Why is the audio out of sync after converting?

Common causes: (1) variable framerate source rendered as constant (use -vsync vfr to preserve VFR); (2) different audio sample rates not resampled (add -ar 48000 ); (3) container limitations (MP4 with variable framerate misbehaves — prefer MKV during editing, encode to MP4 only at the end). Always run ffprobe on both source and output to compare timing.

Why is the audio out of sync after converting?

Common causes: (1) variable framerate source rendered as constant (use -vsync vfr to preserve VFR); (2) different audio sample rates not resampled (add -ar 48000 ); (3) container limitations (MP4 with variable framerate misbehaves — prefer MKV during editing, encode to MP4 only at the end). Always run ffprobe on both source and output to compare timing.

Guide

MKV (Matroska) Video Container: Complete Format Guide

PC By Pablo Cirre • Updated May 7, 2026

MKV (Matroska) Video Container Format — Complete Guide

What Is MKV?

MKV (Matroska Video) is an open, royalty-free multimedia container format developed by the Matroska project, initially released in December 2002. It is based on the Extensible Binary Meta Language (EBML), a binary derivative of XML that provides flexible, forward-compatible structure. MKV can hold virtually unlimited video streams, audio tracks, subtitle tracks, chapters, metadata, and file attachments in a single self-contained file.

Unlike proprietary containers such as AVI (limited to two streams) or MP4 (governed by ISO/MPEG standards with codec restrictions), Matroska imposes no codec constraints and is maintained by an independent non-profit organisation, making it the format of choice for archivists, home-media enthusiasts, and fan-dubbing communities worldwide.

EBML: The Foundation of Matroska

EBML (Extensible Binary Meta Language) gives Matroska its flexibility. Every piece of data in an MKV file is an Element consisting of:

Element ID — variable-length integer identifying the element type
Data Size — variable-length integer giving the byte length
Data — raw content (binary, UTF-8 string, integer, float, or nested elements)

The top-level Matroska structure:

EBML Header
Segment
  ├─ SeekHead          (index of top-level elements)
  ├─ Info              (title, duration, timecode scale, muxing app)
  ├─ Tracks            (video/audio/subtitle track definitions)
  ├─ Chapters          (chapter markers)
  ├─ Tags              (metadata key-value pairs)
  ├─ Attachments       (embedded files — fonts, images)
  └─ Cluster(s)        (actual media data, in time-ordered blocks)

This hierarchical design means readers can skip unknown elements gracefully — future extensions never break existing parsers.

Video Codec Support

MKV is codec-agnostic. Any video codec can be encapsulated; commonly used ones include:

Modern High-Efficiency Codecs

H.265/HEVC — 2× the compression of H.264; dominant in 4K Blu-ray rips and HDR encodes
AV1 — royalty-free, ~30 % more efficient than H.265; used in next-generation streaming and archival
VP9 — Google's open codec; used in YouTube 4K; moderate hardware support

Mainstream Codecs

H.264/AVC — universal hardware support; dominant in internet video; baseline for most devices
VP8 — predecessor to VP9; rarely used in new MKV files

Professional and Legacy Codecs

FFV1 — lossless video codec; used by national archives and film preservation projects
HuffYUV / Lagarith — near-lossless codecs for intermediate editing
MPEG-2 — legacy codec; DVD-rip sources occasionally stored in MKV

Audio Codec Support

Lossless Audio

FLAC — Free Lossless Audio Codec; 40–60 % smaller than raw PCM; popular for archival
TrueHD — Dolby lossless codec; used on Blu-ray; bit-perfect preservation of surround mixes
DTS-HD MA — DTS lossless variant; cinema-standard multi-channel audio
PCM (raw) — uncompressed; maximum quality; large file size

