Video Bitrate vs Card Write Speed

Started by ItsMeLenny, December 09, 2013, 09:58:54 AM

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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

Quote from: Walter Schulz on December 09, 2013, 10:12:51 AM
Where does the first number come from?

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

ilguercio

Canon EOS 6D, 60D, 50D.
Sigma 70-200 EX OS HSM, Sigma 70-200 Apo EX HSM, Samyang 14 2.8, Samyang 35 1.4, Samyang 85 1.4.
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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!
550D - Kit Lens | EF 50mm f/1.8 | Zacuto Z-Finder Pro 2.5x | SanDisk ExtremePro 95mb/s | Tascam DR-100MkII

Walter Schulz

Quote from: NedB on December 09, 2013, 10:27:13 AM45mb/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 :P

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

Quote from: Walter Schulz on December 09, 2013, 10:57:28 AM
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 ;) :P