fix(tts/kokoro-ane): route noise+tail to CPU by default on OS 27

[Alex-Wengg]

Fixes #843.

Problem

On the OS 27 line (observed on iPadOS 27.0 beta, iPad Pro M5), the default routing's two GPU-by-design stages (noise, tail iSTFT) abort intermittently inside MPSGraph under CoreML — an in-process signal-6 abort no fallback can catch, typically within ~an hour of continuous synthesis. Filed with Apple as FB24243070. The same binary and models are stable on OS 26.x.

Fix

• New aneTailCpu preset: noise + tail on .cpuOnly, RNN stages and vocoder unchanged on .cpuAndNeuralEngine — Metal is never invoked. This is exactly the workaround the reporter has been shipping, with imperceptible perf cost for speech-length inputs.
• .default becomes OS-conditional via a testable defaultUnits(for:) seam: aneTailGpu through OS 26 (bit-identical to today's default), aneTailCpu on 27+.

The BNNS iSTFT path that noise/tail fall back to is safe on 27 — the libBNNS segfault is a 26.x-line bug (#817/#844).

Testing

• New KokoroAneComputeUnitsTests: version cutover at 27.0, 26.x unchanged, and aneTailCpu never touching .cpuAndGPU/.all.
• swift build --target FluidAudio clean, swift format lint clean.

[github-actions]

ASR Benchmark Results ✅

Status: All benchmarks passed

Parakeet v3 (multilingual)

Dataset	WER Avg	WER Med	RTFx	Status
test-clean	0.57%	0.00%	4.72x	✅
test-other	1.35%	0.00%	3.03x	✅

Parakeet v2 (English-optimized)

Dataset	WER Avg	WER Med	RTFx	Status
test-clean	0.80%	0.00%	3.05x	✅
test-other	1.22%	0.00%	2.64x	✅

Streaming (v3)

Metric	Value	Description
WER	0.00%	Word Error Rate in streaming mode
RTFx	0.40x	Streaming real-time factor
Avg Chunk Time	2.175s	Average time to process each chunk
Max Chunk Time	2.942s	Maximum chunk processing time
First Token	2.904s	Latency to first transcription token
Total Chunks	31	Number of chunks processed

Streaming (v2)

Metric	Value	Description
WER	0.00%	Word Error Rate in streaming mode
RTFx	0.52x	Streaming real-time factor
Avg Chunk Time	1.742s	Average time to process each chunk
Max Chunk Time	2.013s	Maximum chunk processing time
First Token	1.742s	Latency to first transcription token
Total Chunks	31	Number of chunks processed

_{Streaming tests use 5 files with 0.5s chunks to simulate real-time audio streaming}

_{25 files per dataset • Test runtime: 10m51s • 08/11/2026, 02:38 PM EST}

_{RTFx = Real-Time Factor (higher is better) • Calculated as: Total audio duration ÷ Total processing time
Processing time includes: Model inference on Apple Neural Engine, audio preprocessing, state resets between files, token-to-text conversion, and file I/O
Example: RTFx of 2.0x means 10 seconds of audio processed in 5 seconds (2x faster than real-time)}

Expected RTFx Performance on Physical M1 Hardware:

• M1 Mac: ~28x (clean), ~25x (other)
• CI shows ~0.5-3x due to virtualization limitations

_{Testing methodology follows HuggingFace Open ASR Leaderboard}

[github-actions]

VAD Benchmark Results

Performance Comparison

Dataset	Accuracy	Precision	Recall	F1-Score	RTFx	Files
MUSAN	94.0%	89.3%	100.0%	94.3%	343.5x faster	50
VOiCES	94.0%	89.3%	100.0%	94.3%	352.6x faster	50

Dataset Details

• MUSAN: Music, Speech, and Noise dataset - standard VAD evaluation
• VOiCES: Voices Obscured in Complex Environmental Settings - tests robustness in real-world conditions

✅: Average F1-Score above 70%

[github-actions]

Parakeet EOU Benchmark Results ✅

Status: Benchmark passed
Chunk Size: 320ms
Files Tested: 100/100

Performance Metrics

Metric	Value	Description
WER (Avg)	7.03%	Average Word Error Rate
WER (Med)	4.17%	Median Word Error Rate
RTFx	9.72x	Real-time factor (higher = faster)
Total Audio	470.6s	Total audio duration processed
Total Time	49.6s	Total processing time

Streaming Metrics

Metric	Value	Description
Avg Chunk Time	0.050s	Average chunk processing time
Max Chunk Time	0.099s	Maximum chunk processing time
EOU Detections	0	Total End-of-Utterance detections

_{Test runtime: 1m56s • 08/11/2026, 02:59 PM EST}

_{RTFx = Real-Time Factor (higher is better) • Processing includes: Model inference, audio preprocessing, state management, and file I/O}

[github-actions]

