Remeber that probe which crashed into the surface of Mars because one team was using the metric system and the other was using the imperial system?  Well if you feel like that probe when trying to figure out which memory card to buy for your new flash camcorder, you’re not that far off.  Camcorder and memory card manufacturers are conspiring to confuse you, by using completely different metrics.  The camcorder guys are quoting bitrate in Megabits per second, and the flash memory guys are quoting their write speed in MegaBytes per second.  You can tell the difference by looking at the units, but it’s not that clear to the untrained eye.  Megabits per second (Mbps) will always use a lowercase b. MegaBytes per second (MBps) will always have a capital B.

Well naturally you need to convert to a common metric to compare the speeds.  Any freshman computer scientist will tell you there are 8 bits in a byte.  If you have a 18Mbps then you really have a 2.25 MBps camcorder, so you need a flash card which is faster than 2.25 MBps, but your flash card doesn’t say a MBps on it.  All it has is “Class 4″ on it, so how can you tell the MBps?  Well as luck would have it, that tells us exactly what we need to know. Class 2 is a minimum of 2 MBps sustained writing speed, Class 4 is a minimum of 4 MBps, and Class 6 is a minimum of 6 MBps.  For 2.25MBps you need a Class 4 card.

If you are somewhat confused already, just skip this paragraph, but if you are about to tell me I don’t know what I’m talking about, keep reading.  Yes, in the previous example I cheated.  A 18Mbps bitstream is not really 18Mbps.  That doesn’t include sound, error checking, some other fun stuff which probably raise that bit rate up to 25Mbps.  But it also doesn’t include compression, which would make this problem impossible to solve in this generic sense because video compression is variable.  I’m making the assumption that compression will always negate the extra bits from sound and error checking.  AVCHD will compress anywhere from 2:1 to 20:1 so we’re safe even in the conservative estimate.   It’s possible that your camcorder might do enough compression to work on a lower speed card, but that’s a risky bet.  A faster card should always work whereas a slower card might not work at all.  So if you have to err, err on the high side.

Of course I still recommend you do this the easy way and read the manual (RTFM) for your camcorder.  It should tell you right there in black and white which class of card to use.