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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>
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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

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