High-Quality Lossy Audio

AAC — standard for mobile and streaming; efficient at 128–256 kbps; wide hardware support
Opus — modern open codec; excellent at low bitrates (32–128 kbps); dominates VoIP and podcasting
Vorbis — open-source predecessor to Opus; still widely used in MKV files
AC-3 (Dolby Digital) — 5.1 cinema audio; 192–640 kbps; broad device support
E-AC-3 (Dolby Digital Plus) — enhanced AC-3; higher bitrate and efficiency; used on streaming
DTS — cinema lossy surround; universal AV receiver support

Subtitle Format Support

Matroska's subtitle system is one of its greatest strengths:

Text-Based Subtitles

SubRip (.srt) — plain timing + text; most common format; stored as UTF-8 in MKV
SubStation Alpha / Advanced SubStation Alpha (.ssa/.ass) — rich styling (fonts, colours, outlines, positioning, karaoke effects, per-character animation); dominant in anime fansubs
WebVTT — web standard; stored natively in MKV for future-compatible streaming
HDMV TextST — Blu-ray text subtitles; rarely used in MKV

Bitmap-Based Subtitles

VobSub (.idx/.sub) — DVD bitmap subtitles extracted as image frames; pixel-perfect but large
PGS (Presentation Graphics Stream) — Blu-ray bitmap subtitles; high fidelity; ~5–15 MB per hour

Embedded Fonts for ASS/SSA

MKV's Attachments section holds TrueType (.ttf) and OpenType (.otf) font files. When a player (VLC, mpv, MPC-HC) encounters an MKV with embedded fonts, it extracts them to a temporary directory and uses them for subtitle rendering — guaranteeing visual fidelity regardless of the system's installed fonts.

Chapter Support

Chapters in MKV work similarly to DVD chapter markers:

Chapter 1 "Opening Titles"    00:00:00.000
Chapter 2 "Act One"           00:03:45.200
Chapter 3 "Intermission"      00:58:12.000
Chapter 4 "Act Two"           01:00:00.000
Chapter 5 "Closing Credits"   01:52:40.500

Chapters can be ordered (the player jumps between non-contiguous segments — enabling branching or seamless looping) or unordered (standard linear navigation). This enables DVD-like menu-less chapter selection in a single flat file.

Metadata and Tags

MKV's Tags element supports arbitrary key-value metadata at segment, track, chapter, or attachment level. Standard Matroska tags include:

TITLE, DATE_RELEASE, DIRECTOR, ACTOR, GENRE, COMMENT, DESCRIPTION
TOTAL_PARTS / PART_NUMBER — season/episode numbering for TV series
BPS, DURATION, NUMBER_OF_FRAMES — technical track statistics
RATING, CONTENT_TYPE, LANGUAGE — catalogue information

Cover art (poster images) are stored as attachments with the MIME type image/jpeg or image/png and the filename cover.jpg / cover.png.

Advantages of MKV

Open and royalty-free — no licensing fees; anyone can implement a parser or muxer
Unlimited tracks — no practical limit on video, audio, subtitle, or attachment streams
Lossless audio — FLAC and TrueHD preserve bit-perfect audio fidelity
Advanced subtitle styling — ASS/SSA + embedded fonts for professional-grade subtitles
Future-proof — unknown elements are safely ignored by older parsers; format extensions don't break compatibility
Self-contained — chapters, metadata, subtitles, and fonts all travel with the video in one file
Error recovery — Cluster-level structure allows partial playback of damaged or incomplete files

Disadvantages of MKV

Limited hardware support — most smart TVs, Blu-ray players, gaming consoles, and media streamers cannot play MKV natively; conversion to MP4 is often needed
No browser support — HTML5 video does not accept MKV; web publishing requires MP4 or WebM
No streaming optimisation — MP4's moov atom can be placed at the file start for HTTP pseudo-streaming; MKV has no equivalent mechanism for progressive web download
Larger overhead — EBML element wrapping adds a small size penalty (~0.1–0.5 %) versus optimised MP4 containers
Encoding speed — multi-track muxing with lossless audio is slower than single-track MP4 encoding

