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I wonder what's the smallest component of a video file. I know, in graphics, it's the pixel, in sound,, it's the sample. I also know video is made by frames + audio, so, is it pixels + samples?

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It's a matter of semantics. Both audio and video are expressed as samples, where a continuous analog signal is encoded into discrete chunks. These chunks are expressed as numbers, and the numbers are composed of bytes, and the bytes are composed of bits. But saying that doesn't really give you any deeper understanding of the process.

Audio is encoded in samples where each sample has a magnitude, representing the sound pressure on the microphone at a particular time.

Video is also encoded as samples, where each sample is a representation of the amount of light that falls on a sensor in a certain period. Due to the fact that light is coloured, each sample is usually composed of three components, representing the three visible primaries, red, green and blue (often encoded as YUV, but that's beyond the scope here).

Unlike sound, more than one sample is recorded at a time; an array of them (a grid, aka a "raster") makes up a frame, which is the same as the picture a stills camera would record if it was fired at that particular time. By taking successive still pictures the camera is able to create the illusion of movement due to the persistence of vision. All of the pixels in each picture are recorded at (more or less, search for "rolling shutter" and "interlacing" if you want to dig deeper) the same time.

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