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Which SD cards to use and how much video can they hold, please check my maths


funkytwig
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I has just got a GH3 and in the manual is gives MBPS (mega-bits per second) as the data rate for the various recording formats but does not say how much video can be recorder per MB. I have therefore tried to work out the MB/s for these formats and how much video they can record on a 32GB card. I am doing this for 50 MBPS (MOV FHD/50P) but the same logic can be applied to other data-rates.

  1. First I need to convert MBPS to MB/s, I believe I have to divide MBPS by 8 to get this.
  2. Then I need to know how many MB there are in 32GB which is 32*1024 = 32768
  3. Then I need to work out how many seconds the card will record ( 32768 / MB/s )I then divide by 60 to get how many minutes the card records.

So here is the calculation

  1. 50 MBPS / 8 = 6.25 MB/s (I.e. any class 10 card should be fast enough)
  2. 32768 / 6.25 = 5242.88 seconds / 60 = 87.38 minutes
  3. So a 32GB card can record 87.38 minutes of video at a data-rate of 50 MBPS.

(I did the same calculation for 28 MBPS (AVCHD 50P) and got 156.04 minutes)

 

Does this sound correct?

 

Ben

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i know with my gh2 i get ~60min on a 64gb card. but that is also hacked. on the gh1, hacked, i could get ~2hours on a 32gb card. and the data-rate the gh3 uses, i'm 99% sure is a variable bit rate, so it's only going to be hitting 50mbps in super high detail, well lit scenes. the rest of the time it'll probably much lower. so i'd say your math is pretty close. i just did it for a 176Mbps patch with a 64gb card and came up with 49.65 minutes. which seems about right, because that is assuming a cbr of 176Mbps.

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If it's VBR very dark scenes where you've used high ISO will also crank up the rate. Encoding all the dancing ISO noise takes a lot of data. The higher the bitrate, the more distinct each noise grain and the easier it is to denoise

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