on 2010-04-10 at 20:44 francis keyes wrote:

>Hey All, I'm hoping to use csound to impliment the Fm synthesis
>function as seen in the attached image but I might be in a bit over my
>head.  However, I suspect this is a relatively simple task for an
>expert with csound.  Can anyone give me a hand?


i'm not an "expert with csound", but i'll try to give a hand anyway.
the formula you quote is the generic expression for double frequency
modulation (DFM), that is, two oscillators modulate a carrier of zero
frequency. it's fairly simple to implement using three oscillators.

the modulating oscillators have frequencies fm1 = N1*f0 and fm2 = N2*fo
respectively. their amplitudes are the respective peak frequency
deviations (modulation index times modulating frequency): I1*fm1 and
I2*fm2.

since carrier frequency is zero, the frequency of the carrier
oscillator is just the sum of the output of the two modulators (see
the formula). i attach a very simple instrument written in a couple of
minutes, i hope there are no errors in the code.

the modulation indexes are fixed and the spectrum is therefore static.
the interest of fm is of course the ability to produce dynamic spectra,
so you should use some kind of envelopes for the modulators.

hope this helps.

best,

lj
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