What are the standard evaluation metrics for TTS quality?

The quality of text-to-speech (TTS) systems is typically evaluated using a mix of objective metrics and subjective human judgments. These metrics help developers assess how natural, intelligible, and accurate synthesized speech sounds. Common approaches include measuring acoustic properties, conducting listener surveys, and leveraging automated tools inspired by speech recognition or neural networks. Each method has trade-offs, and combining multiple techniques often provides the most reliable assessment.

Objective metrics focus on quantifiable comparisons between synthesized and reference speech. For example, Mel-Cepstral Distortion (MCD) calculates differences in spectral features (like Mel-frequency cepstral coefficients) to gauge how closely the TTS output matches a human recording. Signal-to-Noise Ratio (SNR) measures background noise levels, while Short-Time Objective Intelligibility (STOI) predicts how understandable the speech is in noisy environments. Tools like Praat analyze pitch (F0) and timing errors, such as jitter or misaligned phoneme durations. However, these metrics often fail to capture nuances like naturalness or emotional expression. For instance, a TTS system might achieve low MCD scores but still sound robotic due to unnatural prosody or pacing.

Subjective evaluations rely on human listeners to rate qualities like naturalness, clarity, and overall preference. The Mean Opinion Score (MOS) is a standard 5-point scale (1: bad, 5: excellent) averaged across multiple raters. Comparative Mean Opinion Score (CMOS) compares two systems directly (e.g., “Which sounds more natural: A or B?”), reducing individual biases. For high-stakes evaluations, MUSHRA (MUltiple Stimuli with Hidden Reference and Anchor) tests are used, where listeners rank samples against a hidden reference recording. These methods are time-consuming and expensive but remain the gold standard for capturing perceptual quality. To address scalability, some teams use neural-based metrics like SpeechLM or ASR (Automatic Speech Recognition) word error rates, which correlate TTS output with human judgments using pretrained models. For example, a low word error rate in ASR transcription suggests high intelligibility, though it doesn’t account for naturalness.

In practice, a combination of methods is ideal. Objective metrics provide quick feedback during development, while subjective tests validate user experience. Emerging neural metrics bridge the gap by automating perceptual assessments, but they require large datasets for training. Developers should prioritize metrics aligned with their use case—for example, intelligibility metrics for accessibility tools versus naturalness scores for voice assistants.