Score-based Continuous-time Discrete Diffusion Models

Haoran Sun, Lijun Yu, Bo Dai, Dale Schuurmans, Hanjun Dai

Keywords: discrete space diffusion, discrete score matching, continuous-time diffusion

Abstract: Score-based modeling through stochastic differential equations (SDEs) has provided a new perspective on diffusion models, and demonstrated superior performance on continuous data. However, the gradient of the log-likelihood function, \ie, the score function, is not properly defined for discrete spaces. This makes it non-trivial to adapt SDE with score functions to categorical data. In this paper, we extend diffusion models to discrete variables by introducing a stochastic jump process where the reverse process denoises via a continuous-time Markov chain. This formulation admits an analytical simulation during backward sampling. To learn the reverse process, we extend score matching to general categorical data, and show that an unbiased estimator can be obtained via simple matching of the conditional marginal distributions. We demonstrate the effectiveness of the proposed method on a set of synthetic and real-world music and image benchmarks.

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TL;DR: a generalized discrete score matching for learning continuous-time diffusion in categorical spaces, with new parameterization and novel analytical sampling.