|What is an "ftyp", anyway?|
An ftyp is a four letter code (sometime including
blanks) that is used to identify the "type" of encoding
used, the "compatibility", or the "intended usage" of a
media file. It only pertains to MP4 or newer QuickTime
(.mov) container file formats. It is somewhat analogous
to the so-called
fourcc code, used for a similar purpose for media
embedded in the AVI container format.
What I've left unanswered here, among other things, are some rather basic questions, some of which I'm not 100% clear on the answers to:
1. Should the "major brand" be duplicated as a "compatible brand"? That would certainly always appear to be true, but isn't that a bit redundant? Seems like, in practice, sometimes this is done and sometimes it isn't.
2. Should an file list as a "compatible brand" a specification for a player which can only perform a "subset" of the features of the major brand? Apparently the answer is yes. Which makes a certain amount of sense. For example, if the major brand was a spec which included the ability to "skip chapters" while watching a video, one certainly might consider it useful to know that the file will also play fine on a slightly more basic player, one that has the ability to do everything except "skip chapters". But certain more extreme examples make one wonder. Apple lists "M4A " as a compatible subtype of "M4V ", which is to say "sure, you can play this video on your 'audio-only' iPod - but don't expect to see any video" ;) That particular fact seems somewhat less than useful, in my own opinion, though I suppose the fact that you could just listen to the audio track of a video file is of some, albeit rather limited, use. 
3. Should an file list as a "compatible brand" a specification which is a "superset" of the major brand? In this case the media file will play properly with all features intact. But it's neither practical nor required to list all of the more "advanced" specifications for this "simple file". Indeed, this would be impossible as more advanced specifications that include all of the features of previous specs are being created all the time. So such a list would never be guaranteed to be up to date. In this case it seems it's the media player's responsibility to recognize that the file's "main brand", or at least one of the compatible ones, is one of the many types that it "knows" how to handle.
 According to this 3GPP specification document (and I'm sure ISO/IEC says this too, but I find the 3GPP spec more readable on this subject):
"... [the inclusion of a brand as compatible indicates] that the file conforms to the specification; it includes everything required by, and nothing contrary to the specification (though there may be other material)" [emphasis mine].
So that would make Apple's decision to include "M4A "as a "compatible brand" for the "M4V " major brand technically correct. The video file indeed includes everything required by (and apparently nothing contrary to) Apple's audio player. It's just that the file contains "extra material" - namely the video content!
| Anyway, this page represents my basic take on this from whole subject from what I've seen and read so far. Please feel free to email me with any comment, suggestions and especially additions and/or corrections. Use the email address displayed at the bottom of the main page.|
- Steve G
Last Update: 12 December 2006
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