Post 3 ended with three numbers: 97.4% test accuracy, 100% precision, 0 false positives. All of it measured on a MacBook.

That was enough to prove the model could learn first crack. It did not prove the model could run where I needed it: next to a coffee roaster, on a Raspberry Pi 5, listening through a USB microphone during a live roast.

The first Pi run was "it kind of works". Export the PyTorch checkpoint to ONNX FP32, copy the 345MB file across, run inference. The result was 9.4 seconds per 10-second audio window. That was not a bug. That was the cost of running an 86M parameter transformer on one ARM Cortex-A76 core at full floating-point precision.

The next attempt exposed a different problem. More threads made the model faster, but the Pi crashed. Oz was already in the SSH session, so the next command was vcgencmd get_throttled. The answer came back as 0x50000: under-voltage and throttling.

So the work was not just "export to ONNX." It became a small edge deployment story: shrink the model, tune the threshold, limit threads, swap the power supply, add active cooling, then decide whether the remaining latency still fit the physics of coffee roasting.

The final number was 2.09 seconds per 10-second window at 4 threads with active cooling. The production profile uses 2 threads and lands at 2.45 seconds, leaving CPU headroom for the rest of the system.

In this post

• What We're Deploying
• The Pi Workflow
• ONNX Export
• The Platform Numbers
• Threshold Sweep
• The Hardware War Story
• The Verdict
• Links
• References

What We're Deploying

The model is an Audio Spectrogram Transformer fine-tuned for binary classification: first_crack vs no_first_crack on 10-second audio windows resampled to 16kHz mono. Post 2 covers the dataset: 973 annotated chunks from 15 roasting sessions, recording-level splits to prevent data leakage, class-weighted training against a 20/80 imbalance. Post 3 covers the training: two hyperparameter attempts, the oscillating loss from a learning rate too aggressive for 587 samples, and the annotation redesign that moved precision from 87.5% to 100%.

The numbers relevant to this post are the historical baseline_v2 numbers from the edge-validation PR:

• 86M parameters, fine-tuned from MIT/ast-finetuned-audioset-10-10-0.4593, pre-trained on 2M AudioSet clips
• 97.4% test accuracy / 100% precision on baseline_v2 (191-sample test set, recording-level isolation)
• ONNX INT8 on Mac: 96.86% accuracy, 1 additional FP. That is the full quality cost of INT8 quantisation on the same test set.
• The model and ONNX deployment path now live at huggingface.co/syamaner/coffee-first-crack-detection

The Pi Workflow

The Pi validation for PR #23 followed a simple division of labour. The ONNX models were exported on the Mac and synced across to the Pi. Oz SSH'd into the device from within the Warp terminal and drove the benchmark and evaluation scripts directly. My role was to read the results and make hardware decisions.

That mattered when the Pi started failing. The SSH session, benchmark output, vcgencmd result, and next command all stayed in one terminal thread. There was no separate debug notebook or copied shell history. The failure and the next experiment were in the same place.

Two scripts drove everything on the Pi:

• scripts/evaluate_onnx.py: runs the test set through the ONNX model and returns accuracy, F1, confusion matrix, and per-sample latency. Designed with no PyTorch inference dependency so it runs on the Pi without a GPU torch install.
• scripts/benchmark_onnx_pi.py: 30 timed runs after 5 warmup runs, using dummy audio to isolate inference latency from file I/O variance.

One setup detail matters: even though inference runs entirely through ONNX Runtime, PyTorch CPU is still needed on the Pi for ASTFeatureExtractor's mel filterbank computation. The install is split deliberately. requirements-pi.txt handles the ONNX and audio dependencies, then torch CPU is added separately:

pip install -r requirements-pi.txt
pip install torch --index-url https://download.pytorch.org/whl/cpu

torchaudio and optimum are explicitly excluded from the Pi install. ONNX Runtime handles everything from the filterbank output onward.

Here is the evaluation session: Oz running evaluate_onnx.py over SSH on the Pi, 4 threads, reading the latency numbers as they arrive:

PR #23 accumulated 36 Copilot comments across 6 review rounds, mostly around type annotations, missing error handling, and edge-case validation. The useful miss was the benchmark script's <500ms target. That target made sense on a Mac, but it would always print FAIL on the Pi. The real question was not whether the model was under 500ms. It was whether the detector could keep up with a roast. The production Pi profile later answers that by widening the hop to 7 seconds.

