Perceived TTFA: the Coval TTS latency benchmark

Coval and Gradium describe a 2026 update to the Coval TTS benchmark that measures perceived time to first audio (TTFA) by combining audio-stream arrival time, including network latency, with any leading inaudible audio before speech begins. The methodological change raised Gradium’s reported TTFA from 171.9 ms to 429.6 ms despite unchanged model and infrastructure performance, because its streams contained a median 225 ms of initial silence. Rather than relying on voice activity detection, which may overlook audible breaths, Coval detects the first audible sound using a normalized RMS loudness threshold over 10 ms windows. The revised metric revealed that models can achieve fast response-arrival times while still feeling slow to listeners if they send silence first, with Cartesia Sonic 3.5 cited as an example. In response, Gradium trained and deployed a new default model that produces audible audio immediately, reducing its median perceived TTFA to 214 ms. Coval notes that results recorded before and after the methodology change are not directly comparable, while providing open-source benchmark code, datasets, methodology, and commit-specific reproducibility information.

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