totally forgotten where and by who. I'm interested in finding it again.

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Nearly, it's VOPM ( http://www.geocities.jp/sam_kb/VOPM/). Another vst that allows you to load in mega drive patches that I've recently 'upgraded' to is FMDrive available at a bloody ridiculously good price (10 euros) here - http://www.alyjameslab.com/alyjameslabfmdrive.html

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Nearly, it's VOPM ( http://www.geocities.jp/sam_kb/VOPM/). Another vst that allows you to load in mega drive patches that I've recently 'upgraded' to is FMDrive available at a bloody ridiculously good price (10 euros) here - http://www.alyjameslab.com/alyjameslabfmdrive.html

too bad its not OSX yet, as the VOPM for OSX cannot load mega drive patches last time i tried

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Guest skibby

 

Nearly, it's VOPM ( http://www.geocities.jp/sam_kb/VOPM/). Another vst that allows you to load in mega drive patches that I've recently 'upgraded' to is FMDrive available at a bloody ridiculously good price (10 euros) here - http://www.alyjameslab.com/alyjameslabfmdrive.html

too bad its not OSX yet, as the VOPM for OSX cannot load mega drive patches last time i tried

 

 

I'm building an outboard version of the ym2612... very slowly. the goal is to be cross platform.

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too bad its not OSX yet, as the VOPM for OSX cannot load mega drive patches last time i tried

Ah just noticed that, probably will come but that company/site is just one guy so might take a little time ! He makes an awesome SN76489 VST too (the Master System chip), actually all his stuff is ace and pretty much cycle accurate so over the months I've ended up buying it all !

 

He's currently working on a Fairlight CMI IIx emulator/VSTi at the moment, my gosh is that going to get him some recognition when he's released it ....

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