fix(tts): reject decoded audio that cannot be speech

[danilogr]

Follow-up to #19, which @pskrunner14 suggested opening separately in #20.

What happens today

A numerical failure in the decoder or codec is written out as a valid WAV that plays as silence: exit 0, plausible duration, normal realtime factor, nothing in the logs. That turns a clear bug into an invisible one, and it is why the M5 fault in #19 went unnoticed.

What this does

Rejects a decoded chunk that cannot be speech and fails the synthesis, rather than writing it.

Two conditions trigger a rejection, because one is not enough. The first is a non-finite sample, the obvious one. But ggml_clamp(x, -1, 1) is the last op of the NanoCodec graph and Metal's clamp maps NaN onto the lower bound, so a fully NaN decode arrives as a finite -1.0f that write_audio converts to a legal -32767. An isfinite check alone never fires on it. The second condition catches exactly that case: a chunk pinned bit-identically to a clamp bound.

Loud audio is unaffected: every sample has to be the same full-scale value, and even a fully saturated waveform crosses zero. Constants that are not full scale stay valid, so digital silence passes.

The check runs on the decoded chunk, before overlap-add crossfades it against its neighbour and it stops being recognisable.

Verification

Apple M5, macOS 26.6.2, metal-tts and metal-speech.

• On unpatched ggml, where the MagpieTTS on the Metal backend emits a constant -32767 DC signal instead of speech (macOS, Apple Silicon) #19 fault is live, synthesis now stops on the first chunk and writes no file. With fix(metal): stop MagpieTTS returning silence on M5-class devices #20 applied, and on CPU, output is unchanged.
• 216 real syntheses (12 texts x 3 voices x 3 seeds, CPU and Metal) produced byte-identical audio with and without the check: 0 rejected, 0 mismatches.
• --cfg-scale to 30 and --temperature to 3.0 on a shouting prompt: peak sample 22,489 of 32,767, zero full-scale samples.
• ctest 8/8 including the new test, pre-commit run --all-files clean.

Cost is 0.3 us per 1024-sample streaming chunk and 18 us for a 2.7 s utterance, against roughly 2.6 s to synthesize it. End-to-end timing over 8 interleaved runs stayed inside noise.

Limitations

• The guard has no partial mode: it passes a chunk through untouched or fails the run. It cannot drop audio.
• The unit test reproduces the overlap-add case but does not guard the production call site; removing that call still leaves the test passing.
• Library and C API callers see only MagpieTTS synthesis failed. The reason goes to stderr, as it does for every failure on that path.
• An empty chunk is a legitimate decode result and is accepted. I could not produce one in normal synthesis, so that path is unexercised.

Summary by CodeRabbit

• Bug Fixes

  • Improved text-to-speech audio validation to detect invalid decoded output, including non-finite samples and unusable constant full-scale audio.
  • Prevented invalid audio from continuing through processing, improving playback reliability.
  • Preserved support for valid speech, silence, clipped waveforms, and empty audio where appropriate.

• Tests

  • Added coverage for audio validation and post-processing behavior across valid and invalid waveform cases.

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📝 Walkthrough

Walkthrough

Magpie TTS now validates decoded audio before overlap-add processing. The validator accepts empty audio, rejects non-finite and uniform full-scale samples, and has dedicated CTest coverage.

Changes

Magpie audio validation

Layer / File(s)	Summary
Decoded audio validation flow src/tts/magpietts/magpietts.h, src/tts/magpietts/magpietts.cpp	Adds magpie_require_valid_audio and invokes it before overlap-add processing. The validator rejects non-finite samples and non-empty uniform full-scale audio.
Audio validation test coverage tests/cpp/tts/test_magpietts_audio_sanity.cpp, tests/cpp/tts/CMakeLists.txt	Adds sanity cases for valid, clipped, non-finite, and post-processed audio, then builds and registers the test with CTest.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: 🟡 Moderate · up to 9c518

Invalid decoded audio is now rejected before it reaches output processing, preventing corrupt WAV output. However, the regression test does not exercise synthesis end-to-end, so a future change could bypass this safeguard without test detection; add a controlled invalid-decoder synthesis test before merging.

