---
license: apache-2.0
language: en
tags:
  - zipformer
  - ctc
  - streaming
  - icefall
  - k2
  - safetensors
base_model: csukuangfj/icefall-asr-librispeech-streaming-zipformer-small-2024-03-18
---

# zipformer-streaming-ctc-small-en

Streaming Zipformer2 encoder with a CTC head, trained on LibriSpeech by the
k2-fsa / icefall project (`--causal 1 --use-ctc 1`, chunk sizes 16/32/64 at
50 Hz, left context 64/128/256), repackaged as `safetensors` for the
[maxi-ml](https://github.com/maxi-tools/maxi-ml) native port
(`asr-models::zipformer`).

- **Source checkpoint:** `csukuangfj/icefall-asr-librispeech-streaming-zipformer-small-2024-03-18`,
  `exp-ctc-rnnt-small`, weights averaged over epochs 28–30 exactly as the
  upstream `ctc-epoch-30-avg-3-chunk-16-left-128.onnx` export
  (`average_checkpoints_with_averaged_model(epoch-27.pt, epoch-30.pt)`).
- **Converter:** `scripts/convert_zipformer_checkpoint.py` in maxi-ml. Only
  the inference parameters are kept (`encoder_embed.*`, `encoder.*`,
  `ctc_output.*`); the transducer decoder/joiner are dropped.
- **Files:** `model.safetensors` (fp32, 22.46 M parameters), `config.json`
  (architecture hyper-parameters), `tokens.txt` (BPE 500 + `<blk>`).
- **Features:** 80-bin Kaldi fbank, 16 kHz, dither 0, snip_edges false,
  high_freq −400.
- **CoreML packages:**
  - `coreml/zipformer-chunk{16,8}-left128.mlpackage`: the icefall streaming
    forward traced with coremltools (`scripts/coreml/convert_zipformer_to_coreml.py`),
    caches in `MLState`. Runs on the GPU; the ANE compiler rejects it.
  - `coreml-ane/zipformer-chunk{16,8}-left128.mlpackage` and
    `…-stateless.mlpackage`: the same step emitted directly as a MIL program
    in the `(B, C, 1, S)` layout the ANE accepts
    (`maxi-ml convert-zipformer-ane`, `maxi_ml_compile_mil::zipformer_emitter`,
    maxi-ml commit bc970670e), caches packed into 14 `MLState` tensors, or
    carried as inputs/outputs in the `-stateless` twin for macOS 27.0, which
    cannot load `MLState` programs. Both have the traced package's I/O
    (`x: [1, 2·chunk+13, 80]` fp32 in, `log_probs: [1, chunk/2, 500]` fp32
    out; the stateless one adds `<cache>` inputs and `<cache>_out` outputs).
    Measured with `examples/zipformer_latency.rs`: 1.6–2.5 ms per 320 ms
    chunk on the ANE across M3 / M4 Pro / M5 Max, 4–6 ms at microphone
    pacing, same WER as the fp32 MLX port on the corpus (one near-tie word
    can differ at chunk 16 — fp16 end to end).
- **License:** Apache-2.0, as upstream.