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Author Topic: Linear Vs Exponential FM  (Read 2844 times)


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Linear Vs Exponential FM
« on: February 20, 2017, 01:18:12 am »
Is there a reason why the Linear & Exponential FM modulation inputs work almost the opposite of how it would in the real world?

So according to the manual the FM on the OSC Driver is a exponential one. Plugging in another oscillator here produces a pretty controlled FM sound which key tracks really well. There is subtle change in pitch when the FM index is changed but not drastic like you would expect from a exponential FM input on a analog modular synth.

The Lin FM input on the other hand changes the pitch quite drastically and also doesn't key track at all. Even when both the carrier and modulator as set to keytrack K1.

Any reason for this?


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Re: Linear Vs Exponential FM
« Reply #1 on: February 21, 2017, 08:33:41 pm »
Good questions. I would like answers for that too.
I also would like to know why the modulation amount is different between EXP and LIN when both is set to max, and the input signal is the same?
According to the manual the maximum amplitude of the modulation should be the same. At least as i understand the manual.


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Re: Linear Vs Exponential FM
« Reply #2 on: September 03, 2018, 03:23:01 am »
Exponential FM is intended to work with a 1volt/octave keyboard voltage controlling both the carrier and modulator VCOs. The so-called "Oscillator Driver" inserts an exponential converter into the signal path, which is intended to receive a keyboard voltage. This is why the vast number of users of Moog modular systems never bothered with the "Linear" FM inputs.

The "Linear" FM inputs produce a **fixed amount of frequency change** in response to an incoming signal. This form of FM is consistent with
"traditional" FM as used in radio broadcast applications, etc. This is also the type of FM used in the FM techniques devused by John Chowning and subsequently licensed by Yamaha. Any number of hardware & software FM symthesizers will do this kind of stuff infintely better than anything
you could get out of an old Moog 55 or for that matter, from Modular V.


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