Fluency NeMo Conformer CTC Aligner

This repository contains the mobile-distribution assets for Fluency's English word-level forced alignment pipeline.

The model is not used as a standalone ASR decoder in the app. Whisper provides the transcript, this model provides frame-level CTC log-probabilities, and the app runs constrained CTC Viterbi to recover word timestamps.

Runtime Pipeline

Whisper transcript
→ text normalization
→ SentencePiece tokenizer
→ native log-mel feature extraction
→ ONNX Runtime Mobile
→ constrained CTC Viterbi
→ word timestamps

Required App Assets

stt_en_conformer_ctc_small_features.onnx
metadata.json
tokenizer/tokenizer.model
tokenizer/vocab.txt
manifest.json

Optional debugging assets are under parity/.

ONNX Contract

Input processed_signal: [batch, 80, feature_frames] float32
Input processed_signal_length: [batch] int64
Output log_probs: [batch, encoded_frames, 1025] float32
Output encoded_len: [batch] int64
CTC blank id: 1024

Feature Contract

Sample rate: 16000 Hz
Channels: mono
Feature type: 80-bin log-mel spectrogram
Window size: 25 ms
Window stride: 10 ms
FFT: 512
Window: Hann
Normalization: per_feature

The ONNX graph intentionally starts after feature extraction. Mobile clients must implement NeMo-compatible log-mel features before invoking ONNX Runtime.

Validation

Validated on a 375.838s video using raw Whisper text fields only:

Windows checked: 7
Segments checked: 110
Matched words checked: 1029
PyTorch feature+model total: 3.106s
ONNX model total: 5.396s
Log-prob mean abs diff: 0.000014
Word center delta p50/p95/max: 0.0ms / 0.0ms / 0.0ms

See parity/parity_1gzKSyOBpZg.json and parity/parity_1gzKSyOBpZg.csv for details.

Download URLs

https://huggingface.co/iamzhangship/fluency-nemo-conformer-ctc-aligner/resolve/main/stt_en_conformer_ctc_small_features.onnx
https://huggingface.co/iamzhangship/fluency-nemo-conformer-ctc-aligner/resolve/main/metadata.json
https://huggingface.co/iamzhangship/fluency-nemo-conformer-ctc-aligner/resolve/main/tokenizer/tokenizer.model
https://huggingface.co/iamzhangship/fluency-nemo-conformer-ctc-aligner/resolve/main/tokenizer/vocab.txt
https://huggingface.co/iamzhangship/fluency-nemo-conformer-ctc-aligner/resolve/main/manifest.json

Checksums

ONNX sha256: 8998fd44bc2374c7d085f7c709fbfbd28ea6db09998622077071faf478db24fd
ONNX size: 53197388 bytes

Use manifest.json for full file sizes and sha256 checksums.

Source Model

NeMo pretrained model: stt_en_conformer_ctc_small
Export graph: Conformer encoder + CTC projection/log-softmax
Export metadata model bytes: 53197388

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