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parakeet.cpp

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Ultra fast and portable Parakeet implementation for on-device inference in C++ using Axiom with MPS+Unified Memory

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// repository documentation

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# parakeet.cpp

Fast speech recognition with NVIDIA's [Parakeet](https://huggingface.co/collections/nvidia/parakeet) models in pure C++.

Built on [axiom](https://github.com/frikallo/axiom) — a lightweight tensor library with automatic Metal GPU acceleration. No ONNX runtime, no Python runtime, no heavyweight dependencies. Just C++ and one tensor library that outruns PyTorch MPS.

**~27ms encoder inference on Apple Silicon GPU** for 10s audio (110M model) — 96x faster than CPU. FP16 support for ~2x memory reduction.

## Supported Models

| Model | Class | Size | Type | Description |
|-------|-------|------|------|-------------|
| `tdt-ctc-110m` | `ParakeetTDTCTC` | 110M | Offline | English, dual CTC/TDT decoder heads |
| `tdt-600m` | `ParakeetTDT` | 600M | Offline | Multilingual, TDT decoder |
| `eou-120m` | `ParakeetEOU` | 120M | Streaming | English, RNNT with end-of-utterance detection |
| `nemotron-600m` | `ParakeetNemotron` | 600M | Streaming | Multilingual, configurable latency (80ms–1120ms) |
| `sortformer` | `Sortformer` | 117M | Streaming | Speaker diarization (up to 4 speakers) |
| `diarized` | `DiarizedTranscriber` | 110M+117M | Offline | ASR + diarization → speaker-attributed words |

All ASR models share the same audio pipeline: 16kHz mono WAV → 80-bin Mel spectrogram → FastConformer encoder.

## Quick Start

```cpp
#include <parakeet/parakeet.hpp>

parakeet::Transcriber t("model.safetensors", "vocab.txt");
t.to_gpu();   // optional — Metal acceleration
t.to_half();  // optional — FP16 inference (~2x memory reduction)

auto result = t.transcribe("audio.wav");
std::cout << result.text << std::endl;
```

## Features

- **Multiple decoders** — CTC greedy, TDT greedy, CTC beam search, TDT beam search (switch at call site)
- **Word timestamps** — Per-word start/end times and confidence scores on all decoders
- **Beam search + LM** — CTC and TDT beam search with optional ARPA n-gram language model fusion
- **Phrase boosting** — Context biasing via token-level trie for domain-specific vocabulary
- **Batch transcription** — Multiple files in one batched encoder forward pass
- **VAD preprocessing** — Silero VAD strips silence before ASR; timestamps auto-remapped
- **GPU acceleration** — Metal via axiom's MPSGraph compiler (96x speedup on Apple Silicon)
- **FP16 inference** — Half-precision weights and compute (~2x memory reduction)
- **Streaming** — EOU and Nemotron models with chunked audio input
- **Speaker diarization** — Sortformer (up to 4 speakers), combinable with ASR for speaker-attributed words
- **C API** — Flat `extern "C"` FFI for Python, Swift, Go, Rust, and other languages
- **Multi-format audio** — WAV, FLAC, MP3, OGG with automatic resampling

See [examples/](examples/) for code demonstrating each feature.

## Install

Prebuilt binaries are attached to each [GitHub release](https://github.com/Frikallo/parakeet.cpp/releases) for macOS arm64, macOS x86_64, and Linux x86_64. Download the tarball for your platform and extract:

```bash
tar -xzf parakeet-v0.1.0-macos-arm64.tar.gz
cd parakeet-v0.1.0-macos-arm64
# On macOS, clear the Gatekeeper quarantine attribute first:
xattr -dr com.apple.quarantine .
./bin/parakeet --help
```

The archive ships a self-contained `bin/parakeet` (and `bin/example-server`) plus `lib/libaxiom` with `@rpath`/`$ORIGIN` set so the binaries resolve their dependencies relative to the install dir — drop the directory anywhere. The C-API headers under `include/parakeet/` are included for embedders.

## Build from source

```bash
git clone --recursive https://github.com/frikallo/parakeet.cpp
cd parakeet.cpp
make build
make test
```

Requirements: C++20 (Clang 14+ or GCC 12+), CMake 3.20+, macOS 13+ for Metal GPU.

> **macOS:** building requires the **full Xcode** install (not just Command Line Tools), because axiom compiles its Metal shaders with `xcrun metal` and `xcrun metallib` — those ship only with Xcode. If you just want to run parakeet, use the prebuilt tarball above; the `.metallib` is embedded into the shipped `libaxiom.dylib` and runs without any Xcode/CLT install on the user side.

## Convert Weights

```bash
# Download from HuggingFace
huggingface-cli download nvidia/parakeet-tdt_ctc-110m --include "*.nemo" --local-dir .

# Convert to safetensors
pip install safetensors torch
python scripts/convert_nemo.py parakeet-tdt_ctc-110m.nemo -o model.safetensors
```

The converter supports all model types: `110m-tdt-ctc` (default), `600m-tdt`, `eou-120m`, `nemotron-600m`, `sortformer`.

```bash
python scripts/convert_nemo.py checkpoint.nemo -o model.safetensors --model 600m-tdt
```

Silero VAD weights:
```bash
python scripts/convert_silero_vad.py -o silero_vad_v5.safetensors
```

