Smartphone video cameras have had a long history of struggling with high frame rates. They reached higher pixels per second by increases in video resolution rather than frame rate.
For example, the earliest 4K 2160p 30fps video recording smartphones were released in 2013, starting with the Galaxy Note 3. (Note: The Acer Liquid S2 had only 24fps and the same chipset as the Note 3, but I am taking the Note 3 as example due to my familiarity with the Samsung product lineage).
The same amount of unencoded data per second as 2160p 30fps would be accomplished with 1080p at 120fps, yet the Note 3 and several Samsung smartphones released thereafter could only record 1080p at 60fps. 120fps was only possible at 720p, so some kind of bottleneck prevented them from recording 1080p at 120fps.
The 1080p 60fps limitation stayed all the way until the S9 (2018) suddenly quadrupled the frame rate to 240fps. The 60fps were proably not a processing limitation, because from my testing, pre-2016 Samsung smartphones were perfectly capable of playing back 1080p 120fps and 720p 240fps video with no lag (original speed, not slowed down).
Yes, I know, playing (decoding) takes less processing power than recording (encoding), but I don't see how 1080p 120fps would be more difficult to process than 2160p at 30fps. They are the same number of pixels per second. 248,832,000 to be exact.
So my closest guess is that it has something to do with the image sensor.
From what I understand, video capture resolution is limited by two things: image sensor resolution and frame buffer size.
This means a 4K 120fps capable image sensor can not capture 8K at 30fps (same data rate) if it lacks the number of pixels (7680x4320 minimum), and the device can not encode 8K video if its GPU has too small of a frame buffer size to hold 7680×4320×8×3 bits of data (width × height × bits per pixel × number of color channels, R G B). The frame buffer needs to be at least large enough to hold a single uncompressed frame.
So what is it that bottlenecks the video recording frame rate of smartphone cameras?
Why can't every smartphone, or other video camera for that matter, that can record in 2160p at 30fps also record in 1080p at 120fps? In other words, why can't resolution be proportionally traded in for frame rate?
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