cstr / canary-ctc-aligner-GGUF

Canary-1B-v2 CTC aligner — GGUF (ggml-quantised)

GGUF / ggml conversions of the auxiliary CTC alignment model that ships inside nvidia/canary-1b-v2 's .nemo tarball, repackaged as a standalone forced-aligner for use with the nfa-align CLI from CrispStrobe/CrispASR@parakeet .

What is this?

When you download nvidia/canary-1b-v2 you get a single canary-1b-v2.nemo tarball. Inside that tarball there are TWO weight files:

• model_weights.ckpt — the main 978 M-parameter Canary encoder–decoder ASR / translation model
• timestamps_asr_model_weights.ckpt — a separate, fully-trained 600 M-parameter Parakeet-style FastConformer + CTC head, trained with the same unified SentencePiece tokenizer as Canary-1B-v2 for 250 000 steps on the same Granary dataset (Canary paper §5)

That second model is what NVIDIA calls the "auxiliary CTC model" used by NeMo Forced Aligner (NFA) to compute frame-aligned word and segment timestamps for Canary's transcript output. It's the official NVIDIA-recommended path for getting word-level timestamps from Canary , since the main encoder–decoder doesn't emit them natively (cross-attention DTW tops out around ~360 ms MAE; CTC forced alignment gives ~30-80 ms).

This repo extracts that auxiliary model as a standalone GGUF and exposes it as a general-purpose multilingual subword CTC ASR + forced aligner in 25 European languages. Because the aligner re-tokenises any input transcript through its own SentencePiece vocabulary, it works as a drop-in alignment tool for transcripts produced by any other ASR system — Canary, Parakeet, Cohere Transcribe, Whisper, even hand-typed text.

License & attribution — IMPORTANT

This model is nvidia/canary-1b-v2 's auxiliary CTC component , redistributed under the same CC-BY-4.0 license as the original. All credit and attribution go to NVIDIA's NeMo team , who:

1. Designed the FastConformer + CTC architecture
2. Trained the model on the Granary dataset (1.7 M hours of multilingual speech)
3. Released nvidia/canary-1b-v2 (and the auxiliary model bundled inside it) under CC-BY-4.0

If you use this GGUF you MUST attribute NVIDIA per the CC-BY-4.0 terms:

This work uses the auxiliary CTC alignment model from NVIDIA's canary-1b-v2 , released under CC-BY-4.0. Cite the technical report: Sekoyan et al., "Canary-1B-v2 & Parakeet-TDT-0.6B-v3: Efficient and High-Performance Models for Multilingual ASR and AST" (arXiv:2509.14128, 2025).

The original model card, paper, and license are at:

• Model card: https://huggingface.co/nvidia/canary-1b-v2
• Technical report: https://arxiv.org/abs/2509.14128
• NeMo Forced Aligner (the tool that uses this model): https://github.com/NVIDIA-NeMo/NeMo
• NVIDIA NeMo: https://github.com/NVIDIA-NeMo/NeMo

The .nemo extraction + GGUF conversion + ggml runtime in this repo are MIT-licensed (matching the whisper.cpp base they extend), but the model weights themselves are CC-BY-4.0 from NVIDIA .

Files

Quick start

# 1. Build the runtime
git clone -b parakeet https://github.com/CrispStrobe/CrispASR
cd CrispASR
cmake -B build -DCMAKE_BUILD_TYPE=Release
cmake --build build -j$(nproc) --target nfa-align

# 2. Download the aligner GGUF
huggingface-cli download cstr/canary-ctc-aligner-GGUF \
    canary-ctc-aligner-q4_k.gguf --local-dir .

# 3. Greedy CTC decode (sanity check — works as a standalone ASR too)
./build/bin/nfa-align \
    -m canary-ctc-aligner-q4_k.gguf \
    -f your-audio.wav -decode -t 8

# 4. Forced alignment of an existing transcript
./build/bin/nfa-align \
    -m canary-ctc-aligner-q4_k.gguf \
    -f your-audio.wav \
    -tt "your transcript text here" -t 8

# Or read the transcript from a file
./build/bin/nfa-align \
    -m canary-ctc-aligner-q4_k.gguf \
    -f your-audio.wav \
    -tf transcript.txt -t 8

Verified accuracy

Tested on samples/jfk.wav (11 s, 22 words). Ground truth from cohere-align (wav2vec2 character-level CTC, the standard reference, ~30-50 ms expected MAE):

Mean Absolute Error (22 words): 78 ms for nfa-align vs 414 ms for canary's cross-attention DTW path. 5.3× tighter word boundaries. That's right at the cohere-align ceiling — for English the wav2vec2 reference is marginally better because its character-level granularity is finer than subword, but the difference is within the encoder frame quantum (80 ms).

For non-English languages where there's no wav2vec2 character-CTC model, nfa-align is the most accurate forced aligner this fork has.

