---
license: mit
pipeline_tag: audio-to-audio
tags:
- audio-separation
- vocal-remover
- stem-separation
- onnx
- onnxruntime
---

# AudioSeparatorONNX

Popular audio source separation models — UVR5, MDX-Net, VR Architecture, BSRoformer — converted to **ONNX** format and packaged as single self-contained files.

## Why ONNX?

The standard way to run these models is through [audio-separator](https://github.com/nomadkaraoke/python-audio-separator) or [UVR](https://github.com/Anjok07/ultimatevocalremovergui), both of which require Python and PyTorch. That's fine for desktop use, but becomes a problem when you want to:

- Ship a **native application** (C++, Swift, Rust, .NET) without a Python runtime
- Run separation on a **mobile or embedded device**
- Build a **server-side pipeline** where spinning up PyTorch per request is too heavy
- Use **CoreML, TensorRT, DirectML, or other hardware accelerators** via ONNX Runtime
- Load a model in any language that has an ONNX Runtime binding

ONNX Runtime handles all of the above with a single lightweight library and no Python dependency. The models in this repo are ready to drop into any ORT-based pipeline.

## What makes this repo different

Most ONNX model repos ship the file and nothing else. Every model here includes two metadata blobs embedded directly inside the `.onnx` file:

| Key | What it contains |
|-----|-----------------|
| `sep_meta` | All inference parameters: arch, stems, STFT config, chunk size, overlap, and for Roformer — `freq_indices` and `num_bands_per_freq` arrays needed for the gather/scatter steps |
| `model_config` | The original training config reconstructed from the model weights — lets you recover a YAML for audio-separator or any other PyTorch pipeline without needing the original sidecar file |

No JSON files, no YAML sidecars, no download_checks.json. One file per model.

---

## ⚠️ Compatibility Notes

### MDX and VR models

These work with audio-separator and UVR out of the box. Just point `model_file_dir` at the folder containing the `.onnx` files.

> **Hash detection:** audio-separator identifies models by MD5 of the last 10 MB of the file. Because `sep_meta` and `model_config` are appended at the end, the hash of these files differs from the originals in the UVR database. If auto-detection fails, pass the parameters explicitly via `mdx_params` or `vr_params` — all values are available in `sep_meta`.

### Roformer models (BSRoformer)

Roformer `.onnx` files are intended for **ONNX Runtime inference only** — audio-separator and UVR run Roformer via PyTorch from the original `.ckpt`, not from ONNX. These files are useful if you are building a custom native pipeline.

The embedded `model_config` key lets you recover the training configuration without the original `.yaml` sidecar — see the extraction section below.

---

## 📦 Reading Embedded Metadata

Both keys are stored as compact JSON strings inside the ONNX `metadata_props` field.

### Python — `onnxruntime`

```python
import json
import onnxruntime as ort

sess = ort.InferenceSession("UVR-DeEcho-DeReverb.onnx", providers=["CPUExecutionProvider"])
meta = sess.get_modelmeta().custom_metadata_map   # dict[str, str]

sep_meta     = json.loads(meta["sep_meta"])
model_config = json.loads(meta["model_config"])

print(sep_meta["arch"])          # "MDX" | "VR" | "ROFORMER"
print(sep_meta["primary_stem"])  # e.g. "No Reverb"
```

### Python — `onnx` library (no inference session)

```python
import json
import onnx

model = onnx.load("UVR-DeEcho-DeReverb.onnx")
meta  = {p.key: p.value for p in model.metadata_props}

sep_meta     = json.loads(meta["sep_meta"])
model_config = json.loads(meta["model_config"])
```