ONNX Export

The trained checkpoint is a standard Hugging Face save_pretrained directory: model weights, config, and the ASTFeatureExtractor preprocessor config. Getting it to the Pi is a two-step process: export the PyTorch graph to a static FP32 ONNX graph, then compress the weights to INT8. Oz executed both steps by invoking the /export-onnx skill shown in Post 1, which runs export, quantisation, and a local benchmark in sequence before marking the step complete.

Step one uses Hugging Face Optimum:

# src/coffee_first_crack/export_onnx.py
from optimum.onnxruntime import ORTModelForAudioClassification

ort_model = ORTModelForAudioClassification.from_pretrained(
    model_dir,
    export=True,
)
ort_model.save_pretrained(str(fp32_dir))

export=True triggers ONNX tracing at load time. The result is exports/onnx/fp32/model.onnx at 345MB. That file ran at 9.4 seconds per window on the Pi at 1 thread, captured in the local validation artifact results/pi5_fp32_eval.json. Usable for offline batch processing, but not for a live roasting session.

Step two uses onnxruntime.quantization.quantize_dynamic:

# src/coffee_first_crack/export_onnx.py
from onnxruntime.quantization import QuantType, quantize_dynamic

quantize_dynamic(
    model_input=str(fp32_path),
    model_output=str(int8_path),
    weight_type=QuantType.QInt8,
)

Dynamic quantisation converts weights to INT8 at export time; activations remain in FP32 at runtime. I used it instead of static quantisation because static quantisation would require a calibration dataset to quantise activations too. The simpler dynamic approach carries a smaller quality penalty. The same model_quantized.onnx benchmarked at 636ms on Apple Silicon runs unchanged on the Pi's ARM Cortex-A76. No recompilation, no platform-specific pass.

The output from one export run:

• exports/onnx/fp32/model.onnx: 345MB, FP32 precision
• exports/onnx/int8/model_quantized.onnx: 89.9MB, INT8 weights

That is a 3.84x size reduction. The full quality and latency comparison across platforms is in the next section.

The feature extractor config is saved into every variant directory independently. Each subdirectory is self-contained. You can copy just exports/onnx/int8/ to the Pi and run inference without the full repo structure or a parent-level config lookup.

There is a second deployment path that avoids any manual copy. The ONNX variants were pushed to HF Hub alongside the PyTorch checkpoint. syamaner/coffee-first-crack-detection includes an onnx/int8/ subfolder. The production inference module, inference_onnx.py, is HF Hub-first by design: _DEFAULT_REPO_ID = "syamaner/coffee-first-crack-detection" and _DEFAULT_SUBFOLDER = "onnx/int8" are the hardcoded defaults. At startup it calls hf_hub_download and caches locally. A Pi with only requirements-pi.txt + torch CPU installed can pull and run the model without any repo cloning or file transfer. The PR #23 validation used local exports synced over via scp because the test split data was also local. A fresh production deployment pulls everything from the Hub.

The Platform Numbers

The v2 Pi5 evaluation, captured locally as results/v2_pi5_int8_4t_eval.json, settled the portability question directly: does INT8 dynamic quantisation produce different accuracy on ARM64 versus Apple Silicon? The answer is no. The Pi5 INT8 model at 4 threads, run against the same 191-sample v2 test set, returns 96.86% accuracy, 96.9% precision, and 1 false positive. That is the same confusion matrix as the Mac INT8 result: [[154, 1], [5, 31]].

†v1 test set (6 roasts, original annotations). v2 2-thread evaluation pending.

The total INT8 quantisation cost, measured against the v2 baseline on the same test set, is 0.54% accuracy and 1 additional false positive. That cost is identical on Mac and Pi5. The identical confusion matrix confirms the ONNX artefact is fully portable: same weights and same predictions on both Apple Silicon and ARM Cortex-A76.

The v1 rows exist for the FP32 vs INT8 latency story: 9.4 seconds at full FP32 precision, 2.4 seconds at 2 threads INT8, 2.07 seconds at 4 threads. The accuracy figures in those rows reflect the older, smaller test set and should not be read as platform quality numbers.

