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The ASR systems used to grade text-to-speech miss 33-67% of very rare words even when a human says them correctly

AspireAI community · arXiv (cs.SD) — BitmanagerAI / lab260
Text-to-speech is normally evaluated by transcribing the output with an automatic speech recognizer and computing word error rate, which quietly measures two systems at once — if the recognizer cannot hear a word, an error there says little about the TTS engine. This paper measures that floor by running the same recognizers on the original human recordings of the test sentences. On LibriSpeech and FLEURS the evaluation recognizer misses 33% to 67% of very rare words even in human speech, and all three recognizers miss 22% to 42%. Adjudicating every word with two more recognizer families cuts apparent TTS error on rare LibriSpeech words from 10-13% to 3-5%, and the paper shows that simply masking out words the recognizer misses in human speech is itself biased: other recognizers find real TTS errors on those words far more often (4-6% versus 0.2-0.3%) and overturn about half of the errors that mask keeps. The authors note that as many as half the remaining adjudicated errors may be spelling variants, so a listening test is still needed.
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