MKV Subtitles & Audio Tracks:
Complete Guide

MKV's Killer Feature: Unlimited Tracks

Unlike MP4, which has practical limitations on track count and format support, MKV (Matroska) was designed from the ground up to handle complex media files:

• Multiple audio tracks: English, Japanese, Spanish, commentary — all in one file with language tags
• Multiple subtitle tracks: Different languages and formats, each toggleable by the player
• Chapter markers: Named chapters with nested editions for scene navigation
• Attachments: Fonts (for styled subtitles), cover art, and other embedded files

This is why MKV is the preferred format for movie collections, anime libraries, and media servers like Plex and Kodi. A single MKV file can be a complete package with every language option included.

Subtitle Formats in MKV

SRT — Simple Text Subtitles

SubRip Text (.srt) is the most basic and widely compatible subtitle format:

• Plain text with timestamps — no styling, colors, or positioning
• Universal player support (VLC, Plex, Kodi, most smart TVs)
• Small file size (a few KB for a full movie)
• Can be extracted from MKV and used as a sidecar file with MP4

SSA/ASS — Styled Subtitles

SubStation Alpha (.ssa) and Advanced SubStation Alpha (.ass) are rich subtitle formats:

• Styling: Custom fonts, colors, bold, italic, outlines, shadows
• Positioning: Subtitles can appear anywhere on screen (top, bottom, sides, overlapping)
• Animation: Fade in/out, movement, karaoke effects
• Font embedding: MKV can embed the required fonts as attachments, ensuring styled subtitles display correctly on any system
• Primary use: Anime fansubs, where styled subtitles are an art form with elaborate typesetting

ASS subtitles can only be fully preserved by burning them into the video during conversion. Converting ASS to SRT strips all styling, leaving only plain text with basic timing.

PGS — Blu-ray Bitmap Subtitles

Presentation Graphic Stream (.pgs) is the subtitle format used on Blu-ray discs:

• Image-based: Each subtitle is a pre-rendered bitmap image, not text
• Cannot be searched or edited: Since they're images, not text, you can't search, copy, or modify them
• Large file size: A full movie's PGS subtitles can be 20–40 MB
• Preservation: Can only be burned into the video or kept in MKV format. MP4 does not support PGS

VobSub — DVD Bitmap Subtitles

The older equivalent of PGS, used on DVDs. Also bitmap-based (image subtitles) with the same limitations: cannot be converted to text, must be burned in or kept in MKV.

Audio Tracks in MKV

MKV supports virtually any audio codec with unlimited tracks:

A typical movie MKV might contain: English DTS-HD 7.1 (default), English AC-3 5.1 (compatibility), Japanese AAC stereo, plus director's commentary in AAC stereo — all in one file.

What Happens When Converting MKV to MP4

MP4 has significant limitations compared to MKV for multi-track content:

Audio Track Selection

Most converters (including ours) keep only the default audio track. If your MKV has 4 audio tracks, only one survives the conversion. The default track is typically the first one or the one flagged as "default" in the MKV metadata.

Subtitle Handling

• Text subtitles (SRT): Can be embedded as MP4 mov_text (limited player support) or extracted as a separate .srt sidecar file
• Styled subtitles (ASS/SSA): Cannot be embedded in MP4 with styling intact. Options: burn into video (permanent, styled) or convert to SRT (loses all styling)
• Bitmap subtitles (PGS/VobSub): Cannot be embedded in MP4 at all. Must be burned into the video or discarded

Chapter Markers

MP4 supports basic chapter markers, but the nested editions and named chapters that MKV provides may be simplified or lost during conversion.

How to Preserve Subtitles During Conversion

Burn-In (Hardcoded)

Rendering subtitles permanently into the video image. The subtitles become part of every frame:

• Pros: Works on every device, preserves ASS/SSA styling, no compatibility issues
• Cons: Cannot be turned off, video must be re-encoded, one language only
• Best for: Anime with styled ASS subtitles, sharing with specific audience

SRT Sidecar File

Extracting SRT subtitles as a separate .srt file that sits alongside the .mp4:

• Pros: Can be toggled on/off in most players, multiple languages possible (one file each)
• Cons: Two files to manage, some players don't detect sidecar files automatically
• Best for: Text-only subtitles that need to be toggleable

Embedded mov_text

MP4's native subtitle format (tx3g/mov_text):

• Pros: Single file, toggleable in some players
• Cons: Plain text only (no styling), limited player support, often fails on smart TVs
• Best for: Simple text subtitles when a single file is required

Anime-Specific: SSA/ASS Styled Subtitles

Anime fansubs are a unique case because ASS/SSA subtitles are an integral part of the viewing experience. They include:

• Karaoke effects for opening/ending songs
• Typesetting that matches on-screen signs and text
• Custom fonts, colors, and positioning for each character
• Translation notes that appear in specific screen regions

When converting anime MKV to MP4, burning in the ASS subtitles is the only way to preserve this styling. Converting to SRT destroys all typesetting and reduces the subtitles to plain white text at the bottom of the screen.

Tools for Managing MKV Tracks

• MKVToolNix: The definitive GUI tool for MKV manipulation. Add, remove, reorder, and extract tracks. Free for Windows, Mac, and Linux.
• FFprobe: Command-line tool to identify all tracks: ffprobe -v error -show_entries stream=index,codec_type,codec_name -of csv=p=0 input.mkv
• VLC: View track information via Tools > Codec Information, switch audio/subtitle tracks during playback.