How it works under the hood

audio (16 kHz mono PCM)
  ↓ NeMo mel preprocessor (128 mels, n_fft=512, hop=160, Hann + log + per-feature norm)
  ↓ Conv2d dw_striding subsampling (8× temporal: 100 → 12.5 fps)
  ↓ 24× FastConformer block:
       FFN1 (Macaron, ½ scale)
       MHA  (rel-pos with Transformer-XL untied biases)
       Conv (pw1 + GLU + dw_k=9 + BN + swish + pw2)
       FFN2 (Macaron, ½ scale)
       LN_out
  ↓ ctc_head: Linear(1024 → 16385)
  → log-softmax → per-frame logits over 16384 vocab + 1 blank

Forced alignment:
  1. Tokenise the input transcript word-by-word using greedy
     longest-prefix matching against the SentencePiece vocab
     (handles the U+2581 ▁ word-boundary marker).
  2. Build the CTC-expanded label sequence:
     [blank, t0, blank, t1, ..., blank, t_{N-1}, blank]
  3. Run Viterbi DP in log-probability space:
     - At each frame t, for each label position j: stay / advance / skip-blank
  4. Traceback → optimal monotone path → per-token frame index
  5. Map per-token frames back to per-word (t0, t1) in centiseconds

The encoder is architecturally identical to nvidia/parakeet-tdt-0.6b-v3 (24-layer FastConformer, no biases on q/k/v/out/ff/conv) — only the head differs (linear CTC vs LSTM TDT). That's why we can reuse the same encoder loading + BN-folding + graph-build code. The only canary-specific piece is the SentencePiece subword Viterbi.

Comparison with other alignment tools

For the 25 European languages canary supports, nfa-align is the only forced-alignment tool with frame-level (~80 ms) accuracy that doesn't require a separate per-language model . The wav2vec2 path ( cohere-align ) is marginally tighter but you'd need 25 separate language-specific GGUFs.

Compatibility — works with any ASR transcript

Because nfa-align re-tokenises words through the auxiliary model's SentencePiece vocab, you can drop in transcripts from any other ASR system :

# Cohere transcript → align with canary CTC
./build/bin/cohere-main -m cohere.gguf -f audio.wav -np > t.txt
./build/bin/nfa-align -m canary-ctc-aligner-q4_k.gguf -f audio.wav -tf t.txt

# Parakeet transcript → align with canary CTC
./build/bin/parakeet-main -m parakeet.gguf -f audio.wav -np > t.txt
./build/bin/nfa-align -m canary-ctc-aligner-q4_k.gguf -f audio.wav -tf t.txt

# Canary transcript → align with canary CTC (the official path)
./build/bin/canary-main -m canary.gguf -f audio.wav -sl en -tl en -np > t.txt
./build/bin/nfa-align -m canary-ctc-aligner-q4_k.gguf -f audio.wav -tf t.txt

# Whisper / OpenAI / hand-typed text → also works
./build/bin/nfa-align -m canary-ctc-aligner-q4_k.gguf -f audio.wav \
    -tt "your transcript here, in any of the 25 supported languages"

Standalone ASR mode

The auxiliary model is a fully-trained CTC ASR in its own right. Pass -decode to use it as a transcriber instead of an aligner:

$ ./build/bin/nfa-align -m canary-ctc-aligner-q4_k.gguf -f samples/jfk.wav -decode
And so, my fellow Americans, ask not what your country can do for you. Ask what you can do for your country.

It's not as accurate as the main canary-1b-v2 encoder–decoder for general transcription (CTC has no language model component, and the 600 M parakeet-style backbone is smaller than canary's 978 M AED), but it's a useful sanity check and works as a fast multilingual fallback ASR if you don't need the full Canary stack.

Supported languages (25)

bg cs da de el en es et fi fr hr hu it lt lv mt nl pl pt ro ru sk sl sv uk

Same coverage as Canary-1B-v2 and Parakeet-TDT-0.6B-v3, since this is the same Granary-trained backbone.

Architecture details

How this was made

1. Inspect canary-1b-v2.nemo : there's a separate timestamps_asr_model_weights.ckpt and timestamps_asr_model_config.yaml inside the tarball. The weights file has 712 tensors, ~625 M parameters, structured as encoder.* + decoder.decoder_layers.0.* .
2. Convert with models/convert-canary-ctc-to-gguf.py : extract just the auxiliary checkpoint (not the main canary), remap NeMo state-dict keys ( encoder.layers.{i}.feed_forward1.linear1.weight → encoder.layers.{i}.ff1.linear1.weight , etc.), squeeze the trailing kernel-1 dim on the CTC weight to make it a plain 2D linear, and write 712 tensors as F16 + F32.
3. C++ runtime in src/canary_ctc.{h,cpp} : mmap the GGUF, fold BN into the depthwise conv at load time, build the encoder graph (identical to parakeet's), add a final mul_mat with the CTC head, return per-frame logits.
4. Subword Viterbi in the same file: greedy longest-prefix tokenisation against the vocab, build the CTC-expanded label sequence, run the standard CTC Viterbi DP, traceback to per-token frames.
5. Quantise with cohere-quantize (the same llama.cpp-style quantiser used for the other GGUFs in this fork). Q4_K alignment is byte-identical to F16 on the verification clip.

Related

• C++ runtime: CrispStrobe/CrispASR@parakeet
• Original NVIDIA model: nvidia/canary-1b-v2 — the .nemo tarball this aligner was extracted from
• Sister GGUF release (canary main model): cstr/canary-1b-v2-GGUF — the encoder-decoder transcription + translation model
• Sister parakeet release: cstr/parakeet-tdt-0.6b-v3-GGUF — the same encoder family with a TDT decoder for free word timestamps
• NeMo Forced Aligner (the official tool that uses this same auxiliary model): NVIDIA-NeMo/NeMo
• Canary technical report: arXiv:2509.14128

License

CC-BY-4.0 , inherited from nvidia/canary-1b-v2 . Use of these GGUF files must comply with the CC-BY-4.0 license including attribution to NVIDIA's NeMo team. See the license for full terms.

The conversion + runtime code is MIT-licensed (matching the whisper.cpp base), but the model weights themselves are NVIDIA's CC-BY-4.0 work.