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
