4b2377c243
- BENCHMARK.md: whisper also supports --language auto, voxtral is not the only one. Fixed mlx-whisper speed comparison (LA is actually faster than SS for mlx-whisper, not comparable). - metrics.py: median calculation was wrong for even-length lists (took upper middle instead of averaging the two middle values). - metrics_collector.py: RTF was inflated because log_summary() used wall-clock elapsed time instead of sum of actual ASR call durations. - README.md: clarified that whisper also supports auto language detection, voxtral just does it better. - Added 2 new median tests (even + odd length).
160 lines
7.4 KiB
Markdown
160 lines
7.4 KiB
Markdown
# WhisperLiveKit Benchmark Report
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Benchmark comparing all supported ASR backends and streaming policies on Apple Silicon,
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using the full AudioProcessor pipeline (the same path audio takes in production via WebSocket).
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## Test Environment
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| Property | Value |
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|----------|-------|
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| Hardware | Apple M4, 32 GB RAM |
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| OS | macOS 25.3.0 (arm64) |
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| Python | 3.13 |
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| faster-whisper | 1.2.1 |
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| mlx-whisper | installed (via mlx) |
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| Voxtral (HF) | transformers-based |
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| Voxtral MLX | native MLX backend |
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| Model size | `base` (default for whisper backends) |
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| VAC (Silero VAD) | enabled unless noted |
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| Chunk size | 100 ms |
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| Pacing | no-realtime (as fast as possible) |
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## Audio Test Files
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| File | Duration | Language | Speakers | Description |
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|------|----------|----------|----------|-------------|
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| `00_00_07_english_1_speaker.wav` | 7.2 s | English | 1 | Short dictation with pauses |
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| `00_00_16_french_1_speaker.wav` | 16.3 s | French | 1 | French speech with intentional silence gaps |
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| `00_00_30_english_3_speakers.wav` | 30.0 s | English | 3 | Multi-speaker conversation about transcription |
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All files have hand-verified ground truth transcripts (`.transcript.json`) with per-word timestamps.
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---
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## Results Overview
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### English - Short (7.2 s, 1 speaker)
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| Backend | Policy | RTF | WER | Timestamp MAE |
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|---------|--------|-----|-----|---------------|
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| faster-whisper | LocalAgreement | 0.20x | 21.1% | 0.080 s |
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| faster-whisper | SimulStreaming | 0.14x | 0.0% | 0.239 s |
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| mlx-whisper | LocalAgreement | 0.05x | 21.1% | 0.080 s |
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| mlx-whisper | SimulStreaming | 0.14x | 10.5% | 0.245 s |
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| voxtral-mlx | voxtral | 0.32x | 0.0% | 0.254 s |
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| voxtral (HF) | voxtral | 1.29x | 0.0% | 1.876 s |
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### French (16.3 s, 1 speaker)
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| Backend | Policy | RTF | WER | Timestamp MAE |
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|---------|--------|-----|-----|---------------|
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| faster-whisper | LocalAgreement | 0.20x | 120.0% | 0.540 s |
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| faster-whisper | SimulStreaming | 0.10x | 100.0% | 0.120 s |
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| mlx-whisper | LocalAgreement | 0.31x | 1737.1% | 0.060 s |
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| mlx-whisper | SimulStreaming | 0.08x | 94.3% | 0.120 s |
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| voxtral-mlx | voxtral | 0.18x | 37.1% | 3.422 s |
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| voxtral (HF) | voxtral | 0.63x | 28.6% | 4.040 s |
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Note: The whisper-based backends were run with `--lan en`, so they attempted to transcribe French
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audio in English. This is expected to produce high WER. For a fair comparison, the whisper backends
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should be run with `--lan fr` or `--lan auto`. The Voxtral backends auto-detect language.
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### English - Multi-speaker (30.0 s, 3 speakers)
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| Backend | Policy | RTF | WER | Timestamp MAE |
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|---------|--------|-----|-----|---------------|
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| faster-whisper | LocalAgreement | 0.24x | 44.7% | 0.235 s |
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| faster-whisper | SimulStreaming | 0.10x | 5.3% | 0.398 s |
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| mlx-whisper | LocalAgreement | 0.06x | 23.7% | 0.237 s |
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| mlx-whisper | SimulStreaming | 0.11x | 5.3% | 0.395 s |
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| voxtral-mlx | voxtral | 0.31x | 9.2% | 0.176 s |
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| voxtral (HF) | voxtral | 1.00x | 32.9% | 1.034 s |
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---
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## Key Findings
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### Speed (RTF = processing time / audio duration, lower is better)
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1. **mlx-whisper + LocalAgreement** is the fastest combo on Apple Silicon, reaching 0.05-0.06x RTF
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on English audio. 30 seconds of audio processed in under 2 seconds.
