"bandlimiting" - what is it, exactly?

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JazzSinger
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2006/02/27 09:31:55 (permalink)

"bandlimiting" - what is it, exactly?

I see that "Wavetable synthesis" has a different meaning to what I am used to. In the P5 WIKI, Rene posted the following:

"One might think that this is equivalent to 'loop' a single wave file, but it's not. To make a transparent, pristine oscillator, a process called 'bandlimiting' needs to be performed. This is, removing some harmonics in the spectra that would generate a nasty distortion when transposed."

Is someone in a position to explain in more technical depth what happens to a waveform when it is bandlimited? And why should certain harmonics cause distortion when transposed?

My AKAI Z8 has pretty good filters and I have used it in classic synth mode without being aware of any distortion. Do AKAI implement bandlimiting but don't mention it?
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    lawapa
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    RE: "bandlimiting" - what is it, exactly? 2006/02/27 21:42:09 (permalink)
    When Dimension loads a wavetable it uses the file to build a representation. Not the file itself. Anything not usuable is discarded? This avoids filtering and saves cpu?

    Rene will be along and clear this up but this was my understanding.
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    techead
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    RE: "bandlimiting" - what is it, exactly? 2006/02/27 22:12:50 (permalink)
    Here is an explanation that I found when trying to understand what René wrote. I think this is the same thing, but if not hopefully he will set us straight.

    http://www.musicdsp.org/files/bandlimited.pdf

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    JazzSinger
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    RE: "bandlimiting" - what is it, exactly? 2006/02/28 04:01:09 (permalink)
    Ah, that's a good article, thanks!

    OK, so basically, aliasing, when reading out at a given sampling rate, will chop up the waveform if it doesn't correspond 100% to a multiple of the nyquist frequency. The closer you get to the nyquist frequency, the bigger the chopping as a proportion of the waveform (it's easier to see on a diagram), until finally the waveform bears no resemblance to the original. In fact, the fundamental frequency completely gets lost in the harmonic noise.

    So: as you go up, you cut the tops. And by the time you are at 20KHz (assuming 44.1) you have just a sine wave, regardless of what the original may have looked like.

    Probably the AKAI reads out the samples at different speeds rather than at a constant 44.1KHz (or whatever has been chosen). Using this technique, you go from digital to analog before filtering (I'm guessing), so the problem doesn't arise. The filters on the Z8 are on a separate option card, which may support my theory.

    Thank you for the link.
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    techead
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    RE: "bandlimiting" - what is it, exactly? 2006/02/28 08:41:38 (permalink)
    I'm not sure if René's approach applies some type of filtering to a loaded wavetable or if it actually converts the waveform (time-domain) to a spectra (frequency-domain) for harmonic analysis and shaping then re-synthesizes. I have no idea what the programming complexities of bandlimiting are like. But I am confident that it makes for a fantastic sounding professional synthesizer and money in René's pocket!
    #5
    René
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    RE: "bandlimiting" - what is it, exactly? 2006/02/28 09:15:36 (permalink)
    Both Dimension Pro and Rapture convert the loaded sample to the frequency domain (spectral), and then resynthesizes the waveform on demand. That has basically the same effect of prefiltering, where the filters are of a huge efficiency, not reachable in realtime applications.



    -René
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