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# Additional Notes on Wavetable Synthesis - PowerPoint PPT Presentation

Additional Notes on Wavetable Synthesis. R.C. Maher ECEN4002/5002 DSP Laboratory Spring 2003. Wavetable Frequency Control. Assume table is length N , holds one period of the signal, and the sample rate is f s Waveform frequency is f s / N. New sample every 1/f s ; Table frequency f s / N.

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R.C. Maher

ECEN4002/5002 DSP Laboratory

Spring 2003

Wavetable Frequency Control
• Assume table is length N, holds one period of the signal, and the sample rate is fs
• Waveform frequency is fs/N

New sample every 1/fs; Table frequency fs/N

Wavetable (N)

Wavetable Details R. C. Maher

Frequency Control (cont.)
• In order to have a waveform frequency different than fs/N, need to re-sample the waveform: this implies sample rate conversion
• Typically use a phase counter or look-up accumulator to hold current re-sampling location. LUA is incremented by the sample increment value, defined by:

Wavetable Details R. C. Maher

Wavetable Frequency Resolution
• Given an [integer:fraction] SI representation, the frequency resolution of the wavetable is determined by the number of fractional bits
• Ex: N=1024, L=24, fs=48000: 2.7940x10-6 Hz
• Ex: N=256, L=8, fs=48000: 0.7324 Hz

Wavetable Details R. C. Maher

Aliasing due to resampling
• The discrete-time signal represented by the wavetable has a spectrum that may occupy the entire bandwidth from 0 - fs/2
• Re-sampling the waveform to change its period can result in aliasing if the wavetable is not properly bandlimited
• Aliasing generally limits how far we can “re-tune” a wavetable

Wavetable Details R. C. Maher

Linear Interpolation
• LUA fraction is used to interpolate the table.
• Effectively convolving stored samples with triangle, so spectrum modified by sinc2 shape
• Need to handle “end of table” issue: LUA may point to region between last stored sample and the start of the table
• Distortion due to difference between “true” value and linear interpolation estimate can be audible

y[L+1]

y[L]

L

L + fract

L+1

Wavetable Details R. C. Maher

Improved wavetable synthesis
• A single stored cycle of a timbre sounds too regular, and sample rate conversion distorts spectral envelope
• Generally make recordings of a real instrument at a variety of musical pitches in order to limit the amount of re-tuning
• Most musical instruments have an aperiodic attack interval followed by a quasi-periodic sustain interval: synthesizer splits signal into a “one-shot” attack spliced to a looped sustain
• Modulate SI and amplitude with some vibrato, tremolo, and random fluctuations

Wavetable Details R. C. Maher

Wavetable Synthesizer

Attack Wavetable

Looped Wavetable

Gain Multiplier

Sample Increment (SI)

Envelope

Modulate with sinusoid for vibrato

Wavetable Details R. C. Maher

Other enhancements
• Layer multiple wavetables
• Stereo wavetables