Special HDMI cable for Apple TV? I'm looking to purchase a HDMI cable online for use with my AppleTV. I asked the seller just to be on the safe side, would this cable work with an AppleTV and he said it will not work. 

From searching the Apple site it seems the only thing the HDMI cable needs is HDCP capability which the seller claims the cable has. The cable is also 1.4 with Ethernet capabilities so it seems to be top of the range. 

Would there be any reason why this cable wouldn't work or do I have to purchase a special Apple one?

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There is no reason why it wouldn't work. In fact I use even cheaper (but shorter) cables myself and they work fine. For reference, I paid €0.75 a piece for a couple of 0.75m HDMI cables with a local online cable shop, and they work fine for 1080p video.

I'd advise to not succumb to the "gold or silver plated" marketing nonsense or the overpriced "premium" HDMI cables, as even the cheap cables transport your binary 1s and 0s just fine.