What are the limitations of current TTS technology from a research perspective?

Current text-to-speech (TTS) technology faces several research-related limitations, particularly in achieving naturalness, handling rare or ambiguous inputs, and scaling efficiently. These challenges stem from gaps in modeling human speech patterns, adapting to diverse linguistic contexts, and balancing computational costs. Below, I’ll outline three key limitations with specific examples and technical context.

First, TTS systems often struggle with prosody—the rhythm, stress, and intonation of speech. While modern neural models like Tacotron or FastSpeech generate intelligible speech, they frequently produce flat or unnatural cadence, especially in longer sentences. For example, the sentence “I didn’t say he stole the money” can carry different meanings depending on which word is emphasized, but many TTS systems fail to infer the correct emphasis without explicit markup. This limitation arises because models are trained on averaged prosodic patterns from datasets, lacking the ability to dynamically adapt to context or speaker intent. Researchers are exploring methods to inject contextual awareness (e.g., leveraging semantic or syntactic cues), but these approaches often require annotated data or complex architectures that are difficult to generalize.

Second, handling rare words, homographs, or multilingual text remains a challenge. TTS systems typically rely on pronunciation dictionaries or grapheme-to-phoneme models, which fail for out-of-vocabulary terms like technical jargon (e.g., “ChatGPT” pronounced as “chat-G-P-T” instead of “chat-jee-pee-tee”) or code-switched phrases (e.g., mixing English and Spanish in one sentence). Homographs like “read” (past vs. present tense) also cause errors unless disambiguated by surrounding text. For instance, a system might mispronounce “He will read the book” versus “He read the book” if context isn’t properly analyzed. While some solutions use external language models or rule-based post-processing, these add complexity and aren’t universally reliable. Research into unified multilingual models or better integration of linguistic knowledge into neural networks is ongoing but incomplete.

Third, computational efficiency and scalability limit real-world deployment. High-quality neural TTS models, such as autoregressive or diffusion-based systems, often require significant GPU memory and inference time, making them impractical for edge devices or low-latency applications. For example, generating one minute of speech in real-time might demand a 10 GB model, which is infeasible for mobile apps. Additionally, supporting multiple languages or voices usually requires training separate models, increasing storage and maintenance costs. While techniques like model pruning or knowledge distillation help, they often degrade output quality. Researchers are exploring lightweight architectures (e.g., non-autoregressive models) and cross-lingual transfer learning, but trade-offs between speed, size, and naturalness persist. These limitations highlight the need for more efficient algorithms and hardware-aware optimization in TTS research.