Sample Based Synthesis

1
Sample Based Synthesis

• Basic Concepts and Methods

2
How Samplers Work

• One or more digitally recorded sounds (samples)
  are played back under MIDI keyboard control.
• Original pitch of sample called root or basis
  note.
• Pitch may be changed by resampling.
• Multiple samples at different root pitches
  usually necessary for instrumental sounds.
• Sample and pitch determined by MIDI NN.

3
How Samplers Work

• Sample amplitudes usually normalized.
• Playback amplitude determined by velocity, ADSR,
  keyboard scaling, aftertouch, etc.
• Duration determined by MIDI note length and ADSR
  settings.
• Short samples may be extended by looping a
  portion of the waveform.
• LFO, Filters, other effects may be used.

4
Pitch Transposition

• To transpose a sample to a specific pitch, you
  need this info, usually stored with sample
• Original sample rate (of recording)
• Original root pitch (basis pitch)
• Pitch is changed by resampling
• Essentially the same as varying playback speed on
  a tape recorder
• Artifacts inherent in this process
• Duration changes with pitch
• Timbre is affected (Mickey Mouse effect)

5
Keymaps Multi-Sample Sets

• To minimize transposition artifacts, multiple
  samples are employed.
• Three or more samples/octave typical
• No sample transposed more than a few semitones up
  or down
• Samples are mapped across the KBD
• Sample maps (keymaps) are indexed by incoming
  MIDI Note number
• Contain name, root pitch, SR, loop points, and
  other info for each sample

6
Sample Layers

• Some samplers allow multiple layers
• Several keymaps in effect simultaneously
• Allows KBD to trigger multiple samples
• Relative amplitudes of layers may be controlled
  (crossfaded) in various ways
• Velocity
• Keyboard scaling
• MIDI Controller

7
Loops

• Most samplers utilized loops to produce sustained
  notes from short samples
• Several types
• Sustain (in effect while key depressed)
• Forward Loop (most common)
• Bidirectional (mirror) Loop (less common)
• Release (in effect during decay)
• Forward and Bidirectional also possible
• Very rarely used

8
Loops Continued

• Playback Order
• Forward Loop
• Start point to loop end point, then
• Loop start to loop end, repeatedly
• Bi-Directional Loop
• Start point to loop end point, then
• Back and forth from Loop end to loop start, loop
  start to loop end, repeatedly

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Tips for Making Good Loops

• Loop a sustained section, if possible
• Amplitude more or less constant
• Pitch and timbre more or less constant
• Loop a number of complete cycles
• For Forward Loops, find zero crossings
• For Mirror Loops, use peak points
• Use Crossfade Loops for sounds that change in
  amplitude, pitch, or timbre

10
Reading Assignment

• Pellman, pp. 278-287