FM synthesis amount knob

[Aro]

If I understand correctly, the fm amount knob increases the frequency of the modulator, which alters the sound? But then, shouldn't going up and down the keyboard alter the sound as well, instead of just altering the pitch. And as well, shouldn't increasing the tuning of the modulator oscillator have the same effect as increasing the fm amount knob?

Thanks

[Niowiad]

FM amount is usually the modulator's level into the carrier, not its pitch.


To anyone interested in the basics of FM synthesis, this is still a good read
http://www.burnkit2600.com/manuals/fm_t ... ations.pdf
Possibly following the examples with Dexed, free vst

[vurt]

FM amount is usually the modulator's level into the carrier, not its pitch.

this.
so, how much the modulator will effect the carrier.
try moving it by hand on some factory patches, it should give you an idea of what is happening if you listen carefully as you increase and decrease the amount slowly over time on a long held tone

[mztk]

better if you've got an envelope to control the amount.

[vurt]

better if you've got an envelope to control the amount.

true for actual usage, im just talking about while learning how it sounds
i find you get a better feel for what its doing if you are the one controlling it.(any new parameter you are learning or even just in a new synth)

[foosnark]

To get more technical, FM can sound different as you go up and down the scale, even with a constant ratio and FM index / "amount".

FM creates sidebands above and below the carrier frequency, and "lower" sidebands can reflect around 0Hz and get phase-reversed in the same way they reflect off the Nyquist frequency (aliasing). So the reflected bands can cancel or stack with the "upper" sidebands... this isn't something I ever thought about myself, but I was at this presentation by John Chowning a couple of weeks ago

https://youtu.be/tpysRrYXxg4

His talk made me want to watch spectrum analyzers a lot more while I design sounds.

[Aro]

FM amount is usually the modulator's level into the carrier, not its pitch.

Thanks for clarifying. It still is a hard concept for me to grasp though.

To anyone interested in the basics of FM synthesis, this is still a good read
http://www.burnkit2600.com/manuals/fm_t ... ations.pdf
Possibly following the examples with Dexed, free vst

I will definitely read this. Thanks!

[Aro]

try moving it by hand on some factory patches, it should give you an idea of what is happening if you listen carefully as you increase and decrease the amount slowly over time on a long held tone

I did this. So far it sounds like it goes from dull to bright overall. But if I move the knob slowly, it also seems to move from dull to bright, to a little less dull than before, to a little more bright with each tick. Not quite sure about the theory behind this. I will think about this more. Thanks for the tip!

[Chris-S]

Thanks for the vid. Can someone explain the formant thing? (1:20:10)

[Aro]

So the reflected bands can cancel or stack with the "upper" sidebands

I will listen for this. Thanks.

[Aro]

To anyone interested in the basics of FM synthesis, this is still a good read
http://www.burnkit2600.com/manuals/fm_t ... ations.pdf
Possibly following the examples with Dexed, free vst

I'm trying to follow along the manual with FM8. Any clue as to what the EG bias might be equivalent to?

[Niowiad]

To anyone interested in the basics of FM synthesis, this is still a good read
http://www.burnkit2600.com/manuals/fm_t ... ations.pdf
Possibly following the examples with Dexed, free vst

I'm trying to follow along the manual with FM8. Any clue as to what the EG bias might be equivalent to?

It has really been a while since I used Dexed, but that EG BIAS switch basically enables control on the operator's level(volume), done with a selected modulation source (modwheel, breath-controller, foot-controller, aftertouch).

So, as an example, if OP2 as a modulator is feeding OP1 as a carrier, enabling EG BIAS to the ModWheel allows you to use the wheel as the actual "FM amount", feeding more of OP2 into OP1.
Please note that EG BIAS in Dexed/DX7 works together with the "A mod sens" knob (aka AMS) of each individual operator, which sets the range of modulation, therefore if AMS is left at zero, the modwheel will have no effect even with ES BIAS engaged.

Doing that in FM8 will probably be something like assigning an operator's level (volume) to the modwheel.


The thing is, that manual was written for the old legendary DX7, which had a very unique (and weird) set of controls, often with unusual names. All the examples give the exact values for those original DX7 controls and it's not very easy to recreate those on other synths as they were supposed to be heard, except for Dexed which is almost a 1:1 vst transposition of the DX7.
https://asb2m10.github.io/dexed/

With that said, I personally used Dexed when I went through that manual 2-3 years ago to make things easier on myself, then never actually used Dexed again. But what I learned with it translates well to any other fm based synth.

[Aro]

All the examples give the exact values for those original DX7 controls and it's not very easy to recreate those on other synths as they were supposed to be heard, except for Dexed which is almost a 1:1 vst transposition of the DX7.

I'll give Dexed a try, but it looks complex. Thanks.

[BlackWinny]

For French readers there is an excellent book from Alain Cassagnau:
https://www.analog-fm-synth.fr/commander-le-livre/

It is very, very, very comprehensive, the most you could find, and totally in French.

A preview of some pages:
https://www.analog-fm-synth.fr/le-synthe-fm-apercu/

The history of this exceptional book:
https://www.analog-fm-synth.fr/le-synthe-fm-la-genese/