## Examples

| Example | Description |
|---------|-------------|
| [basic](examples/basic/) | Simplest transcription (~20 lines) |
| [timestamps](examples/timestamps/) | Word/token timestamps with confidence |
| [beam-search](examples/beam-search/) | CTC/TDT beam search with optional ARPA LM |
| [phrase-boost](examples/phrase-boost/) | Context biasing for domain vocabulary |
| [batch](examples/batch/) | Batch transcription of multiple files |
| [vad](examples/vad/) | Standalone VAD and ASR+VAD preprocessing |
| [gpu](examples/gpu/) | Metal GPU + FP16 with timing comparison |
| [stream](examples/stream/) | EOU streaming transcription |
| [nemotron](examples/nemotron/) | Nemotron streaming with latency modes |
| [diarize](examples/diarize/) | Sortformer speaker diarization |
| [diarized-transcription](examples/diarized-transcription/) | ASR + diarization combined |
| [c-api](examples/c-api/) | Pure C99 FFI usage |
| [cli](examples/cli/) | Full CLI with all options |

## Using as a Library

### CMake `find_package`

After installing (`make install` or `cmake --install build`):

```cmake
find_package(Parakeet REQUIRED)
target_link_libraries(myapp PRIVATE Parakeet::parakeet)
```

### CMake `add_subdirectory`

```cmake
add_subdirectory(third_party/parakeet.cpp)
target_link_libraries(myapp PRIVATE Parakeet::parakeet)
```

### pkg-config

```bash
g++ -std=c++20 myapp.cpp $(pkg-config --cflags --libs parakeet) -o myapp
```

## Architecture

### Offline Models

Built on a shared FastConformer encoder (Conv2d 8x subsampling → N Conformer blocks with relative positional attention):

| Model | Class | Decoder | Use case |
|-------|-------|---------|----------|
| CTC | `ParakeetCTC` | Greedy argmax or beam search (+LM) | Fast, English-only |
| RNNT | `ParakeetRNNT` | Autoregressive LSTM | Streaming capable |
| TDT | `ParakeetTDT` | LSTM + duration prediction, greedy or beam search (+LM) | Better accuracy than RNNT |
| TDT-CTC | `ParakeetTDTCTC` | Both TDT and CTC heads | Switch decoder at inference |

### Streaming Models

Built on a cache-aware streaming FastConformer encoder with causal convolutions and bounded-context attention:

| Model | Class | Decoder | Use case |
|-------|-------|---------|----------|
| EOU | `ParakeetEOU` | Streaming RNNT | End-of-utterance detection |
| Nemotron | `ParakeetNemotron` | Streaming TDT | Configurable latency streaming |

### Diarization

| Model | Class | Architecture | Use case |
|-------|-------|-------------|----------|
| Sortformer | `Sortformer` | NEST encoder → Transformer → sigmoid | Speaker diarization (up to 4 speakers) |

## Benchmarks

Measured on Apple M3 16GB with simulated audio input (`Tensor::randn`). Times are per-encoder-forward-pass (Sortformer: full forward pass).

**Encoder throughput — 10s audio:**

| Model | Params | CPU (ms) | GPU (ms) | GPU Speedup |
|-------|--------|----------|----------|-------------|
| 110m (TDT-CTC) | 110M | 2,581 | 27 | **96x** |
| tdt-600m | 600M | 10,779 | 520 | **21x** |
| rnnt-600m | 600M | 10,648 | 1,468 | **7x** |
| sortformer | 117M | 3,195 | 479 | **7x** |

**110m GPU scaling across audio lengths:**

| Audio | CPU (ms) | GPU (ms) | RTF | Throughput |
|-------|----------|----------|-----|------------|
| 1s | 262 | 24 | 0.024 | 41x |
| 5s | 1,222 | 26 | 0.005 | 190x |
| 10s | 2,581 | 27 | 0.003 | 370x |
| 30s | 10,061 | 32 | 0.001 | 935x |
| 60s | 26,559 | 72 | 0.001 | 833x |

GPU acceleration powered by axiom's Metal graph compiler which fuses the full encoder into optimized MPSGraph operations.

```bash
make bench ARGS="--110m=models/model.safetensors --tdt-600m=models/tdt.safetensors"
```

## Roadmap

### Tier 1 — High Impact

- [x] **Confidence scores** — Per-token and per-word confidence from token log-probs
- [x] **Phrase boosting** — Token-level trie context biasing during decode
- [x] **Beam search** — CTC prefix beam search and TDT time-synchronous beam search
- [x] **N-gram LM fusion** — ARPA language models scored at word boundaries

### Audio & I/O

- [x] **Multi-format audio** — WAV, FLAC, MP3, OGG via dr_libs + stb_vorbis
- [x] **Automatic resampling** — Windowed sinc interpolation (Kaiser, 16-tap)
- [x] **Load from memory** — `read_audio(bytes, len)`, float/int16 buffers
- [ ] **Audio duration query** — Header-only duration without full decode
- [ ] **Progress callbacks** — Stage reporting for long files
- [ ] **Streaming from raw PCM** — Direct microphone buffer feeding

### Tier 2 — Production Readiness

- [x] **Diarized transcription** — ASR + Sortformer → speaker-attributed words
- [x] **VAD** — Silero VAD v5, standalone + ASR preprocessing
- [x] **Batch inference** — Padded multi-file encoder forward pass
- [ ] **Long-form chunking** — Overlapping windows for audio >30s
- [ ] **Neural LM rescoring** — N-best reranking with Transformer LM

### Tier 3 — Ecosystem

- [x] **C API** — Flat C interface for FFI from any language
- [x] **FP16 inference** — Half-precision weights and compute
- [ ] **Model quantization** — INT8/INT4 for mobile deployment
- [ ] **Hotword detection** — Trigger phrase detection
- [ ] **Speaker embeddings** — Speaker verification from Sortformer/TitaNet

## Notes

- Audio: 16kHz mono (WAV, FLAC, MP3, OGG — auto-detected and resampled)
- Offline models have ~4-5 minute audio length limits; use streaming models for longer audio
- GPU acceleration requires Apple Silicon with Metal support

## License

MIT