Wavetable Synthesis

Wavetable Synthesis

  • Idea: Save a per-key waveform, play it back when key is pressed

  • Needs lots of memory (not a problem anymore)

  • Conceptual issues

    • Want note to last as long as key is held: looping is needed

    • May not have a wave for every key: pitch shifting is needed

    • Wave may have inappropriate envelope: envelope generation is desirable

    • May want to modify wave after-the-fact: effects are desirable

Sustain and Looping

  • Want to maintain sustain level for as long as key is held down

  • Infinite time stretching is a thing: usually achieved by "looping"

  • Loop start and end may be marked manually, or can try to infer the sustain region

Building a Loop: Frequency Domain

  • Use DFT on a window of samples

  • Stretch the spectrum as desired

  • Use inverse DFT

  • Need to use overlapping windows to preserve frequency changes over time

  • Smearing is real

Building a Loop: Time Domain

  • Use autocorrelation to try to find a reasonable constant period of the sample

    $$ P = argmax_{t} ~~ x[0..] \cdot x[t..] $$

    (where the signal x is treated as cyclic)

  • Crop the sample so that the start matches the end: usually at zero-crossing

  • May need gain and frequency adjustment to avoid cyclic effects

  • The longer the sample, the more "real" it will sound and the harder it will be to avoid loop effects

Pitch Shifting

  • As we discovered previously, the pitch of the note can be shifted by resampling

  • Knowing the fundamental pitch of the original sample(s) is actually hard (unless the sample comes from a source with "known" frequency). Strongest component of DFT sometimes works.

  • Remember, pitch shifting is frequency stretching.

Resampling Technique

  • Don't want to dynamically resample on every keypress after generating a 93-coefficient FIR anti-aliasing filter (probably)

  • Possibly use small adjustable IIR filter

  • Resample to only a few frequencies (octaves, usually) in advance, then use linear interpolation during synthesis to get final pitch from nearest properly-resampled cache element

  • As long as close to the sample rate, linear interpolation is "good enough"

  • Ideally, just have a sample for every key (fat chance)

Wavetable Envelopes

  • Question: Do you want the "natural" envelope from the sample, a "synthetic" ADSR envelope, or some combination of the two?

  • This is really a question for the musician, so should probably be prepared to use either

  • Ideally, sample will come with start and end of sustain marked; otherwise you have to guess

Soundfonts

  • Standard file format for wavetables, set up for synthesizer consumption

  • File format is immensely complicated, so use a library and think carefully about handling

  • "General MIDI" synthesizers are almost always this: standard GM soundfonts are readily available

General MIDI

  • Standard for cross-manufacturer synth capabilities

  • Most importantly, identifies 128 specific MIDI "programs" with 128 "specific" named sounds

  • Examples: "Alto Sax" (66), "Blown Bottle" (77), "Rock Organ" (19), "Slap Bass 1" (37), "Bagpipe" (110, in "Ethnic" sounds category), "Helicopter" (126)

  • MIDI channel 10 has its own set of GM percussion sounds

Other GM Requirments

  • 24-voice polyphony: 16 instrument + 8 percussion (?)

  • Certain standard MIDI control change messages must be honored

  • General MIDI Level 2 (GM2) extends all this

  • You have heard this a lot

Last modified: Sunday, 10 May 2020, 1:07 AM