One binary. Many sources. Perfectly in sync.

RedSync merges Dolby Vision and HDR10 / HDR10+ into one MKV, syncs audio, subtitles and chapters taken from different sources, and injects a DV RPU onto an HDR10 base with the active-area crop worked out automatically. A single command line tool for Linux and Windows.

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RedSync is a command line tool for building Dolby Vision and HDR hybrid MKV files and for syncing tracks across releases. It wraps dovi_tool, hdr10plus_tool, ffmpeg and mkvmerge behind one command, measures audio offsets so tracks line up, and keeps every language tag, title and flag intact.

Use it to:

• Add Dolby Vision to an HDR10 file (inject a DV RPU and produce a DV HDR10 / DV HDR10+ hybrid)
• Make a DV + HDR hybrid from a small DV rip and a full-size HDR10 video
• Combine a DV source and a separate HDR10+ source into one DV HDR10+ hybrid
• Convert HDR10+ to Dolby Vision
• Sync audio, subtitles or chapters from one release onto the video of another
• Remux tracks from several MKV sources into one file with correct timing
• Set the sync offset by hand when you already know it (--shift)
• Drive any of the above from a script and read the result back as JSON (--json)

Contents

• Install
• Requirements
• Quick start
• How to add Dolby Vision to an HDR10 file
• How to sync audio from a different release
• Scripting RedSync
• How the sync stays accurate
• How the hybrid crop is automatic
• Build from source
• Credits

Install

Download the binary for your platform from the releases page and run it. It is self-contained and needs no admin rights.

Linux

chmod +x RedSync
./RedSync -h

Windows (terminal)

.\RedSync.exe -h

Then check your tooling in one step:

RedSync doctor

Requirements

RedSync calls a few external programs. Run RedSync doctor at any time to see what is present and what is missing.

Tool	How it is provided
dovi_tool, hdr10plus_tool	Bundled inside the binary. Nothing to install.
ffmpeg, ffprobe	Fetched automatically on first run if not already on your system.
mkvmerge, mkvextract, mkvpropedit (MKVToolNix)	You install these.
mediainfo	You install this.

Install the two you need:

Linux (Debian / Ubuntu)

sudo apt install mkvtoolnix mediainfo

Other distributions: dnf install mkvtoolnix mediainfo (Fedora), pacman -S mkvtoolnix-cli mediainfo (Arch).

Windows

winget install MoritzBunkus.MKVToolNix MediaArea.MediaInfo

If a tool is missing, RedSync doctor prints the exact command to fix it.

Quick start

Pick interactively. Point at a folder and choose a version:

RedSync ./my-clips/

RedSync numbers each source, lists the versions you can build (with a recommended best pick based on the highest video tier, best audio and most subtitles), and lets you compose one with short codes: v1 video from source 1, a2 audio from source 2, s2 subtitles from source 2, c1 chapters from source 1, dv2 Dolby Vision from source 2.

Quick two-file sync. First file is the video, the second hands over its audio, subtitles and chapters:

RedSync film_a.mkv film_b.mkv --sync

Narrow it with --audio-only, --subs-only or --chapters-only.

Pulling subtitles from more than one source? Add --unique and RedSync keeps one subtitle track per language/forced/SDH combination instead of muxing every track from every source. Sources are deduped in the order they're given, so nothing is lost - if source A has 4 subtitle languages and source B has 8, and one of source A's languages isn't in source B at all, that one still makes it into the output instead of being dropped in favor of source B's larger set:

RedSync film_a.mkv film_b.mkv film_c.mkv --sync --unique

Region matters here too: a French track tagged fr-CA and one tagged fr-FR count as different languages, not duplicates. RedSync reads that from Matroska's BCP-47 language tag (mkvmerge's identify output), since plain ffprobe collapses both down to a bare fre.

Spell out three or more sources:

RedSync --video film_a.mkv --audio film_b.mkv --subtitles film_c.mkv --chapters film_a.mkv

--subs / --subtitles and --chapters / --chaps are interchangeable.

Inspect a file:

RedSync analyze *.mkv

Add --dry-run to any command to print the plan and the exact mkvmerge line without writing anything, and --out-dir <folder> to choose where the result is written.

On a real hybrid run the layer extractions, the RPU work and the offset measurement run in parallel, and the large intermediate files are deleted as soon as they are used, so nothing piles up in the cache.

Every run ends with a timings breakdown - how long probing, offset measurement, the hybrid build and the final mux each took, plus the total - so it's obvious where the time actually went.

How to add Dolby Vision to an HDR10 file

Take the HDR10 video and the Dolby Vision from two sources and build a DV HDR10 hybrid, keeping the HDR10 source's own audio, subtitles and chapters:

RedSync hybrid --hdr movie_hdr10.mkv --dv movie_dv.mkv

The DV source can be a smaller rip (for example 1080p Dolby Vision onto a 2160p HDR10 base). The RPU is the same metadata at any resolution, and RedSync scales the active-area crop to the base for you.

