Video & SD Cards - where is the speed bottleneck for video?

Started Apr 11, 2012 | Discussions thread
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Chas Tennis
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Video & SD Cards - where is the speed bottleneck for video?
Apr 11, 2012

Despite discussions indicating the critical nature of the card speed I find that for my high speed video applications so far all cards from class 2(or 4?) to 10 have worked. I'd like to better understand why and which video operations - recording or playback - might require the highest speed performance from the card. (I'm not primarily interested in which card to buy but what is going on.)

I use a Casio Ex FH100 camera. It records high speed video with much reduced resolution. The user's manual indicates that the Approximate Data Rate required for most high speed recording modes is 50 Megabits/second, the max rate listed in the table. HD video requires 30 Megabits/second. The manual indicates that an "Ultra High Speed Type SD memory card is recommended" and mentions that one reason is to avoid dropping frames when recording. I interpreted that Ultra High Speed to mean about a class 10. The camera will indicate when dropping frames and I have never seen that in my high speed video applications. Playback would require less data rate since low resolution frames taken at 240 fps are played back at 30 fps. As one way to communicate videos I bought some cheap 2 Gb cards to loan. These cards seem to work.

Video compression might be important for data rate. ? My understanding of video compression is basically that the camera records one frame and then records the difference between that frame and each of the next so many frames, for example, the next 30 frames (frame count varies for compression method). Then the camera records another reference frame and repeats the process. If a camera were on a tripod it might produce more compressible video than if it were panning. I don't understand video compression that well.............but it seems that the required data rate could vary considerably based on the action in the video. ? Most of my videos are taken on a tripod or steady hand held without panning.

HD video with panning might be the most demanding for card speed?

Anyway, the basic question is why do all cards seem to work for my application and what operation mode or situation requires the highest performance from the card?

Chas Tennis

Casio Exilim EX-FH100
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