Good analysis. The issue with the Black (and this goes for all of them, Hero4 or Hero3) is that despite being able to record at 4k or 80fps, the recording speed of the card is maxed out before this is useful. Therefore the video must be compressed more in order to be a smaller throughput to the card. If this is 4k, you are going to get more pixels, but also more noise and less color depth, if it is 80 fps it is going to be more frames, but again more noise and less color depth ans the compression is cranked up to get the data on the card fast enough.
GoPro Hero 4 Silver vs Black - Hypoxic Comparison
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