Turn an HDR10+ file into DV HDR10+ from a single source:

RedSync hybrid --hdr10plus movie_hdr10plus.mkv

Combine a DV source and a separate HDR10+ source. If you have one file with Dolby Vision and a different file with HDR10+, RedSync can graft the HDR10+ dynamic metadata onto the DV video to make a DV HDR10+ hybrid:

RedSync hybrid --dv movie_dv.mkv --hdr10plus movie_hdr10plus.mkv

This keeps the DV file's video and borrows only the HDR10+ metadata, so the DV RPU (and its active-area crop) is preserved untouched. It needs a DV source that already carries an HDR10 base layer - profile 8.1 (or 7). A profile 5 source (typical of iTunes rips) is single-layer DV with no HDR10 base, so its pixels can't stand in for HDR10; RedSync detects that and points you at the other direction instead, which keeps the HDR10+ video and grafts the DV onto it:

RedSync hybrid --hdr movie_hdr10plus.mkv --dv movie_dv.mkv

Both directions produce a DV HDR10+ file - they differ only in which source's video pixels survive.

Want only the elementary stream to mux yourself? Add --hevc-only.

How to sync audio from a different release

RedSync --video keep_this_video.mkv --audio other_release.mkv

RedSync measures the real offset from the audio and applies it. If the two releases run at different frame rates it corrects the drift with a linear factor instead of a single delay that slips over time. Subtitles and chapters sync the same way.

Already know the offset? Skip the measurement and set it yourself with --shift (milliseconds, negative allowed). The value is applied as a constant delay to every source being synced onto the video:

RedSync a.mkv b.mkv --sync --shift -320

Scripting RedSync

Every command is non-interactive when you pass explicit flags, and --json turns the result into a single machine-readable object on stdout while all the decorative output stays on stderr - so a wrapping script can parse stdout cleanly.

RedSync analyze --json *.mkv                       # tracks, fps, HDR/DV per file
RedSync doctor --json                              # tool availability + paths
RedSync hybrid --dv dv.mkv --hdr10plus hdr.mkv --json
RedSync a.mkv b.mkv --sync --json                  # output path, offsets, timings

A run's JSON reports the output filename, the final dynamic-range tag, the delay (and any frame-rate stretch) measured for each source, the exact mkvmerge command, and per-stage timings. Add --dry-run to get all of that without writing anything, or --quiet to keep the pretty output but drop the spinners.

How the sync stays accurate

The offset between two tracks comes from their audio. RedSync decodes a short window from each, builds a log-mel energy envelope, and cross-correlates them with an FFT. The peak is the delay, and how sharp that peak is says how far to trust it.

It checks two points in the runtime. The same delay at both means a constant offset that goes straight into mkvmerge --sync. Different delays mean the sources run at different speeds, so RedSync applies a linear factor from the exact frame-rate ratio and the alignment holds from first frame to last.

How the hybrid crop is automatic

A Dolby Vision display reads the RPU active area to know where the real picture sits. When you put DV metadata from a cropped source onto a letterboxed base, the active area has to describe those black bars, or the display tone-maps the black.

RedSync fits the DV picture inside the base frame, takes the leftover as the bars, splits it evenly, and writes that into the RPU. A 2160p base over a 1608 picture gives (2160 - 1608) / 2 = 276 pixels top and bottom, from the geometry alone. Per-frame active areas in the source RPU are carried through and scaled, so variable-aspect titles keep their changes.

Build from source

You need Go 1.24 or newer.

Install Go on Linux

wget https://go.dev/dl/go1.24.0.linux-amd64.tar.gz
sudo rm -rf /usr/local/go && sudo tar -C /usr/local -xzf go1.24.0.linux-amd64.tar.gz
echo 'export PATH=$PATH:/usr/local/go/bin' >> ~/.profile && source ~/.profile
go version

Install Go on Windows (terminal)

winget install GoLang.Go
go version

Build

git clone https://github.com/720pixel/RedSync
cd RedSync
make build        # native binary
make release      # linux + windows into dist/

Credits

RedSync would not exist without these projects:

• dovi_tool by quietvoid - all Dolby Vision RPU work (extract, edit, inject, generate). Bundled in the binary.
• hdr10plus_tool by quietvoid - HDR10+ metadata extraction. Bundled in the binary.
• audio-offset-finder by the BBC - the audio cross-correlation approach RedSync's offset engine is based on.

Also relies on FFmpeg, MKVToolNix and MediaInfo.

License

MIT, see LICENSE. Bundled tools keep their own licenses, listed in NOTICE.

RedSync · Dolby Vision and HDR hybrid muxing, audio and subtitle sync, for Linux and Windows