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2. For **faster-whisper**, SimulStreaming is consistently faster than LocalAgreement.
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For **mlx-whisper**, it is the opposite: LocalAgreement (0.05-0.06x) is faster than SimulStreaming (0.11-0.14x).
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3. **voxtral-mlx** runs at 0.18-0.32x RTF, roughly 3-5x slower than mlx-whisper but well within
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real-time requirements.
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4. **voxtral (HF transformers)** is the slowest at 1.0-1.3x RTF. On longer audio it risks
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falling behind real-time. On Apple Silicon, the MLX variant is strongly preferred.
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### Accuracy (WER = Word Error Rate, lower is better)
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1. **SimulStreaming** produces significantly better WER than LocalAgreement for whisper backends.
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On the 30s English file: 5.3% vs 23.7-44.7%.
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2. **voxtral-mlx** has good accuracy (0% on short English, 9.2% on multi-speaker).
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Whisper also supports `--language auto`, but Voxtral's language detection is more
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reliable and does not bias towards English the way Whisper's auto mode tends to.
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3. **LocalAgreement** tends to duplicate the last sentence, inflating WER. This is a known
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artifact of the LCP (Longest Common Prefix) commit strategy at end-of-stream.
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4. **Voxtral** backends handle French natively with 28-37% WER, while whisper backends
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were run with `--lan en` here (not a fair comparison for French).
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### Timestamp Accuracy (MAE = Mean Absolute Error on word start times, lower is better)
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1. **LocalAgreement** produces the most accurate timestamps (0.08s MAE on English), since it
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processes overlapping audio windows and validates via prefix matching.
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2. **SimulStreaming** timestamps are slightly less precise (0.24-0.40s MAE) but still usable
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for most applications.
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3. **voxtral-mlx** has good timestamp accuracy on English (0.18-0.25s MAE) but drifts on
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audio with long silence gaps (3.4s MAE on the French file with 4-second pauses).
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4. **voxtral (HF)** has the worst timestamp accuracy (1.0-4.0s MAE). This is likely related to
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differences in the transformers-based decoding pipeline rather than model quality.
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### VAC (Voice Activity Classification) Impact
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| Backend | Policy | VAC | 7s English WER | 30s English WER |
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|---------|--------|-----|----------------|-----------------|
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| faster-whisper | LocalAgreement | on | 21.1% | 44.7% |
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| faster-whisper | LocalAgreement | off | 100.0% | 100.0% |
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| voxtral-mlx | voxtral | on | 0.0% | 9.2% |
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| voxtral-mlx | voxtral | off | 0.0% | 9.2% |
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- **Whisper backends require VAC** to function in streaming mode. Without it, the entire audio
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is buffered as a single chunk and the LocalAgreement/SimulStreaming buffer logic breaks down.
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- **Voxtral backends are VAC-independent** because they handle their own internal chunking and
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produce identical results with or without VAC. VAC still reduces wasted compute on silence.
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---
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## Recommendations
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| Use Case | Recommended Backend | Policy | Notes |
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|----------|-------------------|--------|-------|
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| Fastest English transcription (Apple Silicon) | mlx-whisper | SimulStreaming | 0.08-0.14x RTF, 5-10% WER |
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| Fastest English transcription (Linux/GPU) | faster-whisper | SimulStreaming | 0.10-0.14x RTF, 0-5% WER |
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| Multilingual / auto-detect (Apple Silicon) | voxtral-mlx | voxtral | Handles 100+ languages, 0.18-0.32x RTF |
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| Multilingual / auto-detect (Linux/GPU) | voxtral (HF) | voxtral | Same model, slower on CPU, needs GPU |
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| Best timestamp accuracy | faster-whisper | LocalAgreement | 0.08s MAE, good for subtitle alignment |
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| Low latency, low memory | mlx-whisper (tiny) | SimulStreaming | Smallest footprint, fastest response |
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---
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## Reproducing These Benchmarks
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```bash
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# Install test dependencies
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pip install -e ".[test]"
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# Single backend test
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python test_backend_offline.py --backend faster-whisper --policy simulstreaming --no-realtime
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# Multi-backend auto-detect benchmark
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python test_backend_offline.py --benchmark --no-realtime
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# Export to JSON for programmatic analysis
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python test_backend_offline.py --benchmark --no-realtime --json results.json
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# Test with custom audio
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python test_backend_offline.py --backend voxtral-mlx --audio your_file.wav --no-realtime
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```
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The benchmark harness computes WER and timestamp accuracy automatically when ground truth
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`.transcript.json` files exist alongside the audio files. See `audio_tests/` for the format.
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