How a DSP Engineer Fixed Imperceptible Vibrato in a Voice Design App

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A developer building a voice design app added a pitch-variation slider to reduce robotic-sounding speech, but found the vibrato effect was nearly inaudible even at maximum settings. The root cause was a modulation depth of only ±24 cents — less than a quarter of a semitone — which is too subtle to perceive in a voice with natural overtones and formants. The team implemented vibrato using a time-varying fractional delay, where the rate of change of delay time shifts pitch via the Doppler effect, oscillating at 4.5 Hz to mimic natural human vibrato. Increasing the maximum delay from 1.0 ms to approximately 3.0 ms raised modulation depth to around ±65 cents, making the effect clearly audible. The article concludes that correct DSP parameters alone are insufficient — perceptual thresholds must guide the design of audio effects.

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