Author Topic: If you ever wanted to know how DX7-Operatorvolumes translate to MIs...  (Read 2131 times)

SMF

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Sometimes it's quite handy to be able to translate the modulation-indices of one FM-synth to the other. And sometimes you are just curious why you seem unable to replicate some timbre with a different FM-synth, as well...

The following was fine-tuned by ear and verified using a spectrum-analyzer:

DX-7          PreenFM 2(*)
  00.00
  50.00no matching value: 0.01 is too much 0.00 is too less
100.00no matching value: 0.01 is too much 0.00 is too less
150.00no matching value: 0.01 is too much 0.00 is too less
200.01
250.01no matching value: 0.02 is too much 0.01 is too less
300.03??hmm?? bigger jump than expected..
350.06
400.08
450.12
500.15
550.27
600.42
650.63
700.95
751.25
802.45
853.33
905.00
956.67
9910.00

(*) These values are averages over different octaves. The reason for this is: The PreenFM2 seems to have a (quite strongly) varying modulation-index in between different octaves. To get the same spectrum but shifted upwards by one octave you have to decrease the according MI a little bit (sometimes even relatively drastic and not subtle changes are required to keep the spectrum decently steady...). This is neither noticeable on a real DX7, nor on dexed nor FM8. And it's a little bit unexpected, too. (hhmm,... just thinking... could this eventually be due to the high-pass-filter required in between each of the operators for "real" FM with more than 2-Ops?)

Another interesting finding, when trying to estimate the above table: Looking at the spectra obtained by modulation-indices of 0.25, 0.50, 0.75, 1.00, 2.00, 3.00, 4.00 and 5.00, from an ideal 2-OP FM without the high-pass-filter and comparing this to the spectral output of the PreenFM2, I wonder if the MIs really are the modulation-index or just something quite close? Alternatively the highpass-filter could introduce some IMD here, too. The effect is strange and like this: The wrong partials get canceled out and the wrong partials get amplified (mainly in the low frequency-end), resulting in the inability to do the expected FM-highpass-spectra with really high mod-indices. Hmm,...

best,
Stefan

Xavier

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Re: If you ever wanted to know how DX7-Operatorvolumes translate to MIs...
« Reply #1 on: December 26, 2020, 10:40:17 PM »
As i never owned a DX7 i'm not able to comment the comparison.

hhmm,... just thinking... could this eventually be due to the high-pass-filter required in between each of the operators for "real" FM with more than 2-Ops?

The high-pass filter is added only on operators where it's usefull.
If you want to study its impact, you can use for example ALGO 10 without using ALGO 5 & 6.
OP4 has a high pass filter, OP2 hasn't.

I wonder if the MIs really are the modulation-index or just something quite close?

In the Pfm2 they're supposed to be exactly the modulation-index of the FM formula.
I say "supposed" because they have changed so many times since i wrote the first implementation... and i don't remember exactly  ;)
« Last Edit: December 26, 2020, 10:42:09 PM by Xavier »

ericd

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Re: If you ever wanted to know how DX7-Operatorvolumes translate to MIs...
« Reply #2 on: December 27, 2020, 12:17:07 AM »
This is a great reference - thanks for posting.