I’’m synthesising a simple sine wave using the IFFT. I’m controlling the pitch of the sine wave by controlling the bin number and the phase for that bin number.
I’ve synthesised an F whose frequency is very close to a bin centre frequency and everything looks good.
however when I detune that same note away from the bin centre frequency I start to get additional harmonics showing up. Like this: (Note the -30db peak just after the significant peak, and the appearance of signal all the way to the left.)
Why is this occurring???
Answer
Note that additive overlapped windows of pure sinewaves of different phases will not produce a pure sinewave. The difference may include something like a low frequency beat note, which might explain the low frequency content in the 2nd spectrum. Modulation side-bands might explain the nearby higher peak. See trigonometric sum and difference formulas.
Splitting the magnitude of your between bin frequency between the two (or more) nearest bins using Sinc interpolation might help reduce your artifacts somewhat, depending on how you do your delta phase correction/interpolation between windows.
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