Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: Notesfiles $Revision: 1.7.0.8 $; site uiucuxc Path: utzoo!watmath!clyde!cbosgd!ihnp4!inuxc!pur-ee!uiucdcs!uiucuxc!aurenz From: aurenz@uiucuxc.Uiuc.ARPA Newsgroups: net.music.synth Subject: Re: Stupid Question About FM Message-ID: <101000003@uiucuxc> Date: Tue, 17-Sep-85 12:54:00 EDT Article-I.D.: uiucuxc.101000003 Posted: Tue Sep 17 12:54:00 1985 Date-Received: Thu, 19-Sep-85 05:07:32 EDT References: <817@mit-vax.UUCP> Lines: 66 Nf-ID: #R:mit-vax.UUCP:-81700:uiucuxc:101000003:000:2292 Nf-From: uiucuxc.Uiuc.ARPA!aurenz Sep 17 11:54:00 1985 > What, exactly *is* FM sound synthesis and why is it so great? > Somebody out there must know! I expect that I am simply > missing something obvious because I do know: > > 1) What a Fourrier transform is. > 2) What a Z-transform is > 3) What FM is (as a process) > 4) Just about any other "signal processing" jargon. If you understand FM as a process (e.g. as applied to radio communication), then you understand FM as a synthesis technique. All math is the same, only the operating parameters are different. For example: 1) In FM radio, the modulator signal is in the audio band (20-20Khz) and the carrier is around 100Mhz. With FM synthesis, both carrier AND modulator are in the audio band, and are generally related by small integer ratios (e.g. 2:1, 3:1 etc.) Non-integer ratios are also used to produce non-harmonic spectra (i.e. metal sounds). 2) In FM radio, the modulation index tends to stay constant; in FM synthesis this index varys with time. Doing this varys the spectra over time, which is what "real" and interesting sounds do. 3) So for simple simple case of sinusoidal modulation (one carrier one modulator): +-------+ | Mod | +-------+ | V +-------+ | Car | +-------+ | V Out you can use the standard bessel functions to compute the output spectra. Of course, when you stack your modulators the computation gets much trickier. The reason FM is "so great" is that it's a relatively cheap way to generate very complex spectra with relatively few parameters (as opposed to additive synth, which needs quite a lot). So in that respect, FM is a more "powerful" technique than additive synth. The drawback of FM is, as mentioned in (3), it's very difficult to transform between (parameters <-> spectra) for all but very simple cases. Hence the need for much heuristic knowledge of the behaviour of FM spectra. By contrast, in additive synthesis one can basically "lift" the necessary synth parameters from a 2d spectral plot. Well, that's enough hot air for now. Hope it helps! ----------------------------------------------------------- Scot Aurenz { ihnp4! pur-ee! } uiucdcs!uiucuxc!aurenz