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Old Apr 29, 2009, 10:29 PM   #1
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Hi everyone,

I've visited this site a number of times and find it very helpfulwith friendly people. I am fairly novice at photography, but excited to learn more. I am looking to get a decent point &shoot that will also do HD video (looking at Canon SX200 IS and others). The question I can not seem to find an answer to regards the speed of the SD card required to capture HD video without dropping frame rates.

I have a Transcend TS4GSD150 SD card that has a Read / Write speed of 18/17 MB/s, but I have no idea if it is fast enough to capture 1280 x 720 x 30fps movies without dropouts. Is there a way to know (other than experimentation) how fast of a card you need to keep up with this fast video stream? Does the camera buffer come into play or is it strictly a card speed issue? How fast of a card do you need to do 1920 x 1080 x 30fps video? I know you will not have much HD video recording time on an SD card, but I find it very convenient to have one camera instead of 2.

Thanks for your help!
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Old Apr 30, 2009, 8:41 AM   #2
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That card is someone of an "oddball" in that it's not SDHC (or if it is, they didn't market it that way).

The original Secure Digital Card specification only allowed for Secure Digital cards up to 1GB in size. The spec was later extended to allow larger cards. But, some manufacturers used their own methods to try and make larger cards that can give you some compatibility issues with some devices. See this section of a Wikipedia page about it:

http://en.wikipedia.org/wiki/Secure_...d_larger_cards

You've got a variety of factors that influence write speed to media, including the speed a camera is capable of writing to a card (sometimes the bottleneck is the camera's electronics versus the card speed). I also wouldn't assume that card is as fast as you think it is either (manufacturer claims are under optimum conditions, using optimum block sizes, etc., and write speed is usually much slower than read speed with flash media).

If you did run into any compatibility issues with it (and you may not), flash memory cards are very inexpensive now. I'd suggest sticking with a Class 6 SDHC card from a major manufacturer (for example, Sandisk Extreme III cards are very popular).

In the Transcend lineup, you can get a 4GB Class 6 SDHC card for around $10 now (and one including an SDHC card reader is only a few bucks more). Here are some listings for Transcend SDHC cards at my favorite vendor for memory cards:

Transcend SDHC Cards at newegg.com

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Old Apr 30, 2009, 10:21 PM   #3
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Thanks much for the thoughtful reply. Isn't it possible to compute the theoretical write speed needed to record HD video. I don't know all the math, but if you are capturing each frame at 1280 x 720 pixels then multiply by 30 frames per second you have 27,648,000 pixels per second. I'm not sure how that translates to MB/s, but it would seem to me that you could figure out the write speed required for recording this data assuming no bottlenecks anywhere. You could then select the card you want with a better idea if it will perform up to your expectations. Does that make sense?
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Old May 1, 2009, 4:13 AM   #4
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The type if video files being used and compression being used comes into it. Not all processes are conducted in parallel with some cameras either (so, some cameras may need faster cards so that they can write some of their buffer contents to media before doing other tasks, then come back and empty more of the contents to media and repeat).

You have to take each camera on a case by case basis. But, a Class 6 SDHC card is usually fast enough to prevent a full buffer with most newer cameras that are capable of recording longer video clips (keeping in mind that you'll have speed differences between Class 6 Cards, too). Manufacturers sometime publish a list of tested cards for a given camera model, too.
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Old May 2, 2009, 8:43 AM   #5
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Thanks JimC. I was hoping it would be a little more cut and dried, but I appreciate knowing the truth of the matter.
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