---
license: mit
language:
- ru
pipeline_tag: automatic-speech-recognition
base_model: ai-sage/GigaAM-v3
base_model_relation: quantized
tags:
- coreml
- apple-neural-engine
- on-device
- offline
- russian
- asr
- ctc
- gigaam
- macos
---

# GigaAM v3 (e2e CTC) — Core ML

Core ML conversion of Sber's GigaAM v3 `v3_e2e_ctc` for Apple silicon. Runs on the Neural
Engine, produces Russian text with punctuation and capitalization out of the box.

## Attribution

| | |
|---|---|
| Original model | [GigaAM v3](https://github.com/salute-developers/GigaAM) by Sber (`salute-developers`), MIT — weights on [ai-sage/GigaAM-v3](https://huggingface.co/ai-sage/GigaAM-v3) |
| Core ML conversion | Ivan Ushakov ([Inhum](https://github.com/Inhum)) |

Converted from the original PyTorch checkpoint (`gigaam.load_model("v3_e2e_ctc")`), not from
an intermediate ONNX export. Weights are redistributed under the original MIT license.

## Files

| File | Size | SHA-256 |
|---|---|---|
| `gigaam_v3_e2e.mlpackage/Data/com.apple.CoreML/model.mlmodel` | 469 KB | `8e0c43329503b3b2fc41f6e594f44d7b35acdd0bd61a91bf5741557d716be957` |
| `gigaam_v3_e2e.mlpackage/Data/com.apple.CoreML/weights/weight.bin` | 421 MiB | `706153a1e14e2b26f328f394f3eded2689865eaf2d653c021006a4f3ab3a0ec6` |
| `gigaam-melfb.f32` | 40 KB | mel filterbank, 64 × 161 float32, little-endian |
| `gigaam-window.f32` | 1.3 KB | analysis window, 320 float32, little-endian |
| `gigaam-vocab.txt` | 1 KB | 256 sentencepiece pieces, one per line, line number = token id |

The three small files are not optional. The model takes **mel features, not audio**, and emits
token ids — without the filterbank, the window and the vocabulary the package cannot be used.

## Input / output

| | Name | Shape | Type | Notes |
|---|---|---|---|---|
| in | `features` | `[1, 64, 2499]` | float32 | log-mel, fixed 25 s window |
| in | `feature_lengths` | `[1]` | int32 | real frame count before padding |
| out | `log_softmax` | `[1, 625, 257]` | float32 | log probabilities, 256 pieces + blank at id 256 |
| out | `_to_copy_6` | `[1]` | int32 | encoded length, valid frames in the output |

Output names are generated by the converter and carry no meaning. Matching them by rank —
the 3-D tensor is the logits — is more robust than matching by name.

## Preprocessing

The feature extractor must reproduce the checkpoint's own configuration exactly:

```
sample rate  16000 Hz, mono
n_fft        320
win_length   320   (Hann, periodic)
hop_length   160
center       false
n_mels       64    (HTK scale, 0–8000 Hz, unnormalized)
power        2
output       log(clamp(mel, 1e-9, 1e9))
frames       floor((samples - 320) / 160) + 1
```

Do not compute the mel filterbank from a formula. GigaAM ships its own filterbank inside the
checkpoint, it overwrites the standard torchaudio one on load, and it matches no standard
recipe. `gigaam-melfb.f32` and `gigaam-window.f32` are those exact tables.

Audio is padded with zeros to the 25 s window; the unpadded frame count goes into
`feature_lengths`. Longer audio has to be chunked by the caller.

## Decoding

Greedy CTC: argmax per frame → collapse repeats → drop blank (id 256) → concatenate pieces,
where a leading `▁` marks a word boundary and becomes a space. Token id 0 is `<unk>` and is
skipped. The sentencepiece library is not needed at run time — the flat vocabulary is enough.

## How it was converted

Environment: Python 3.13, `torch==2.7.*`, `torchaudio==2.7.*`, `coremltools>=9`, `gigaam` from
its git repository. Target `ct.target.macOS14`, weights in fp16, format `.mlpackage`.

