CPP Song Key + BPM Finder

CPP-Song-Key-BPM-Finder is a C++ command-line project that uses a Python analysis bridge (librosa + numpy) to detect:

1. tempo (BPM)
2. musical key (major/minor)
3. confidence and analysis metadata

Why this version is more accurate

The project now uses a proper chroma-based key estimation pipeline with Krumhansl-Schmuckler key profiles instead of using tuning estimation as a key proxy.

Repository layout

• src/main.cpp: single C++ entrypoint
• include/bridge_runner.hpp: C++ bridge execution utilities
• scripts/PyBridge.py: Python analysis engine
• CMakeLists.txt: build configuration
• requirements.txt: Python dependencies

Prerequisites

• C++17 compiler (g++, clang++, or MSVC)
• CMake 3.16+
• Python 3.9+
• pip

Setup

1. Install Python dependencies:

python3 -m venv .venv
.venv/bin/pip install -r requirements.txt

2. Configure and build:

cmake -S . -B build
cmake --build build

Usage

Run human-readable output:

KEY_BPM_PYTHON=.venv/bin/python ./build/Key-BpmFinder <audio_file>

Run JSON output from the same executable:

KEY_BPM_PYTHON=.venv/bin/python ./build/Key-BpmFinder --json <audio_file>

Example output

Human-readable mode:

BPM: 121.53
Key: A minor
Key score: 0.8652
Key confidence: 0.0421
Sample rate: 44100
Duration (seconds): 34.18

JSON mode:

{"bpm": 121.53, "key": "A minor", "key_score": 0.8652, "key_confidence": 0.0421, "sample_rate": 44100, "duration_seconds": 34.18}

Troubleshooting

• Error: Audio file not found: check the file path.
• Error: Could not find PyBridge.py: ensure scripts/PyBridge.py exists in the repository.
• ModuleNotFoundError: No module named 'librosa': install dependencies with python3 -m pip install -r requirements.txt.
• If you use a virtual environment, set KEY_BPM_PYTHON to that Python executable.
• If an audio backend error appears, install ffmpeg and retry.

Notes

• Key-BpmFinder copies scripts/PyBridge.py into the build output directory automatically.
• Key estimation is probabilistic; confidence is included so low-confidence detections are visible.