FM Synth Modulation Index -Sound Forge - PC - MAGIX Community Forums Archive

Subject:FM Synth Modulation Index Posted by: GPD Date:10/13/2005 10:30:03 AM Does anyone have any idea how to translate a modulator's amplitude (in dB) to the common FM modulation index, or deltaF? What is the equation here? Does SF set 0dB as some arbitrarily large FM index and then scale proportionally from 0 to this value accross all possible dB levels? This is fairly confusing, and juding by searching the forums, it seems I am not the only one who feels this way. Maybe it would be nice to be able to specidicy modulator "strength" in terms of I or deltaF instead of dB value that is very hard to predict exactly the effect of.
Subject:RE: FM Synth Modulation Index Reply by: mpd Date:10/13/2005 12:12:30 PM I have never thought about this before, but the help documentation is a bit odd, especially if you know how an FM circuit works...
Subject:RE: FM Synth Modulation Index Reply by: ForumAdmin Date:10/13/2005 1:22:18 PM It's based on the old OLP synths, not the textbook FM equation. The non-feedback sine->sine equation looks more like: fm(n) = gcsin[wcn + gmsin(wmn)] gc = carrier gain wc = carrier freq. gm = mod gain (scaled by wc) wm = mod freq. If you are programatically-minded and interested in creating your own synth algorithm, I'll mention that Sound Forge 8.0b allows you to read and write audio data from a script. J.
Subject:RE: FM Synth Modulation Index Reply by: GPD Date:10/13/2005 4:59:53 PM Thanks for the explaination. Could you take me one step further, and lead me through two example calcs? 1.) Carrier is 256Hz, Modulator is 1Hz. What modulator dB do I need to get a change from 255 to 257hz? 2.) Carrier is 128Hz, Modulator is 2Hz. What modulator dB do I need to get a change from 120 to 136Hz?
Subject:RE: FM Synth Modulation Index Reply by: GPD Date:10/13/2005 5:01:58 PM If it is easier, and yo wouldn't mind an example script that accomplishes one of these would be awesome also.
Subject:RE: FM Synth Modulation Index Reply by: ForumAdmin Date:10/14/2005 12:56:22 PM See my example here. J.
Subject:RE: FM Synth Modulation Index Reply by: GPD Date:10/14/2005 1:17:50 PM THX!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! That is awesome. About to try it now.
Subject:RE: FM Synth Modulation Index Reply by: GPD Date:10/14/2005 1:57:44 PM I get a complier error. Can you verify that it works for you?
Subject:RE: FM Synth Modulation Index Reply by: ForumAdmin Date:10/14/2005 2:24:46 PM Fixed in the original posting.

Subject:FM Synth Modulation Index

Posted by: GPD

Date:10/13/2005 10:30:03 AM

Does anyone have any idea how to translate a modulator's amplitude (in dB) to the common FM modulation index, or deltaF?

What is the equation here? Does SF set 0dB as some arbitrarily large FM index and then scale proportionally from 0 to this value accross all possible dB levels?

This is fairly confusing, and juding by searching the forums, it seems I am not the only one who feels this way.

Maybe it would be nice to be able to specidicy modulator "strength" in terms of I or deltaF instead of dB value that is very hard to predict exactly the effect of.