Add native Coqui XTTS v2 voice cloning

[DrewThomasson]

Summary

• add a native Coqui XTTS v2 family for offline multilingual TTS and zero-shot voice cloning
• implement reference mel frontends, conditioning encoder + Perceiver, speaker encoder, 30-layer autoregressive GPT, and speaker-conditioned HiFiGAN decoder with ggml
• add BPE tokenization, request validation, session/registry integration, conversion tooling, documentation, and F16/Q8_0 GGUF packages
• preserve the conditioning Perceiver in F32 to avoid half-precision NaNs and keep convolution/embedding/logit-sensitive tensors at safe precision in Q8_0
• match Hugging Face repetition-penalty semantics, including XTTS's synthetic prefix ids, so generation emits natural EOS instead of becoming rough or overlong

Model artifacts

Hugging Face PR: https://huggingface.co/audio-cpp/audio.cpp-gguf/discussions/9

• xtts-v2-q8_0.gguf: 616,802,944 bytes, SHA-256 e07fd16d7365575fc9fe3946a70e9510f73f904983459d8eef060e91e62cfd4a
• xtts-v2-f16.gguf: 976,290,816 bytes, SHA-256 1db573bd06510f9fb8513913cc4a20b3b4340d23f918dbaf7dde695dda1ce055

Both packages embed the tokenizer, config, CPML license, and this model spec, and both pass the conditioning → GPT → HiFiGAN native probe.

Validation

• complete CPU build with model and unit-test targets enabled
• native speaker embedding vs Coqui: correlation 0.999999999993, RMSE 1.66e-7
• native HiFiGAN vs Coqui for identical inputs: correlation 0.99999999983, RMSE 6.31e-7
• Q8_0 GPT prefill vs Coqui: logits correlation 0.9999105, identical argmax token 808
• final Q8_0 CLI synthesis: 24 kHz mono, 2.965 seconds, natural EOS
• independent Whisper ASR of final demo: Hello? This is a native voice clone. for input Hello, this is a native voice clone.
• CTest: 54 passed, 4 skipped for unavailable model fixtures, one unrelated existing macOS /var vs /private/var alias failure in fun_asr_nano_assets_test

The XTTS model and generated output are governed by the Coqui Public Model License 1.0.0 and are non-commercial.

[DrewThomasson]

Native Q8_0 synthesis demo

Generated end-to-end by this PR's audiocpp_cli using the published Q8_0 GGUF and Coqui's bundled English reference clip. The input text is: “Hello, this is a native voice clone.” The MP4 contains H.264 video plus 24 kHz AAC audio so it plays inline on GitHub.

xtts-v2-audiocpp-demo.mp4

Independent Whisper-base.en transcription: “Hello? This is a native voice clone.”

[DrewThomasson]

Same as usual if this passes your requirements this also needs to be merged

https://huggingface.co/audio-cpp/audio.cpp-gguf/discussions/9

👍

[DrewThomasson]

Synthetic voice-cloning demo — alternate reference

This is an explicitly labeled synthetic test using the user-supplied DavidAttenborough.wav reference. It was generated end-to-end by this PR's native Q8_0 audiocpp_cli; it is not a recording of David Attenborough.

Spoken text: “This is a synthetic voice clone generated locally by audio dot C P P. It is not a real recording of David Attenborough.”

xtts-v2-attenborough-demo.mp4

[DrewThomasson]

Hm that output is good but... perhaps quantization the model has lowered the voice cloning ability