Video Bitrate vs Card Write Speed

[ItsMeLenny]

I'm a bit confused as to why on my 550d I can get 45mb/s video bitrate (fullHD PAL / 1080p 25fps) yet the max card write speed is only 21mb/s (and then my card probably doesn't even write half that).

Is this a case of me confusing megabit with megabyte?

[Walter Schulz]

Where does the first number come from?

Ciao
Walter

[ItsMeLenny]

Where does the first number come from?

I played back the video on my computer and checked out the average bitrate.

[ilguercio]

Is this a case of me confusing megabit with megabyte?

It is.

[NedB]

@ItsMeLenny: Yes, that's the confusion. 45mb/s is 45 MegaBITS per second, which is less than 6 MegaBYTES per second. The SD card limit is about 21MegaBYTES per second. Probably the bitrate viewer you used to check the average "bitrate" shows you just that, not the "byte"-rate.

Cheers!

[Walter Schulz]

45mb/s is 45 MegaBITS per second, which is less than 6 MegaBYTES per second.

And that's why a decent Class 10 card will be sufficient recording Canon's h.264 implementation at 1080p/30 without any hickups.

Ciao
Walter

[ItsMeLenny]

Ok, my bad

While on the subject though, which uses lower case b?
Mb = megabit/megabyte
MB = megabit/megabyte

And I should get myself a faster card and see how much I can ramp the bitrate up.

[Walter Schulz]

http://en.wikipedia.org/wiki/Megabit

Faster card? 21 MByte/s is interface limit. Nothing you can do about.

Ciao
Walter

[ItsMeLenny]

Faster card? 21 MByte/s is interface limit. Nothing you can do about.

Yes but my card only writes at about 12.

So in that case, something I can do about