Threshold Sweep

The default classification threshold is 0.5. Any window where the model outputs P(first_crack) >= 0.5 is flagged. For an isolated classifier, that default is fine. For a roasting assistant, the consequences are asymmetric.

A false negative means the system waits one more 10-second window before detecting first crack. A false positive could incorrectly flag background noise or a drum knock as first crack. That could trigger an automated action (timer, fan relay, alert) at the wrong moment in the roast. The tradeoff is not symmetric, and the threshold should reflect that.

The sweep ran from 0.50 to 0.95 on the Pi5 INT8 model, captured locally as results/pi5_threshold_sweep.json on the v1 test set:

The numbers between 0.50 and 0.65 are identical. The model's output probabilities are far from the decision boundary in most cases. At 0.80, one FP is eliminated. The remaining FP, a no_first_crack chunk scored at 0.941, survives all the way to 0.90. Only at 0.95 does it drop out, reducing FP to zero at the cost of recall dropping to 77.3% (5 additional missed windows).

The deployed Pi profile does not use 0.95. It uses 0.90, paired with a pop-confirmation layer. The logic is in configs/default.yaml:

# configs/default.yaml
pi_inference:
  window_size: 10.0
  overlap: 0.3           # 30% -> 7s hop, comfortable margin for 2-thread latency
  threshold: 0.90        # precision=0.952, recall=0.909, F1=0.930
  min_pops: 3            # 3 positive windows required within the confirmation window
  confirmation_window: 30.0  # seconds
  onnx_threads: 2        # leaves 2 cores free for MCP server + agent UI

A single window scoring above 0.90 does not trigger the detector. Three positive windows within a 30-second span must agree. An isolated false positive from background noise cannot survive the confirmation requirement. For a false positive to trigger the system, three separate audio windows, 7 seconds apart, must all independently score above 0.90. I did not see that pattern in the test set. The 0.941 outlier appears once, in one chunk, not in three consecutive windows of a real roasting session.

Two further notes on the Pi profile. First, the overlap drops from 70% (default, 3-second hop) to 30% (7-second hop). With 2-thread inference at 2.45 seconds per window, a 3-second hop would leave almost no idle time and risk falling behind the audio stream. The wider hop provides 4.5 seconds of headroom per cycle. Second, the thread count is 2, not 4, because the Pi runs more than just the detector. The MCP server and agent UI share the same device; limiting ONNX to 2 cores prevents inference from starving the rest of the stack.

The Hardware War Story

The first sustained inference run on the Pi used an Apple 96W USB-C charger. It was what was on the desk. The benchmark ran at 1 thread without issue. At 2 threads, the Pi5 crashed mid-run.

Oz was already in the SSH session. The next command was vcgencmd get_throttled:

$ vcgencmd get_throttled
throttled=0x50000

0x50000 sets bits 16 and 18 in the throttle register: under-voltage has been detected and throttling has occurred. The Pi5 was drawing more current than the charger could supply and the firmware was cutting clock speeds to compensate. When the deficit is severe enough, the board halts entirely.

The Apple 96W charger negotiates 5V at 3A on its USB-C output, which is 15 watts. The Raspberry Pi 5 under sustained 2+ thread inference draws up to 5V/5A, which is 27 watts. This is in the RPi5 hardware specification. The 5V/5A USB Power Delivery profile the Pi requires is not what most laptop chargers, including the Apple 96W, negotiate. The naming is misleading: "96W" refers to the high-voltage MacBook profile, not the 5V rail.

Oz handled the workaround immediately: dropped onnx_threads to 1 and re-ran. Single-thread inference stayed within the 15W budget and the benchmark completed. But 1-thread inference at 9.4 seconds per window is not a deployment target. The fix required hardware, not configuration.

Gemini was brought in to cross-reference the RPi5 throttle register documentation and the official PSU specification. The human made the call: the official Raspberry Pi 27W USB-C Power Supply (5V/5A) was the required fix, not a config change.

