yttml

A collection of Rust utilities and crates for working with YouTube styled subtitles, an attempt to rewrite the YTSubConverter in Rust.

[!NOTE] This project is still in early development and is not yet ready for use, however it is designed to be a more modern and stable alternative to the original YTSubConverter.

Motivation

If you've ever tried to work with YouTube subtitles, you may have noticed that the official YouTube subtitle editor is quite limited in terms of features.

However, the YouTube captioning system actually supports a wide range of features that are not exposed in the official editor, the format is called YTT (YouTube Timed Text), version 3 (also called SRV3).

This format supports almost all the features you would expect from an advanced format like SSA (SubStation Alpha), including text formatting, custom fonts, text alignment, text rotation, karaoke timing, and more.

The original YTSubConverter was a C# utility that allowed you to convert between various subtitle formats, focusing primarily on converting SSA to SRV3. However, the CLI interface was quite clunky and the codebase was not very maintainable, and there was a complete lack of documentation on the SRV3 format.

This project was created to provide a more modern and maintainable alternative to the original YTSubConverter, and to fully document and study YouTube's proprietary subtitle format.

The SRV3 (YouTube Timed Text) Format

SRV3 is a proprietary format used by YouTube for storing subtitles. It is based on the TTML standard, but with slight modifications. The format is XML based, and is used to store subtitles for YouTube videos. The format however does not only support timed text, but also a range of other features such as:

• Text formatting (bold, italic, underline, strikethrough)
• Text color and background transparency
• Custom text alignment
• 6 different font styles
  • Carrois Gothic SC
  • Comic Sans MS
  • Courier New
  • Lucida Console
  • Monotype Corsiva
  • Roboto
  • Times New Roman
• Custom font sizes and colors
• Outlines, glows and drop shadows
• Custom text rotation
• Karaoke timing
• Vertical text
• Ruby (furigana/bopomofo) text

The usage of this format is popular among fansubbers and captioners especially in Japanese media and anime, as it allows for a wide range of text formatting options that can emulate and even overlay original text in a video. You will usually see these kinds of styled subtitles in fansubbed anime, Japanese music videos, or VTuber clips on YouTube.

This makes the SRV3 format similar to Advanced SubStation Alpha instead of the more common SubRip or WebVTT formats in terms of features.

Documentation for the SRV3 format can be found here.

Example videos showcasing the capabilities of the SRV3 format

• PinocchioP - Anonymous M (Thai - animation caption track)
• Shinra-bansho - Aria on the line of fate MV
• Kizuna AI - AIAIAI MV
• ZUTOMAYO - Fastening MV
• GYARI, Kotonoha Akane & Aoi - Aoi-chan is going to eat ChocoMint no matter what (Non-English example since the English version has been automatically unformatted due to YouTube's subtitle editor being used)
• 32ki, Hatsune Miku & Kasane Teto - Mesmerizer (Thai captions only)
• x0o0x - Kisaragi Station (Use British English subtitles for the styled version)
• Kikuo ft. Hatsune Miku - Love me, Love me, Love me
• Most, if not all hololive music videos are officially subtitled in this format, with karaoke timing and custom text formatting
• Various news clips from American news networks which provide real-time captioning with a teletype effect like this
• Various VTuber clips, example here

Crates

• srv3-ttml: A serde parser for the SRV3 format.
• yttml (this crate): A command-line utility for reading SRV3 subtitles and converting them (work in progress)

Testing

To download SRV3 subtitles from YouTube, you can use the yt-dlp utility, and download the video with the --write-subs, --sub-langs and --sub-format=srv3 flags.

You may also directly download from the API using this URL format:

https://www.youtube.com/api/timedtext?v=<video ID>&lang=<language code>&fmt=srv3

License

This project is licensed under the MIT license.