How Phoneme Coarticulation Prevents TTS Clicks

Phoneme boundary coarticulation models are essential components of modern Text-to-Speech (TTS) synthesis that prevent digital artifacts such as clicks, pops, and unnatural acoustic jumps. By accurately predicting and simulating how adjacent speech sounds blend together in human vocal tract dynamics, these models smooth transitions between discrete phonetic units. This article explores the acoustic mechanisms behind phonetic boundary artifacts and details how coarticulation modeling ensures seamless, continuous, and natural-sounding synthetic speech.

The Cause of Clicks and Acoustic Jumps

In raw text processing, speech is represented as a discrete sequence of phonetic symbols. Early concatenative TTS systems assembled speech by stitching together pre-recorded acoustic segments representing these individual phonemes.

Without coarticulation modeling, placing two independent acoustic units side by side creates sharp discontinuities at the boundary. These discontinuities present as sudden steps in amplitude, abrupt phase mismatches, or jagged shifts in spectral envelopes (such as formant frequencies). When rendered through a digital-to-analog converter, an instantaneous jump in the waveform creates a high-frequency transient spike, which human ears perceive as an audible click, pop, or robotic glitch.

Mimicking Human Vocal Tract Dynamics

In natural human speech, the articulators—the tongue, lips, jaw, and velum—move continuously. Because of physical inertia, the vocal tract begins shaping the mouth for an upcoming sound before the current sound has finished producing. This physical phenomenon is called coarticulation.

Coarticulation models incorporate context-dependent phonetic variations (such as context windows, diphones, triphones, or learned neural embeddings) to simulate this inertia. Rather than treating a phoneme as a static acoustic block, the model defines it as a dynamic trajectory that depends on both its preceding and succeeding neighbors.

Spectral Envelope and Formant Smoothing

A key mechanism of coarticulation models is the interpolation and continuous prediction of spectral envelopes. Vowels and voiced consonants are characterized by distinct resonant peaks known as formants.

When transitioning between phonemes, coarticulation models enforce smooth trajectories across these formant frequencies rather than allowing them to jump abruptly from one state to another. By modeling the natural transitional curves of formants (like the transition from the glide /w/ to the vowel /i/), the synthetic voice maintains continuous harmonic structure, preventing harsh spectral transitions that the ear flags as unnatural.

Phase Continuity and Waveform Alignment

Audible clicks often occur at the waveform level due to phase cancellation or sudden displacement jumps. When two waveforms are joined where the phase does not match, the audio signal experiences an unnatural discontinuity across the zero-crossing axis.

Modern coarticulation modeling, especially when paired with modern neural vocoders (such as HiFi-GAN or WaveNet), models the continuous temporal dependency of acoustic waveforms. By learning overlapping temporal receptive fields, the model predicts the waveform phase continuously across boundaries. This continuous generation ensures that pressure waves do not suddenly invert or shear, eliminating the physical source of clicking noises.

Dynamic Energy and Pitch Contours

Unnatural jumps also manifest through disjointed energy levels and pitch (fundamental frequency or F0) contours. Human pitch shifts gradually across syllables, and acoustic energy naturally dips slightly at unvoiced consonant boundaries while remaining supported during voiced transitions.

Coarticulation models apply context-aware smoothing to both fundamental frequency and root-mean-square (RMS) energy. By shaping the decay of energy at the end of a phoneme and matching it to the attack profile of the subsequent phoneme, the system prevents abrupt volume jumps and unnatural pitch fluctuations that disrupt speech cadence.

Conclusion

Phoneme boundary coarticulation models bridge the gap between discrete text representations and the fluid physics of human speech. By enforcing continuous formant trajectories, ensuring phase and waveform alignment, and dynamically smoothing energy and pitch contours, these models eliminate the high-frequency transients and spectral steps responsible for clicks and acoustic jumps in synthetic speech.