Fix Cohere Transcribe hallucinating sentences on silent audio - #3924
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For most supported languages, digital silence decodes to a hallucinated sentence (measured with the 14-language int8 model: "Herr Präsident, meine Damen und Herren!" for de, "Sous-titrage Société Radio-Canada" for fr, "谢谢" for zh; only en returns an empty string), because the decoder runs regardless of the input content. GetFrames() returns per-feature normalized fbank features for this model family, which are scale invariant: digital silence measures a max |feature| of about 0.09 while audio with content stays above 4 regardless of its level. CohereHasSignal() uses that to short-circuit silent clips in DecodeStreams() and return an empty transcript before the encoder and the decoder run, mirroring the all_silent short-circuit of the Qwen3-ASR recognizer.
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📝 WalkthroughWalkthroughThe Cohere offline recognizer now detects digitally silent audio before decoding and returns an empty result. Tests cover silent and signal-containing feature buffers. CMake registers the new test. ChangesCohere silence detection
Estimated code review effort: 2 (Simple) | ~10 minutes Merge Risk: ⚪ Minimal · up to Cohere transcription now returns an empty transcript for digitally silent audio while preserving normal decoding for audio with signal. No current merge-blocking risk is identified. Sequence Diagram(s)sequenceDiagram
participant DecodeStream
participant CohereHasSignal
participant EncoderDecoder
DecodeStream->>CohereHasSignal: inspect raw fbank features
CohereHasSignal-->>DecodeStream: signal present or absent
DecodeStream->>EncoderDecoder: decode only when signal is present
Suggested reviewers: 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
✨ Finishing Touches🧪 Generate unit tests (beta)
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Inline comments:
In `@sherpa-onnx/csrc/offline-recognizer-cohere-transcribe-impl-test.cc`:
- Around line 26-27: Add test cases for CohereHasSignal covering values 0.999f,
1.0f, and 1.001f, with expectations matching its strict greater-than-1.0f
threshold. Keep the existing signal coverage unchanged.
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sherpa-onnx/csrc/CMakeLists.txtsherpa-onnx/csrc/offline-recognizer-cohere-transcribe-impl-test.ccsherpa-onnx/csrc/offline-recognizer-cohere-transcribe-impl.h
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Root cause
OfflineRecognizerCohereTranscribeImpl::DecodeStreams()runs the encoder and the decoder regardless of the input content. For a clip of digital silence, most supported languages decode to a hallucinated sentence (deterministically, measured with the 14-language int8 model):""(the only correct one)"Herr Präsident, meine Damen und Herren!""Sous-titrage Société Radio-Canada""谢谢""으음.""ạ""نهايه الدرس""El juego se queda en la pista."The Qwen3-ASR recognizer in this repo already short-circuits this case: an all-silent clip returns an empty result before any decoding happens (#3907). Cohere Transcribe has no equivalent check.
Fix
CohereHasSignal()decides whether the clip contains any signal, andDecodeStreams()returns an empty transcript when it does not, before the encoder and the decoder run.The features returned by
GetFrames()are per-feature normalized fbank values, which are scale invariant — the recording level does not matter. Measured in this model's own feature domain: digital silence gives a max |feature| of about 0.09, while the official test wavs for all 8 supported languages stay between 3.3 and 4.5 regardless of their level; the threshold sits at 1.0.As with the Qwen3-ASR fix, audio with a noise floor is still decoded; this change only covers clips that are exactly silent.
Verification
sherpa-onnx-cohere-transcribe-14-lang-int8-2026-04-01, CPU, Windows:""""""(unchanged)The new
offline-recognizer-cohere-transcribe-impl-testcovers the silence detector (thresholds on both sides, a single loud sample, empty input).Summary by CodeRabbit
Bug Fixes
Tests