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Digital Video Anatomy

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Take a look at YouTube’s recommended upload settings. Do you know what all this means?

Pixel Aspect Ratio vs. Frame Aspect Ratio

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How a Camera Captures Light

Chroma Subsampling

So you can see that there’s already a form of resolution reduction inherent to how the camera captures different colors of light, this is sort of a “chroma subsampling”. However, after the electrical charges from photons striking the sensor are converted to digital values and demosaicing is performed, there’s another form of color compression and that’s what most people refer to when they reference chroma subsampling.

Frame.io has a great visual explanation here.

Edit Lists

“Edit Lists” are a Apple-specific mp4 extensions; they are atoms that basically allow you to pick portions of the video file for playback. You could, for example, play on loop, the middle two seconds of a 15 second video. This isn’t commonly used for most people, but YouTube’s warning to avoid them should make sense now.

Atoms

“Atoms” (or “boxes” in the ISO spec) are data within the video file container that hold specific information about the video file’s parameters. These descriptive atoms differ from the actual media data (the individual frames of video or samples of audio) itself.

Though the atom’s location should be determined at the compression and muxing stage, software does exist to move the “moov” atom’s location after compression has happened. This hierarchical structure of atoms containing data separate from the atoms describing that data is part of what makes editing easy with the Quicktime format. In fact, the descriptive bits and the media bits don’t even have to reside in the same .mov file. Media can be ‘redescribed’ by changing description media atoms rather than having to rewrite the media file.

For example, the most common “moov” atom is sometimes called the “movie atom” and includes information on video length, track count, timescale and compression. Perhaps most importantly, it’s also the index with information about where the actual media file to be played is stored. Within the “moov” atom sits a “trak” sub-atom for each of the movies tracks, and within each trak atom sits a “mdia” atom with even further defined specifics. That moov atom is crucial for the playback of the entire clip, and the end user will not be able to scrub the playhead or jump to a location in the clip without it. For this reason, in some web streaming situations it’s crucial to load it first so you’ll see parameters in encoding software for  “progressive download,” sometimes called “fast start,” or “use streaming mode.” “Muxing” is the term used for merging your video track, audio tracks, and subtitles all into one container.

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