### C — byte-scan (no ORT, no Python)

Both keys are stored near the **end** of the ONNX protobuf, after all weight tensors. You can extract either by scanning the last 64 KB without loading any weights:

```c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

/*
 * Returns a heap-allocated null-terminated JSON string for the given key,
 * or NULL if not found. Caller must free() the result.
 *
 * Increase SCAN_SIZE to 524288 for large Roformer models whose
 * freq_indices array may exceed 64 KB.
 */
char* read_onnx_meta_key(const char* path, const char* key) {
    FILE* f = fopen(path, "rb");
    if (!f) return NULL;

    const size_t SCAN_SIZE = 65536;
    fseek(f, 0, SEEK_END);
    long sz = ftell(f);
    size_t read_sz = (sz < (long)SCAN_SIZE) ? (size_t)sz : SCAN_SIZE;
    fseek(f, sz - (long)read_sz, SEEK_SET);

    char* buf = (char*)malloc(read_sz + 1);
    if (!buf) { fclose(f); return NULL; }
    size_t n = fread(buf, 1, read_sz, f);
    fclose(f);
    buf[n] = '\0';

    size_t klen = strlen(key);
    char* found = NULL;
    for (size_t i = 0; i + klen < n; i++)
        if (memcmp(buf + i, key, klen) == 0) found = buf + i;
    if (!found) { free(buf); return NULL; }

    char* p = found + klen;
    while (p < buf + n && *p != '{') p++;
    if (p >= buf + n) { free(buf); return NULL; }
    char* start = p;

    int depth = 0;
    while (p < buf + n) {
        if      (*p == '{') depth++;
        else if (*p == '}') { if (--depth == 0) break; }
        p++;
    }
    if (depth != 0) { free(buf); return NULL; }

    size_t len = (size_t)(p - start) + 1;
    char* out  = (char*)malloc(len + 1);
    memcpy(out, start, len);
    out[len] = '\0';
    free(buf);
    return out;
}

int main(void) {
    char* sep  = read_onnx_meta_key("model.onnx", "sep_meta");
    char* cfg  = read_onnx_meta_key("model.onnx", "model_config");
    if (sep) { printf("sep_meta: %s\n", sep); free(sep); }
    if (cfg) { printf("model_config: %s\n", cfg); free(cfg); }
    return 0;
}
```

---

## 🔧 Using `model_config` in your ONNX pipeline

The `model_config` key contains the original training configuration. If you are building a custom ONNX Runtime pipeline around Roformer inference, you can read it directly at runtime instead of shipping a separate YAML:

```python
import json
import onnxruntime as ort

sess = ort.InferenceSession("deverb_bs_roformer_8_384dim_10depth.onnx",
                            providers=["CPUExecutionProvider"])
meta = sess.get_modelmeta().custom_metadata_map

sep_meta     = json.loads(meta["sep_meta"])       # chunk sizes, freq_indices, etc.
model_config = json.loads(meta["model_config"])   # arch params: dim, depth, n_fft, ...

# Everything you need to run the pipeline is in these two dicts.
# No separate YAML or JSON sidecar required.
n_fft      = sep_meta["n_fft"]
hop_length = sep_meta["hop_length"]
chunk_size = sep_meta["chunk_size"]
overlap    = sep_meta["overlap"]
```

---

## 🔬 `sep_meta` Reference

### MDX-Net

```json
{
  "arch":           "MDX",
  "primary_stem":   "Vocals",
  "secondary_stem": "Instrumental",
  "sample_rate":    44100,
  "n_fft":          7680,
  "hop_length":     1024,
  "dim_f":          3072,
  "dim_t":          256,
  "compensate":     1.021,
  "overlap":        0.25
}
```

**ONNX I/O** — Input `input (1, 4, dim_f, dim_t)`: `[real_L, imag_L, real_R, imag_R]` · Output `output` same shape

### VR Architecture

```json
{
  "arch":           "VR",
  "primary_stem":   "No Reverb",
  "secondary_stem": "Reverb",
  "sample_rate":    44100,
  "vr_model_param": "4band_v3",
  "bins":           672,
  "window_size":    512,
  "is_vr51":        true,
  "nn_arch_size":   218409,
  "model_capacity": [32, 128],
  "band_params": {
    "1": {"sr": 11025, "hl": 480, "n_fft": 960,  "crop_start": 0,   "crop_stop": 245},
    "2": {"sr": 22050, "hl": 480, "n_fft": 1920, "crop_start": 245, "crop_stop": 432},
    "3": {"sr": 44100, "hl": 480, "n_fft": 3840, "crop_start": 432, "crop_stop": 567},
    "4": {"sr": 44100, "hl": 960, "n_fft": 7680, "crop_start": 567, "crop_stop": 673}
  }
}
```

**ONNX I/O** — Input `input (1, 2, bins+1, window_size)`: stereo multi-band magnitude · Output `output` same shape (source mask)

### BSRoformer

The ONNX graph covers `band_split + transformer + mask_estimators`. STFT and iSTFT are handled by the caller.

