opensrt

A powerful CLI tool that generates .srt subtitle files from video files using local Whisper transcription with advanced audio processing.

Features

• Fully offline — All processing happens locally, no cloud dependencies
• Advanced audio processing — ffmpeg + noisereduce for clean voice extraction
• Accurate timestamps — stable-ts provides precise word-level timing
• VAD (Voice Activity Detection) — Silero VAD for detecting speech boundaries
• Dual-model mode — Use small for accuracy + base for timestamps
• Customizable output — Font and color selection for subtitles
• GPU acceleration — Automatic CUDA detection for faster processing

Prerequisites

• Python 3.10+
• ffmpeg (must be installed separately)

Installing ffmpeg

macOS:

brew install ffmpeg

Ubuntu/Debian:

sudo apt install ffmpeg

Windows: Download from https://ffmpeg.org/download.html

Installation

pip install opensrt

Or for development:

git clone <repo> && cd opensrt && pip install -e .

Quick Start

# Basic usage — generates video.srt next to video.mp4
opensrt generate video.mp4

# Use defaults (skip font/color prompts)
opensrt generate video.mp4 --withdefaults

Usage

opensrt generate <video_path> [OPTIONS]

Options

Examples

# Specify model and language
opensrt generate video.mp4 --model small --language en

# Skip noise reduction (faster)
opensrt generate video.mp4 --no-denoise

# Stronger noise reduction
opensrt generate video.mp4 --nr-strength 0.9

# Tighter subtitle gaps (0.5s instead of 0.8s)
opensrt generate video.mp4 --gap 0.5

# Custom font and color
opensrt generate video.mp4 --font "Times New Roman" --color Yellow

# Fully automated (no prompts)
opensrt generate video.mp4 --withdefaults

Model Information

On first run, the Whisper model will be downloaded automatically:

Model Selection Guide

• tiny/base: Fast, good for quick previews
• small: Best balance of speed and accuracy (recommended)
• medium/large: Most accurate, slower processing

How It Works

video.mp4
    │
    ▼
┌─────────────────────────────────────────────┐
│ 1. Audio Extraction (ffmpeg)                │
│    - Convert to 16kHz mono WAV              │
└─────────────────────────────────────────────┘
    │
    ▼
┌─────────────────────────────────────────────┐
│ 2. Denoising (if --denoise enabled)        │
│    - ffmpeg: frequency filtering + gate    │
│    - noisereduce: statistical subtraction  │
│    - Output: clean voice-only audio        │
└─────────────────────────────────────────────┘
    │
    ▼
┌─────────────────────────────────────────────┐
│ 3. Transcription (stable-ts + Whisper)      │
│    - VAD: detect speech boundaries         │
│    - Transcribe with word-level timestamps  │
│    - Regroup by gap/punctuation            │
└─────────────────────────────────────────────┘
    │
    ▼
┌─────────────────────────────────────────────┐
│ 4. SRT Generation                           │
│    - Apply font + color styling            │
│    - Write to .srt file                    │
└─────────────────────────────────────────────┘
    │
    ▼
video.srt

Output

The generated .srt file is saved next to the input video with the same base name:

• /videos/lecture.mp4 → /videos/lecture.srt

Troubleshooting

"ffmpeg not found" error

• Install ffmpeg (see Prerequisites section)

Slow transcription

• Use --model small instead of medium/large
• Use --no-denoise if audio is already clean
• Ensure GPU is available (CUDA)

Poor timestamp accuracy

• Use --denoise to clean audio first
• Try --gap 0.5 for tighter subtitle splitting