## 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