[music-dsp] additive synthesis

Sampo Syreeni decoy at iki.fi
Mon Oct 8 08:36:06 EDT 2001

On Sun, 7 Oct 2001, Philipp Wassibauer wrote:

>What are the differences between harmonics and resynthesized sound based
>on frequency analysis?

That depends a lot on what sort of analysis and synthesis we are talking
about. If you use straight, windowed FFT, you'll end up with a perfect
reconstruction but the data you'll get is just about useless for
perceptually significant/easy modification. If you use MacAulay-Quatieri
(MQ), you'll get data which is inharmonic, and so quite expensive to
resynthesize. It's useful for modification, especially with quasi-periodic
input, but noise tends to throw the algo off. Then there are hybrid
algorithms, like Spectral Modelling Synthesis (SMS), which attempt to
first break off the noise and model it separately (white noise to a time
variant LPC filter), and then apply something like MQ to decompose the
quasi-periodic part. Most useful, though still computationally intensive
and conceptually difficult to master.

>Would harmonic sounds sound good of just like an organ?

Like an *electronic* organ. Harmonic sounds are mostly boring, at least
when used in isolation.

>I was looking at the Kawai K5000, is that only harmonics or
>resynthesized sound too?

K5000 is a hybrid. It uses a bank of upto 6 half-FFT's (the upper or lower
64 harmonics, but not both, of a full 128 point FFT) to synthesize
periodic waveforms (period=FFT frame length), and controls the 64 partials
of each of those via envelopes, on a FFT frame-by-frame basis. However, it
also permits each of the 6 half-FFT's to be detuned wrt one another, and
employs an extra subtractive filter on top of that. Those of us who read
the Computer Music Journal will recognize this setup as a funky
implementation of FFT accelerated 6-part group additive synthesis plus a
low order filter thrown in for kicks.

All in all, it's an ideal toy for those who *really* want to tinker --
more parameters than you'd *ever* like to see. It can also sound very
real, after you've learned how to use it. There is also a variant of K5000
with sample playback capability added, for sounds like cymbals which are
really difficult to get right with additive.

Sampo Syreeni, aka decoy - mailto:decoy at iki.fi, tel:+358-50-5756111
student/math+cs/helsinki university, http://www.iki.fi/~decoy/front
openpgp: 050985C2/025E D175 ABE5 027C 9494 EEB0 E090 8BA9 0509 85C2

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