SD cards

Posted on 5 November 2012

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The GH2 supports SD, SDHC and SDXC cards at a maximum Highspeed busrate of (50Mhz, 4bit-bus = ) 25MB/s. To take advantage of this and to support the highest bitrate patches the general advise is to buy Sandisk cards.

Sandisk Extreme 45MB/s

I am using the Sandisk Extreme SDHC 45MB/s 32 GB cards (not the Extreme PRO 45 cards!!!), which is recommended for the patches. SDHC is available in sizes from 4 up to 32 GB and might not be read by SDSC readers/camera’s because it uses FAT32 instead of FAT16. It is fine in my camera.

The package of this card states a few speeds:

  • It is class 10, meaning it has a minimum non-fragmented sequential write speed of 10MB/s. There also is a class 16 defined, but It seems like no card manufacturer is using a higher number then class 10, which I think is stupid, because it is the only reliable number on the card!
  • It is UHS speed class 1 (the logo looks like U with the 1 in it) meaning exactly the same: it has a minimum speed of 10 MB/s.
  • It is UHS-I (the capital i), meaning it supports a clock frequency of 100 MHz (a quadrupling of the original Default Speed), which in four-bit transfer mode could transfer 50 MB/s. There is also UHS104 (208MHz clock, up to 104MB/s) and UHS-II (theoretical maximum of 312 MB/s, more pins).
  • It is 300x. This means: up to 300x 150kB/s (cd-rom) read speed in optimal condition.
  • It is 45MB/s maximum read speed, measured by SanDisk itself, in optimal condition.

Conclusion: you should be able to get a minimum 10MB/s = 80mbps read and write speeds without any problem. But: this also depends on the size of the packets, most cards seem to hold up pretty good until the packets become as small as 64KB or smaller. This can be really well tested with http://usbflashspeed.com/. This seems to be the problem when spanning.

Don’t take every number as a given, as this test shows the minimum write speed might even drop far under 10MB/s with some cards, like the SanDisk Extreme Pro 45 UHS-1 with only 0,8MB/s minimum write speed. But the good thing here is: I can’t find a class-10 sign on this card, maybe this is for a reason! This minimum write speed might not be as important as avarage speed when filming (because the buffer of the camera will take care of short speed-drops). But it is when spanning between files, maybe because then the buffer can’t be used for a short period of time.

In order to import the cards at the full 45MB/s peak-potential of this card, you will need at least a UHS-I compatible cardreader (in other words: a reader that supports FAT32, a 100MHz clock and four-bit mode). This is defined in spec 3.01 (May 18, 2010), so if you have an older reader (non-UHS-1) you might be lucky and get 50 Mhz * 4 busses = 25 MB/s (highspeed), 25 Mhz * 4 busses = 12,5 MB/s, or you might be stuck to 25MHz x 1 bit bus / 8 bit = 3MB/s (do these cards even work with 1-bus, or do the need 4 bit bus?), or if your reader is really old, you might only be able to read SDSC cards (Fat-16) at even lower speeds.

Read test

If I connect my GH2 with the USB cable to my computer it transfers about 10 MB/s to my harddrive. This might be the USB bus of the camera, or it might be that the camera only reads at Normal Speed (25 MHz * 4 busses =  12,5 MB/s). This combined wit the class 10 minimum read speed might give a 10 MB/s avarage transfer speed. At this speed it takes about an hour to transfer one 32 GB SD-card to my harddrive.

I bought an expresscard reader for my MBP to import my cards to my computer. It should be using the PCI connectors instead of the USB connector (expresscard has both USB and PCI connectors) like this sonnet card, so it should support speeds up to UHS-II standards (312 MB/s). I’ve tested my cards and it reads the card at a 25 MB/s average speed. Not a bad average, knowing that the card is a 10 MB/s minimum – 45MB/s max speed card. But it still takes 20 minutes for one 32 GB card to transfer.

I was hoping for 10 minutes transfer time. The fact that it’s slower could be due to my SD-card’s average speed, but it could also mean that my cardreader only uses the USB bus instead of PCI (58 MB/s theoretical maximum) and/or doesn’t support UHS (it might not support bus-speeds higher then the High-speed 50 MHz).

Sandisk Extreme Pro 95MB/s

There also is a Extreme Pro 95MB/s card, this is an SDXC card which (due to it’s exFAT system) is not limited to 4GB file-sizes and has even higher readout speeds. But, it also is said that the SDXC standard is less reliable because it’s exFAT system only has one single file allocation table. I’m not really sure if the GH2 is formatting these cards using exFat or Fat32. My guess is Fat32, because it also needs spanning at 4GB. Until I am sure about this, I will not buy SDXC cards.

I guess this card is using UHS104, so you will need a cardreader that supports UHS104 (208Mhz, 4bit-bus) in order to take advantage of the full speed range of this card. Since the GH2 is only HS (50Mhz, 4bit-bus = 25MB/s), the advantage for GH2 users seems to be that this card would have a higher minimum write speed at lower packet sizes, allows for spanning at higher bitrates (or, should we say: spanning is not needed anymore, since the 4GB limit has disappeared?).

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Posted in: GH2