preenfm Forum

PreenFM => preenfm2 and preenfm3 => Topic started by: SMF on December 26, 2020, 07:30:50 PM

Title: If you ever wanted to know how DX7-Operatorvolumes translate to MIs...
Post by: SMF on December 26, 2020, 07:30:50 PM

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(*)
0	0.00
5	0.00	no matching value: 0.01 is too much 0.00 is too less
10	0.00	no matching value: 0.01 is too much 0.00 is too less
15	0.00	no matching value: 0.01 is too much 0.00 is too less
20	0.01
25	0.01	no matching value: 0.02 is too much 0.01 is too less
30	0.03	??hmm?? bigger jump than expected..
35	0.06
40	0.08
45	0.12
50	0.15
55	0.27
60	0.42
65	0.63
70	0.95
75	1.25
80	2.45
85	3.33
90	5.00
95	6.67
99	10.00

(*) 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

Title: Re: If you ever wanted to know how DX7-Operatorvolumes translate to MIs...
Post by: Xavier on December 26, 2020, 10:40:17 PM

As i never owned a DX7 i'm not able to comment the comparison.

Quote from: SMF on December 26, 2020, 07:30:50 PM

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.

Quote from: SMF on December 26, 2020, 07:30:50 PM

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  ;)

Title: Re: If you ever wanted to know how DX7-Operatorvolumes translate to MIs...
Post by: ericd on December 27, 2020, 12:17:07 AM

This is a great reference - thanks for posting.