HD Video & Card Speed Debunking Unit

category: Cameras • 3 min read

Now both Canon and Nikon produce dSLRs with video. Let’s not forget the other guys, Olympus, Panasonic and Pentax. Canon seems to be the most advanced in video. The new Canon 7D does HD 1080p in 24 frames, 25 frames or 30 frames. Nikon only goes up to HD 720p.

With all these video modes, there is so much BS floating around, that I wanted to clear the air about the relationship between the memory speed and the different video modes. So, at great personal expense, I called back the “Debunking Unit”, those guys don’t come cheap. Big warning, there’s math involved.

Sandisk and a few other manufacturers have come up with UDMA6 modes. That’s 90 Megabytes per second for the reads, the writes can be a lot slower depending on the card and the camera.

  • Please remember that when a card manufacturer claims a speed, it’s not in a camera but attached to a computer and a wide bus connector.
  • The speed of a memory card will also depend on the camera. The most prominent example is when Sandisk created the Extreme III SDHC for the Nikon D90 where it could do 30Mb/sec, but 25Mb/sec in any other camera. The card was still sold and rated as a 30Mb/sec card.

So first, let’s do some math, instead of assertions. Please note that these calculations apply to Compact Flash, aka CF cards, and Secure Disk, aka SD cards.

For some simplification, let’s take the highest current video resolution 1080p with a resolution of 1920x1080 at 30 frames per second.

1920 times 1080                            = 2073600 pixels per frame
30 frames per second = 2073600 times 30    = 62208000 pixels per second
12 bits per pixel times 62208000 pixels    = 746496000 bits per second
746496000 bits divided by 8 bits per byte  = 93312000 bytes per second
9331200 bytes                              = 91 Megabytes per second

What does this mean? Do I need at 90Mb/sec card? No, it only means that we need to process 91 megabytes each second so we need one or two very fast CPUs, but we are not finished:

  1. We didn’t take into account the stereo sound track. The sound track is 8k per channel, so we have to add 16k for the stereo sound or 0.012 Megabyte per second.
  2. We didn’t take into consideration the video format. There are many video formats from the MPEG-2 to MPEG-4… The compression ranks from 50% at the low end to 95% at the high end. Your camera manufacturer will tell you which compression and which is format used. The compression on a Canon 7D with MPEG-4, as per Canon, is 16 to 1 so now the memory card speed needed is:

    91 Megabytes Divided by 16 = 5.7 Megabytes per second

Speed of the Memory Card

Memory card speed is expressed as a multiple of the speed of an original CD or 150Kb/sec or 0.146Mb/sec, so

5.7 Megabytes divided by 0.146 = 40x
  • Canon states in their Canon 7D manual that the memory card speed for video should be at least 8 Megabytes per second or 55X for a memory card. Engineers, from Canon or any other place, like to cover their ass, so make sure that:

    • There’s a minimum card speed for the video mode only.
    • There’s no minimum card speed for the still picture mode.
    • In still mode, the speed of the camera in frames per second is not affected by the speed of the memory card.
    • In still mode, the speed of the memory card only matters when clearing the memory buffer inside the camera.
    • Very few cameras support the new UDMA6 bus for the 90Mb/sec cards.
    • Find somebody that has tried it before spending your hard earned money. There are many forums dedicated to your camera. You can ask and people will tell you their experience.
    • My opinion is that if you get a 60X card for Compact Flash or a Class 6 SD card, you will be more than OK for the video.
    • If in doubt RTFM aka “Read The Fine Manual”.
    • The camera’s manufacturer has done extensive tests to determine the minimum memory card speed in video mode.