Duration diversity and quality (FDD) Additionally, we evaluate the quality and diversity of
duration samples at the distribution level, similar to our audio evaluation of diversity and quality via
FSD. We produce one sample per utterance from a duration model and collect all sampled phoneme
durations, possibly many per-utterance, into an empirical distribution. We compare means and
variances of this sampled distribution versus the means and variances of the training distribution,
labeled µ, s, and µ′, s′ respectively. We define the Fréchet duration distance (FDD) as the Fréchet
distance between the distributions

(10)
treated as though they were Gaussians. FDD depends on the sampled durations accurately reflecting
the training distribution of real durations. As for FSD, this metric is specific to unconditional
text-to-speech generation.

√
(µ − µ′)2 + s + s′ − 2

ss′,

C.3 Duration model evaluation with standalone metrics
Voicebox-Text-GuidedMultilingual UniversalSpeechGenerationatScale