Device and Software Compatibility

Environment	Status
VLC (all platforms)	Full native support
mpv	Full native support
Windows Media Player (Win 11)	Requires codec pack or MKV extension
Plex / Kodi / Jellyfin	Full; transcodes for incompatible clients
Smart TVs (LG, Samsung, Sony)	Partial — depends on embedded codec and track count
Apple TV (tvOS)	Via Infuse or Plex only
Roku / Fire TV	Via Plex or VLC app; not native
PlayStation 5	Limited — some MKV files play, many do not
Xbox Series X	Partial; H.264 + AAC MKV usually works
iOS / iPadOS	Via VLC, Infuse, or nPlayer apps
Android	Via VLC, MX Player, or Kodi

MKV vs. MP4: When to Choose Each

Choose MKV when:

Archiving a film or TV series with multiple audio languages and subtitle tracks
Lossless audio quality (FLAC, TrueHD) is required
Rich subtitle styling with embedded fonts is needed (anime, fansubs)
Long-term preservation where codec flexibility matters

Choose MP4 when:

Universal device compatibility is the priority
Uploading to YouTube, Vimeo, or any streaming platform
Playing on smart TVs, gaming consoles, or mobile without extra apps
Sharing with general audiences unfamiliar with media players

Converting MKV Files

Converting MKV to MP4 is lossless for the video stream (stream copy) when both containers support the same codec (e.g., H.264 or H.265). Audio tracks are copied directly if they are AAC or MP3; AC-3, FLAC, and TrueHD typically require re-encoding to AAC for MP4 compatibility. KaijuConverter handles MKV input natively, selecting the optimal output profile automatically.

Use Cases

Home Media Library — Plex and Kodi users store rips of purchased Blu-rays in MKV with original H.265 video, DTS-HD MA audio, and multiple subtitle languages. One self-contained file replaces a physical disc.

Anime Fansubs — Fansubbing groups release MKV files with ASS subtitles featuring custom fonts, karaoke effects, and per-character styling impossible in any other widely-used format.

Film Preservation — Archivists use MKV + FFV1 (lossless video) + FLAC (lossless audio) for digital film preservation. FLAC reduces audio storage by 50–60 % versus raw PCM while maintaining mathematical bit-for-bit accuracy.

Educational Content — University lecture recordings archived in MKV bundle multiple language dubs and accessibility subtitle tracks. A single 2 GB file replaces five separate per-language MP4 files.

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Frequently Asked Questions

Most smart TVs have limited MKV support. LG and Samsung TVs often play simple MKV files (H.264 + AAC) but fail on H.265, Opus, FLAC, or multi-track files. The reliable solution is a media server (Plex, Jellyfin) that transcodes MKV on-the-fly, or converting the file to MP4 with H.264 beforehand.

Most smart TVs have limited MKV support. LG e Samsung TVs often reproduzir simples MKV arquivos (H.264 + AAC) mas fail on H.265, Opus, FLAC, ou multi-track files. The reliable solution is a media server (Plex, Jellyfin) that transcodes MKV on-the-fly, ou convertendo o arquivo to MP4 com H.264 beforehand.

Most Smart-TVs have limited MKV support. LG und Samsung TVs often abspielen einfach MKV Dateien (H.264 + AAC) aber fail on H.265, Opus, FLAC, oder multi-track files. The reliable solution is a media server (Plex, Jellyfin) that transcodes MKV on-the-fly, oder umwandelnd die Datei to MP4 mit H.264 beforehand.

Most smart TVs have limited MKV support. LG y Samsung TVs often reproducir simple MKV archivos (H.264 + AAC) pero fail on H.265, Opus, FLAC, o multi-track files. The reliable solution is a media server (Plex, Jellyfin) that transcodes MKV on-the-fly, o convirtiendo el archivo to MP4 con H.264 beforehand.

