FCP HDV Preset captures at 3.2 mbs?? at DVinfo.net

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Old February 21st, 2006, 01:47 PM   #1
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FCP HDV Preset captures at 3.2 mbs??

I thought HDV (1080i) was 25 mbs. How come when I edit in FCP5 using the HDV preset, my clips all say 3.2 mbs for their data rate? Is it not capturing NATIVE HDV from the camera via firewire? Looks to me like it's the same picture quality as straight off the camera but if it's cut down to 3.2 mbs it can't be, can it?
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Old February 21st, 2006, 04:53 PM   #2
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BIT MATH... Yeah!

It actually says 3.2 MB/sec (megabytes.) That is exactly the same as 25 Mb/sec (megabits). Capital "B" means bytes and lower-case "b" means bits. There are 8 bits in a byte.

So 25,000,000 bits divided by 8 = 3,125,000 bytes

Take 3,125,000 bytes and divide by 1,000,000 and you will get 3.125 MB.
FCP simply rounds that up to 3.2MB/sec.

However, there are 1024 bytes in a Kilobyte, and 1024 Kilobytes in a "megabyte." This is one of the reasons a DVD-R advertised as 4.7Gb is actually only 4.3Gb.
So 3,125,000 should actually be divided by (1024x1024) 1048576 to give you 2.98 MB/sec, but I guess FCP does the simple math.
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Old February 21st, 2006, 09:16 PM   #3
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Thanks Tim!
This has been gnawing away at me for a while and now. Thanks to your explanation, it all makes sense! I guess I wasn't paying attention to the big "B" or little "b".
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