Sequence Diagram(s)

sequenceDiagram
  participant decode_and_stream_chunk
  participant magpie_require_valid_audio
  participant overlap_add_processing
  decode_and_stream_chunk->>magpie_require_valid_audio: validate decoded audio
  magpie_require_valid_audio-->>decode_and_stream_chunk: valid or invalid result
  decode_and_stream_chunk->>overlap_add_processing: process valid audio

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❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 14.29% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 7 functions across 3 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

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Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: rejecting decoded TTS audio that cannot represent speech.
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Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Actionable comments posted: 1

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Inline comments:
In `@tests/cpp/tts/test_magpietts_audio_sanity.cpp`:
- Around line 103-105: Update the test around the two writeDecodedAudio calls
and flush to check each returned bool and record one test failure whenever any
call returns false, while preserving the existing processing and assertions.

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In `@tests/cpp/tts/test_magpietts_audio_sanity.cpp`:
- Line 93: Add a synthesis-level regression test near the existing audio sanity
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[danilogr]

A couple of thoughts on this PR:

Motivation

• This PR turns a silent failure (invalid audio frames) into an explicit one so that users and developers can more quickly diagnose what would otherwise be a literally silent failure (pun intended because the audio file would be created, silent pipeline failure, with a silent audio content)

• Claude Code found this issue while we were debugging MagpieTTS on the Metal backend emits a constant -32767 DC signal instead of speech (macOS, Apple Silicon) #19. Now that fix(metal): stop MagpieTTS returning silence on M5-class devices #20 has landed, there isn't a direct way to reproduce the issue fixed by this PR. That being said, I still believe this should be merged in. The M5 fault is one instance of a class, not the reason for the guard. Without this fix, any decoder or codec graph path can hand the audio writer numbers it converts without complaint. This could happen whenever a new fused kernel or backend is added. A recent example is fix(ggml): guard the half_snake fusion against allocator buffer reuse #28, a separate graph-level fault in the same NanoCodec decoder.

Risks and Latency

• On the risk of it firing when it should not, I measured both ends (silence to really loud audio) and neither gets near a bound. Loud: --cfg-scale 30 --temperature 3.0 on a shouting prompt peaks at 22,489 of 32,767, with zero full-scale samples. Quiet: across pause-heavy and near-silent prompts the longest run of identical samples is 32, against the 1024 a chunk would need, and real near-silence sits near zero rather than at a bound. Across 216 real syntheses on CPU and Metal, nothing was rejected and the audio was byte-identical with and without the check.

• Cost is 0.3 µs per 1024-sample chunk.

[danilogr]

@pskrunner14 @anand-nv @rmittal-github. I need your help with a design decision that could simplify this PR:

This PR has to check two conditions instead of one because ggml_clamp(ctx, x, -1.0f, 1.0f) at nanocodec/model.cpp:811 and :899 maps NaN to -1/1, so a NaN decode arrives at the audio writer as a finite -1.0f. That is why an isfinite check there would not fire, and why the guard also has to treat a chunk pinned to a bound as invalid. While I can't generate an audio packet with -1.0f or 1.0f, these are legitimate sample values that we are forced to read as a fault signature because of ggml_clamp in nanocodec/model.cpp:811.

If the clamp preserved NaN, or were not needed, isfinite alone would be enough and the second condition could go.
I think that there is a better fix for this so I will send a new PR with that fix (and I might just close this).

[danilogr]

@pskrunner14 @anand-nv @rmittal-github here is the alternative I mentioned: #43. It moves the clamp after the NaN check inside the decoder, so isfinite alone is enough and we stop reading -1.0f as a fault signature.