victor basumatary
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Differences Between MP4 & QuickTime

I’ve been working on Xray and I recently found out that the MP4 spec is not exactly the same as the QuickTime spec. Well duh, why would you have a different spec if they’re exactly the same, right? For whatever reason I expected them to only differ in atoms not already defined by the QuickTime spec but what do you know, there are several boxes in the MP4 spec that are different to the atoms in the QuickTime spec!

Just in case you’re confused, “atoms” and “boxes” mean the same thing. The QuickTime spec calls them atoms and the MP4 spec calls them boxes.

This blog post will be updated whenever I find any differences.

meta

This is the first box that I found to be different. While working on Xray I was testing the QuickTime renderer on .mp4 and .mov files and noticed that the .mov file wasn’t being read correctly. It was obvious that either too many or too few bytes were being read somewhere. After a few hours of thinking and a few minutes of refactoring the parser now could tell me where the misread was happening. And lo and behold, it was the meta atom.

The QuickTime spec defines the meta atom as a regular atom but the MP4 spec defines the meta box as a full box. The difference is just 4 bytes; the version and flags fields.

mvhd

The movie header box is also slightly different. MP4 uses 64 bits instead of 32 for the creation time, modification time and duration fields ONLY if version is 1.

keys

The keys atom is a full atom but the keys box is a regular box with no version and flags fields.

elst

In MP4 files the edit list entries use 64 bits for the edit duration and media time fields if version is 1 and 32 bits if version is 0.

Thank you for reading so far. Once again, the contents of this website is fictional. Take care.