sanoTTS: The Smallest Real-Time Neural TTS on a General-Purpose Microcontroller

This paper describes an audited neural text-to-speech stack that runs from phoneme IDs to 22.05-kHz PCM on general-purpose microcontrollers. Its deployed graph has 567,008 parameters, and its two int8 blobs occupy 679,832 bytes. On an ESP32-S3, the complete duration-acoustic-inverse-STFT path generates 4.54 s of speech in 1.02 s (0.22x real time) without a neural accelerator. The same portable C core runs offline at 5.72x real time on an FPU-less ESP32-C3. To our knowledge, this is the smallest complete phoneme-to-waveform neural TTS graph demonstrated in real time on a general-purpose microcontroller without a neural accelerator. We derive the students from the conditional-VAE objective of their Piper/VITS teachers and state the duration, latent-interface, waveform, adversarial, and joint-distillation losses used in training. The size and speed come with an audible cost: on unseen text, the embedded stack distilled from en_US-kristin-medium scores 2.54 SCOREQ and 2.80 UTMOS, compared with 4.68 and 4.42 for its teacher. A separate English quality package uses the stronger en_US-amy-medium teacher. Its 1,454,284-parameter Pareto point scores 4.13 SCOREQ and 4.10 UTMOS; a 1,834,380-parameter variant scores 4.16 SCOREQ. A controlled capacity study with Kristin identifies the decoder, rather than the output representation, as the main constraint. Two evaluation failures also affected the work: a narrow, templated test set overstated one early student's SCOREQ by 1.35, and aggregate quality predictors missed a sibilant failure that was evident in listening and in a phoneme-resolved spectral probe. Checksums cover the reported model blobs, runtime ports, and golden vectors.