Here is the SSH debugging session: Oz reading vcgencmd get_throttled, diagnosing the 0x50000 flag, and adjusting threads as an interim workaround while the hardware decision was made:

After the PSU swap, 4-thread inference ran without crashes. But a second flag appeared under sustained load:

$ vcgencmd get_throttled
throttled=0xe0000

0xe0000 sets bits 17, 18, and 19: ARM frequency capped, throttling has occurred, soft temperature limit active. The CPU core temperature hit 77°C under continuous inference. The Pi5 has no heatsink by default; sustained transformer inference is a thermal stress test the passive design cannot pass.

The official Raspberry Pi Active Cooler brought operating temperature to 45°C under sustained 4-thread inference, a 32°C reduction. At 45°C, vcgencmd get_throttled returns 0x0. Stable.

The final hardware requirements from that debugging session are encoded in AGENTS.md:

### RPi5 Hardware Requirements
- PSU: Official RPi5 27W (5V/5A) USB-C. Standard chargers (5V/3A incl. Apple 96W) cause under-voltage crashes.
- Cooling: Active cooler mandatory. Sustained inference hits 77°C+ without it.
- Threads: Default 2 via `--threads` flag. 4 threads needs 27W PSU + active cooler.

The entire debugging session, including SSH, vcgencmd output, and the subsequent AGENTS.md update, stayed inside one Warp terminal session.

The Verdict

The production Pi deployment runs inference_onnx.py --profile pi_inference: 2 ONNX threads, 30% overlap (7-second hop), threshold 0.90, min_pops: 3 confirmation. Under those parameters, with 27W PSU and active cooler, per-window latency is 2.45 seconds p50. That leaves 4.5 seconds of idle time between inference calls at the 7-second hop, which is enough headroom for the MCP server and agent UI sharing the same board.

For this use case, that is sufficient. First crack is not a millisecond-precision event. It is a sustained acoustic phase lasting 60 to 90 seconds. A detector that confirms within 30 seconds of onset, using three positive windows at 7-second intervals, fits the timing of a real roast. The 2.07-second 4-thread result is the performance ceiling with optimal hardware; 2.45 seconds at 2 threads is the stable production target.

The numbers also show the limit of transformers at the edge. An 86M parameter model that takes 9.4 seconds at FP32 on a low cost ARM board is not a general-purpose edge architecture. It works here because the inference cadence is slow and the event it detects lasts long enough to be caught across multiple windows. Deploy the same model against a task requiring sub-second response and you would need a much smaller architecture or dedicated inference hardware. INT8 quantisation helped: 3.84x size reduction, 4.5x latency improvement over FP32. It does not turn a large transformer into a microcontroller-class model.

Post 5 covers the other half of making the model usable: getting it in front of people who are not running SSH sessions into a Raspberry Pi. Publishing the model card and dataset card, building a Gradio Space that works inside a container, and the four platform bugs, including an SSR/asyncio crash on Python 3.13, each diagnosed by pasting HF Space logs to Oz and getting fixes back in seconds.

Links

Project:

• GitHub Repository
• Hugging Face Model
• Hugging Face Dataset
• Live Gradio Space

Tools:

• Warp: The Agentic Development Environment
• Oz: Warp's AI Agent
• Warp Block Sharing

References

1. ONNX Runtime & Quantisation

• ONNX Runtime Dynamic Quantisation: Documents quantize_dynamic with QuantType.QInt8, the approach used for the INT8 export. Explains the distinction between dynamic (weight-only) and static (requires calibration data) quantisation.
• Hugging Face Optimum ONNX Export: ORTModelForAudioClassification.from_pretrained(export=True) triggers the ONNX tracing used in export_onnx.py.

2. Raspberry Pi 5 Hardware

• Raspberry Pi 5 Product Brief (PDF): Specifies the 5V/5A (27W) USB-C PD requirement for full-load operation. Standard 5V/3A supplies are explicitly noted as insufficient for sustained workloads.
• vcgencmd Documentation (Raspberry Pi): Documents get_throttled and the bitmask flags: bit 16 (under-voltage detected), bit 17 (ARM frequency capped), bit 18 (throttled), bit 19 (soft temp limit).

3. Edge Deployment & Model Optimisation

• Hugging Face Audio Classification Fine-Tuning Guide: Context for the AST model architecture and ASTFeatureExtractor preprocessing requirements that carry through to edge deployment.