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Notes from the desk of Shane Dunne, software development consultant

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sarah [2017/09/05 22:27]
shane [SARAH: FFT-based synthesizer plugin] 		sarah [2017/09/30 15:38] (current)
shane
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   - 1024 samples of each waveform (one cycle) are generated once, using exactly the same mathematical expressions used in the LFOs, resulting in mathematically "exact" waveforms having 512 harmonics.   - 1024 samples of each waveform (one cycle) are generated once, using exactly the same mathematical expressions used in the LFOs, resulting in mathematically "exact" waveforms having 512 harmonics.
   - Each mathematically-exact waveform is transformed using //juce::dsp::FFT// to produce a frequency-domain representation, which is a new 1024-element array, where each element ("coefficient") represents the relative amplitude and phase of all 512 harmonics. These three "frequency-domain" tables (one each for triangle, square, and sawtooth wave shapes) are kept in memory.   - Each mathematically-exact waveform is transformed using //juce::dsp::FFT// to produce a frequency-domain representation, which is a new 1024-element array, where each element ("coefficient") represents the relative amplitude and phase of all 512 harmonics. These three "frequency-domain" tables (one each for triangle, square, and sawtooth wave shapes) are kept in memory.
-  - Each oscillator instance has its own 1024-element array. In preparation to sound a note, it copies the coefficient data out from the appropriate common frequency-domain table to its own array, then performs an //in-place **inverse** FFT// to re-create the appropriate time-domain wave table. To prevent aliasing, only the coefficients for harmonics below the //Nyquist frequency// (one-half the sampling rate) are copied; higher-order coefficients are set to zero. +  - Each oscillator instance has its own 1024-element array. In preparation to sound a note, it copies the coefficient data out from the appropriate common frequency-domain table to its own array, then performs an //in-place **inverse FFT**// to reconstruct the appropriate time-domain wave table. To prevent aliasing, only the coefficients for harmonics below the //Nyquist frequency// (one-half the sampling rate) are copied; higher-order coefficients are set to zero.
-  - To sound a note, the oscillator resamples its own 1024-element array (wave table).+
  
 ===== Harmonic Shaping ===== ===== Harmonic Shaping =====
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 Because SARAH is using coefficient adjustments and the inverse FFT to //simulate// filtering, I decided to call this //Harmonic Shaping// rather than "filtering". I'm not at all convinced this is a sensible alternative to conventional (time-domain) digital filtering, but it is certainly interesting, and allowed me to implement simple time-varying timbres (using both an envelope generator and an LFO) with just a few simple changes to the existing anti-aliased oscillator code. Because SARAH is using coefficient adjustments and the inverse FFT to //simulate// filtering, I decided to call this //Harmonic Shaping// rather than "filtering". I'm not at all convinced this is a sensible alternative to conventional (time-domain) digital filtering, but it is certainly interesting, and allowed me to implement simple time-varying timbres (using both an envelope generator and an LFO) with just a few simple changes to the existing anti-aliased oscillator code.
 +
 +===== The details =====
 +See [[sarah_oscillator_details|this page]] for the gritty details of SARAH's oscillator implementation.
 +
 +===== Skinning =====
 +As of 29 September, 2017, SARAH uses a single-view GUI instead of the multi-tabbed approach inherited from [[https://github.com/getdunne/VanillaJuce|VanillaJuce]]. See [[sarah_skinning|this page]] for some thoughts on the two approaches.
  
  
  
sarah.1504650435.txt.gz · Last modified: 2017/09/05 22:27 by shane

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