Containers are files that wrap around video and audio tracks—indexing and organizing the streams for playback—in addition to providing advanced features, like chapters similar to those on DVDs.
By default HandBrake saves all files in the MP4 container, which is both widely compatible and offers many advanced features. MP4 is the native file format for a number of consumer devices (iPod, PSP, Apple TV) and is supported out of the box by QuickTime and iTunes. If you are encoding for playback in any of those situations, it is recommended.
If you need features not found in MP4, HandBrake can also save files in the widely popular MKV container.
Different container formats allow for a wide combination of audio and video tracks, with some also allowing for the inclusion of chapter, subtitle, and advanced playback flags to alter how a video is displayed. A simple comparison of the container formats that HandBrake supports is included below. A more complete description can be found at Wikipedia:Container Format (digital).
- In an MP4, you can store MPEG-4 video created by ffmpeg or x264.
- It stores audio in the AAC format. It is also possible to pass through Dolby Digital 5.1 Surround Sound (AC3) audio from a DVD in an MP4 file, although it is a new standard and not widely supported. Perian and VLC can decode it, and the AppleTV can pass it through to a surround sound receiver.
- MP4 also supports chapters, for which HandBrake uses Apple's chapter format.
- MP4 also is capable of storing Variable Frame Rate video.
- Be aware that chapters and AC3 audio can only be seen by QuickTime if you change the file extension from .mp4 to .m4v. HandBrake will do this for you automatically when you enable those features, although this can be disabled in the preferences if you really want .mp4 at the end of the name.
- The MP4 format can also be optimized for "fast start" progressive downloads over the Web.
- It can also include "soft" text subtitles that can be turned on or off, instead of always being hard burned into the video frames.
- In an MKV, you can store MPEG-4 video created by ffmpeg or x264, or Theora video.
- It stores audio in the AAC, MP3, or Vorbis formats. It can also pass through the Dolby Digital 5.1 (AC3) and Digital Theater Systems (DTS) surround sound formats used by DVDs.
- It supports chapters, as well as Variable Frame Rate video.
- It can include "soft" subtitles that can be turned on or off, instead of always being hard burned into the video frame. These can either be bitmap images of the subtitles included on a DVD (known as vobsub) or text.
MP4 vs M4v
They are the exact same file, the only difference is the extension. MP4 vs M4v.
For MP4 files, HandBrake will automatically use the extension M4V when you pass-through audio (AC3), use SRT subtitles or have chapter markers enabled.
You can simply change the file extension between .mp4 and .m4v as the file is exactly the same. There are no differences in the content or container itself.
You can disable the option to automatically set this in HandBrakes preferences.
Note: QuickTime or any QT based application will not be able to play a MP4 file with these features enabled, unless it has the extension ".m4v"