The conversion is not a one-liner, and most of the work is in getting a clean graph out of
PyTorch:

1. Export `forward_for_export(features, lengths)` (encoder + head), not `forward()` — the
   latter expects a raw waveform and calls the preprocessor internally.
2. Do not call `transcribe()` before tracing. Its `@inference_mode` caches rotary cos/sin as
   inference tensors and the trace fails afterwards. Warm the model up through
   `forward_for_export` under `no_grad` instead.
3. Trace inside `model.encoder.onnx_export_mode()`.
4. `torch` 2.13 is incompatible with coremltools 9 (`aten::Int` bug) — use 2.7.x.
5. `torch.jit.trace` fails even on 2.7 (the same integer bug in positional encoding). Use
   `torch.export.export` followed by `ep.run_decompositions({})`.
6. The example input must be `.contiguous()` (EXIR rejects a non-contiguous dim order), but do
   not put `.contiguous()` inside `forward` — that introduces an alias node.
7. The decomposed graph still contains 32 `aten.alias` nodes, which coremltools cannot lower.
   They are no-ops: walk the FX graph, `replace_all_uses_with(node.args[0])`, `erase_node`,
   then `lint()` and `recompile()`.

Dynamic input length did not survive conversion, hence the fixed 25 s window.

```python
import gigaam, torch
import coremltools as ct

model = gigaam.load_model("v3_e2e_ctc", device="cpu"); model.eval()
wav, length = model.prepare_wav("sample_25s.wav")
with torch.no_grad():
    features, feat_len = model.preprocessor(wav, length)
    features = features.contiguous()

class W(torch.nn.Module):
    def __init__(s, m): super().__init__(); s.m = m
    def forward(s, features, feature_lengths):
        return s.m.forward_for_export(features, feature_lengths.to(torch.long))

w = W(model).eval()
with model.encoder.onnx_export_mode(), torch.no_grad():
    ep = torch.export.export(w, (features, feat_len.to(torch.int32)))
    ep = ep.run_decompositions({})
    gm = ep.graph_module
    for node in list(gm.graph.nodes):
        if node.op == "call_function" and "alias" in str(node.target):
            node.replace_all_uses_with(node.args[0])
            gm.graph.erase_node(node)
    gm.graph.lint(); gm.recompile()
    mlm = ct.convert(ep, minimum_deployment_target=ct.target.macOS14)
mlm.save("gigaam_v3_e2e.mlpackage")
```

## Usage

```swift
import CoreML

let cfg = MLModelConfiguration()
cfg.computeUnits = .all                     // let the system pick ANE/GPU/CPU
let model = try MLModel(contentsOf: compiledURL, configuration: cfg)

// features: [1, 64, 2499] float32 log-mel, zero-padded to the 25 s window
// realFrames: frame count before padding
let lens = try MLMultiArray(shape: [1], dataType: .int32)
lens[0] = NSNumber(value: Int32(realFrames))

let out = try model.prediction(from: MLDictionaryFeatureProvider(
    dictionary: ["features": features, "feature_lengths": lens]))

// 3-D output is the logits; the other one is the encoded length
```

A `.mlpackage` has to be compiled before use — either `MLModel.compileModel(at:)` once at
install time, or `xcrun coremlcompiler compile gigaam_v3_e2e.mlpackage .` ahead of time.

## Measurements

MacBook Air, Apple silicon, 8 GB, 25 s of audio:

| | PyTorch (CPU) | Core ML (ANE/GPU) |
|---|---|---|
| Inference | ~1200 ms | **102 ms** (≈245× real time) |
| Artifact | 433 MB checkpoint | 422 MB `.mlpackage` |
| Peak RAM | ~1.1 GB | lower (not measured precisely) |
| argmax token match vs PyTorch | — | 98.4% (fp16 rounding) |
| Decoded text | reference | identical, punctuation included |

## Limitations

- Fixed 25 s input window; dynamic shapes did not survive the conversion. Longer audio must be
  chunked, and chunk boundaries need de-duplication if the chunks overlap.
- Input is mel features, not audio. The feature extractor is the caller's responsibility and
  must match the tables shipped here.
- fp16 weights: 1.6% of argmax tokens differ from the PyTorch reference. Decoded text was
  identical on the samples tested, but this is not a guarantee.
- Russian only. English words are usually transliterated into Cyrillic.
- Greedy CTC only. No beam search, no language model, no timestamps, no diarization.
- Deployment target macOS 14. Not tested on iOS.

## Used by

[Voica](https://github.com/Inhum/voica) — a macOS menu-bar dictation app; this model is its
offline engine.

## Support

A by-product of a personal project, published because it may be useful. Issues are read, but
answers may be slow or absent, and feature requests are not accepted. There is no commitment to
convert future GigaAM releases.