AV1 is the most efficient (royalty-free, ~30% smaller than H.265) but encoding is slow. H.265 (HEVC) saves ~30–50% over H.264 and is supported by every modern phone and desktop. H.264 remains the safest baseline for legacy compatibility. Rule of thumb: archives → AV1, daily use → H.265, broadest reach → H.264.

Not if you use stream copy (remuxing). A stream copy simply moves the compressed video and audio data from the MKV wrapper into an MP4 wrapper without re-encoding — zero quality loss, near-instant processing. Quality loss only occurs if re-encoding is needed (e.g., converting FLAC audio to AAC for MP4 compatibility).

Not if you usar stream copy (remuxing). A stream copy simply moves the comprimido video e audio data de the MKV wrapper em an MP4 wrapper sem re-encoding — zero quality loss, near-instant processing. Quality loss only occurs if re-encoding is needed (e.g., convertendo FLAC audio to AAC para MP4 compatibilidade).

Not if you verwenden stream copy (remuxing). A stream copy simply moves the komprimiert video und audio data von the MKV wrapper in an MP4 wrapper ohne re-encoding — zero quality loss, near-instant processing. Quality loss only occurs if re-encoding is needed (e.g., umwandelnd FLAC audio to AAC für MP4 Kompatibilität).

Not if you usar stream copy (remuxing). A stream copy simply moves the comprimido video y audio data de the MKV wrapper en an MP4 wrapper sin re-encoding — zero quality loss, near-instant processing. Quality loss only occurs if re-encoding is needed (e.g., convirtiendo FLAC audio to AAC para MP4 compatibilidad).

CRF (Constant Rate Factor) is the best default for offline files: ffmpeg picks the bitrate frame-by-frame to maintain perceived quality. Two-pass is only better when you must hit an exact final size (DVD targets). Constant bitrate is for streaming with a fixed channel. For "smallest at quality X" always use CRF.

Use mkvmerge (part of MKVToolNix, free) to mux an existing .srt or .ass file into your MKV. Drag the MKV and subtitle file into mkvmerge, ensure both tracks are selected, and click Start Muxing. The output MKV contains both video and embedded subtitles.

Common causes: (1) variable framerate source rendered as constant (use <code>-vsync vfr</code> to preserve VFR); (2) different audio sample rates not resampled (add <code>-ar 48000</code>); (3) container limitations (MP4 with variable framerate misbehaves — prefer MKV during editing, encode to MP4 only at the end). Always run <code>ffprobe</code> on both source and output to compare timing.

The most common reason is lossless audio: if your MKV contains FLAC or TrueHD audio, the audio track alone can be 300–800 MB per hour. Converting audio to AAC 192 kbps and remuxing into MP4 typically reduces file size by 20–40 % with imperceptible quality loss for most viewers.

The most common reason is sem perdas audio: if your MKV contém FLAC ou TrueHD audio, the audio track alone can be 300–800 MB per hour. convertendo audio to AAC 192 kbps e remuxing em MP4 tipicamente reduces tamanho do arquivo by 20–40 % com imperceptible quality loss para most viewers.

The most common reason is verlustfrei audio: if your MKV contains FLAC oder TrueHD audio, the audio track alone can be 300–800 MB per hour. umwandelnd audio to AAC 192 kbps und remuxing in MP4 typically reduces Dateigröße by 20–40 % mit imperceptible quality loss für most viewers.

The most common reason is sin pérdidas audio: if your MKV contains FLAC o TrueHD audio, the audio track alone can be 300–800 MB per hour. convirtiendo audio to AAC 192 kbps y remuxing en MP4 typically reduces tamaño de archivo by 20–40 % con imperceptible quality loss para most viewers.

Yes if you only change the container: <code>ffmpeg -i in.mkv -c copy out.mp4</code>. This remuxes the stream without re-encoding, takes seconds even for hours of footage. Limitations: codec must be supported by the target container (e.g. you cannot put H.264 in WebM, only VP8/VP9/AV1). To shrink size you must re-encode.