Pipeline:
1. STFT per channel → interleave channels → `stft_repr` shape `(n_full_freqs, T, 2)`
2. Gather `freq_indices` from `stft_repr` → flatten → `x_flat` shape `(1, frames, n_freq_indices*2)`  ← **ONNX input**
3. ONNX forward → `masks` shape `(1, 1, n_freq_indices, frames, 2)`
4. `scatter_add` masks back to `stft_repr` positions, divide by `num_bands_per_freq` → `masks_avg`
5. Multiply `stft_repr * masks_avg` → iSTFT per channel → audio

```json
{
  "arch":               "ROFORMER",
  "roformer_type":      "BSRoformer",
  "primary_stem":       "No Reverb",
  "secondary_stem":     "Reverb",
  "sample_rate":        44100,
  "num_channels":       2,
  "chunk_size":         112455,
  "native_chunk_size":  352800,
  "hop_length":         441,
  "n_fft":              2048,
  "frames":             256,
  "n_freq_indices":     2050,
  "n_full_freqs":       2050,
  "overlap":            2,
  "freq_indices":       [0, 1, 2, "..."],
  "num_bands_per_freq": [1, 1, 1, "..."]
}
```

| Field | Description |
|-------|-------------|
| `chunk_size` | Samples per inference chunk (export size — smaller to reduce RAM during export) |
| `native_chunk_size` | Original training chunk size — use for best quality if RAM allows |
| `frames` | STFT frame count — `x_flat` must have exactly this many time frames |
| `freq_indices` | Indices into `stft_repr` to gather before the ONNX forward pass |
| `num_bands_per_freq` | How many bands cover each frequency bin — scatter normalization denominator |

**ONNX I/O** — Input `x_flat (1, frames, n_freq_indices*2)` · Output `masks (1, 1, n_freq_indices, frames, 2)`

---

## 🚀 Quickstart — MDX and VR with `audio-separator`

```bash
pip install "audio-separator[cpu]"   # CPU / Apple Silicon
pip install "audio-separator[gpu]"   # Nvidia CUDA
```

```bash
# CLI
audio-separator mix.wav \
  --model_filename UVR-DeEcho-DeReverb.onnx \
  --model_file_dir /path/to/models \
  --output_dir ./output
```

```python
# Python API
from audio_separator.separator import Separator

sep = Separator(model_file_dir="/path/to/models", output_dir="./output")
sep.load_model("UVR-DeEcho-DeReverb.onnx")
sep.separate("mix.wav")
```

If hash auto-detection fails (see compatibility note above), pass the parameters manually:

```python
sep = Separator(
    model_file_dir="/path/to/models",
    mdx_params={"hop_length": 1024, "segment_size": 256, "overlap": 0.25},
)
sep.load_model("UVR-MDX-NET-Inst_HQ_5.onnx")
```

---

## 📋 Requirements

```
onnxruntime >= 1.16
```

For reading metadata without running inference:
```
onnx >= 1.14
```

---

## 📄 License

[MIT License](LICENSE). Check individual model licenses before commercial use.

---

## 🙏 Acknowledgments

- [Ultimate Vocal Remover (UVR5)](https://github.com/Anjok07/ultimatevocalremovergui)
- [audio-separator](https://github.com/nomadkaraoke/python-audio-separator)
- [ONNX Runtime](https://onnxruntime.ai/)
- [BS-RoFormer](https://github.com/lucidrains/BS-RoFormer)