Offline VBx Pipeline Results

Speaker Diarization Performance (VBx Batch Mode)

Optimal clustering with Hungarian algorithm for maximum accuracy

Metric	Value	Target	Status	Description
DER	10.4%	<20%	✅	Diarization Error Rate (lower is better)
RTFx	10.95x	>1.0x	✅	Real-Time Factor (higher is faster)

Offline VBx Pipeline Timing Breakdown

Time spent in each stage of batch diarization

Stage	Time (s)	%	Description
Model Download	24.721	25.8	Fetching diarization models
Model Compile	10.595	11.1	CoreML compilation
Audio Load	0.076	0.1	Loading audio file
Segmentation	28.078	29.3	VAD + speech detection
Embedding	95.597	99.8	Speaker embedding extraction
Clustering (VBx)	0.108	0.1	Hungarian algorithm + VBx clustering
Total	95.833	100	Full VBx pipeline

Speaker Diarization Research Comparison

Offline VBx achieves competitive accuracy with batch processing

Method	DER	Mode	Description
FluidAudio (Offline)	10.4%	VBx Batch	On-device CoreML with optimal clustering
FluidAudio (Streaming)	17.7%	Chunk-based	First-occurrence speaker mapping
Research baseline	18-30%	Various	Standard dataset performance

Pipeline Details:

• Mode: Offline VBx with Hungarian algorithm for optimal speaker-to-cluster assignment
• Segmentation: VAD-based voice activity detection
• Embeddings: WeSpeaker-compatible speaker embeddings
• Clustering: PowerSet with VBx refinement
• Accuracy: Higher than streaming due to optimal post-hoc mapping

_{🎯 Offline VBx Test • AMI Corpus ES2004a • 1049.0s meeting audio • 123.8s processing • Test runtime: 2m 22s • 08/11/2026, 03:02 PM EST}

[github-actions]

PocketTTS Smoke Test ✅

Check	Result
Build	✅
Model download	✅
Model load	✅
Synthesis pipeline	✅
Output WAV	✅ (165.0 KB)

_{Runtime: 1m25s}

_{Note: PocketTTS uses CoreML MLState (macOS 15) KV cache + Mimi streaming state. CI VM lacks physical GPU — audio quality and performance may differ from Apple Silicon.}

[github-actions]

Speaker Diarization Benchmark Results

Speaker Diarization Performance

Evaluating "who spoke when" detection accuracy

Metric	Value	Target	Status	Description
DER	15.1%	<30%	✅	Diarization Error Rate (lower is better)
JER	24.9%	<25%	✅	Jaccard Error Rate
RTFx	16.01x	>1.0x	✅	Real-Time Factor (higher is faster)

Diarization Pipeline Timing Breakdown

Time spent in each stage of speaker diarization

Stage	Time (s)	%	Description
Model Download	14.426	22.0	Fetching diarization models
Model Compile	6.183	9.4	CoreML compilation
Audio Load	0.072	0.1	Loading audio file
Segmentation	19.651	30.0	Detecting speech regions
Embedding	32.752	50.0	Extracting speaker voices
Clustering	13.101	20.0	Grouping same speakers
Total	65.540	100	Full pipeline

Speaker Diarization Research Comparison

Research baselines typically achieve 18-30% DER on standard datasets

Method	DER	Notes
FluidAudio	15.1%	On-device CoreML
Research baseline	18-30%	Standard dataset performance

Note: RTFx shown above is from GitHub Actions runner. On Apple Silicon with ANE:

• M2 MacBook Air (2022): Runs at 150 RTFx real-time
• Performance scales with Apple Neural Engine capabilities

_{🎯 Speaker Diarization Test • AMI Corpus ES2004a • 1049.0s meeting audio • 65.5s diarization time • Test runtime: 4m 6s • 08/11/2026, 03:17 PM EST}

[github-actions]

Supertonic3 Smoke Test ✅

Check	Result
Build	✅
Model download (incl. VectorEstimatorVariants/ int4 buckets)	✅
Model load	✅
Synthesis pipeline (--ve-variant int4)	✅
Output WAV	✅ (364.7 KB)

_{Runtime: 0m37s}

_{Note: CI VMs lack a physical Neural Engine; the ANE-bucketed VectorEstimator falls back to CPU here. This validates download + variant resolution + synthesis, not ANE residency/perf.}

[github-actions]

Sortformer High-Latency Benchmark Results

ES2004a Performance (30.4s latency config)

Metric	Value	Target	Status
DER	30.3%	<35%	✅
Miss Rate	28.2%	-	-
False Alarm	0.9%	-	-
Speaker Error	1.2%	-	-
RTFx	20.1x	>1.0x	✅
Speakers	4/4	-	-

_{Sortformer High-Latency • ES2004a • Runtime: 2m 56s • 2026-08-11T19:22:36.093Z}