Mathematical Check

[James.Northrup@gmail.com]

I am trying to check the math on the amount of time per 8 GB card using DVCPRO HD codec. But it's not working. Could someone point out my miscalculation?

On a 8 GB card you should get 8 minutes @ 100 Mb/s. This is in the HVX manual.

Therefore,

8 min * 60 sec/min = 480 sec //Uniform Units
100 Mb/sec * 1 Gb/1024 Mb = .01 Gb/sec //0.0977 Gb/sec
.01 Gb/sec * 1 GB/8 Gb = .0125 GB/sec //Bits to Bytes

Finally,

Data Rate * Advertised storage in Time = Total bytes

480 sec * .0125 GB/sec = 6 GB !!!


I'm sure this is an error on my part because I calculated the same thing for XDCAM and got nearly the same percentage error.

[BobDiaz]

First of all, 100Mb is saying 100 mega-bits per second for video. Audio adds an extra 3Mb/second to that. There is also some additional overhead added to all of this, some for file storage and some for the video overhead. The notation MB is saying MegaBytes.

Berry was nice enough to give me the exact numbers for a 4GB card:

Yeah but you're forgetting that it's not just the video file. For each clip there's a video file, four audio files, an XML file, a BMP thumbnail file, then there's the directory structure and its attendant text file, and ... well, I think that's about it.

A chock-full 4GB card reports in Windows Explorer that it is 3,720,918,946 bytes.

The audio folder contains four audio MXF files, each is 24,217,152 bytes, taking up a total size on disk of 96,993,280 bytes.

The CLIP folder contains one XML file of 2,892 bytes, taking up a total size on disk of 32,768 bytes

The ICON folder contains one BMP file of 14,454 bytes, taking up a total size on disk of 32,768 bytes

The VIDEO folder contains one MXF file of 3,624,032,992 bytes, taking up a total size on disk of 3,624,042,496 bytes

The exact runtime of the file is 00:04:11.28



Let's keep this simple, 00:04:11.28 = 251.28 seconds.
4GB / 251.28 seconds = 0.016 GB per second with all the overhead added in.

Depending on how you wish to calculate Mb (Megabits), 0.016 GB/sec = 0.016 GB per second x 1,000 x 8 = 128 Mb/second OR 0.016 GB/sec = 0.016 GB per second x 1,024 x 8 = 131 Mb/second. Computer people like to use 1024 and the rest of us use 1,000 for the conversion.


The problem is that a 4GB card is less than 4GB. This is also true for hard drives on a PC, so don't panic. Just remember that a 4GB card uses about 7% of it's space for file overhead. These are things like the FAT (File Allocation Table), Directory, Error Detection, and Correction information.


Taking into account the non-computer conversion of 1,000 for going from mega to giga, here's what I get:

8 GB x 1,000 = 8,000 MB

8,000 MB x 8 bits per byte = 64,000 Mb (Megabits)

64,000 Mb / 128Mb per second (with overhead) = 500 seconds

500 seconds / 60 = 8.3 minutes (roughly)

This is at the maximum data rate.

I hope this helps.


Bob Diaz

[James.Northrup@gmail.com]

That's great Bob. I'll check your math, but you clarified a lot of foggy ideas for me.

But what you are saying about the extra 3 Mb/s for audio. This must be true of tape based DVCPRO HD, right? P2 DVCPRO HD and tape-based DVCPRO HD are exactly the same compression formats, right?

Thanks again.

[evinsky]

Yes.