SDHC HD video cards: do we need them?
Specifically for video? How can you designed a flash card specifically for high definition video?
Apparently, you can, according to Transcend. The new SDHC HD cards are Class 6, which means minimum write speeds of 6MB/second, which is fine but when you look at the supporting information that comes with the announcement, you actually find you don’t need anything like this sort of speed anyway.
Using Transcend’s example, the highest capture rate even for 1080p (1920×1080-pixel) video captured in H.264 at 30 frames per second (30fps) is 13Mbps, which is only 1.625MB/second so having 6MB/second write speed is really a bit of a waste. More important would be fast read times because you’re going to have to get the video off the card and onto your PC’s hard drive for editing.
So, really, if you’re thinking of buying an HD video camera that captures video to SDHC flash memory, you don’t need a high-speed Class 6 card to capture it with – you could easily get away with cheaper Class 4 (4MB/second) or even Class 2 (2MB/second) cards. The reason is that the maximum data recording rate (as mentioned by Transcend) is only 1.6MB/second – so anything faster than Class 2 won’t make any difference to recording video.
And that’s at the maximum recording rate – if you choose a lower quality but longer runtime recording rate, the actual recording rate is even lower, making a Class 6 card even less necessary.
It might however make a difference to getting that video off the card as the higher the class, the faster the read speeds of that card should be too but also, the higher the cost.
Basically, it pays to know the maximum data transfer rate of your device, then purchase a flash card on that basis – not always on raw speed.
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