Specifications

Technical Appendix

Hard Drive Size Inconsistencies and DVMS Storage Formula Calculation

If you query your PC’s operating system for the hard drive size, you may be surprised to find it doesn’t match

the Manufacturer’s rating. For example, your PC’s hard drive was advertised to be 10.24GB of capacity. Windows

reports that only 9.85GB are usable.

The problem arises from the fact that HD manufacturers use the decimal measurement system whereas the

Operating System will use binary.

The more scientifically accurate standard is really binary, which states that a Gigabyte of information is

1,073,741,824 bytes of storage. Hard drive manufacturers consider 1 Billion bytes as one Gigabyte.

Since the purpose of the guides and tables are to simplify specification and purchasing, all tables are based on

the decimal measurement of Gigabytes. That means that when our chart says that you need a 27GB drive to

record x number of images for x days, you can be assured that the HD you purchase will accommodate that.

Otherwise, you’d have to convert the “scientifically accurate” figures to how the hard drives are advertised.

Note: PAL notes and figures in RED

All DVMS units (100 thru 1600) grab a 720 x 243 (NTSC) or 720 x 288 (PAL) pixel image per field. Multiply by 2

bytes/pixel gives 349,920 (NTSC) or 414,720 (PAL) total bytes per uncompressed image field.

How the formulae were Derived

To calculate the Size of the hard drive needed to satisfy the requirements, this formula was used:

((H x R) / C) x 1.26 = S (For NTSC)

((H x R) / C) x 1.49 = S (For PAL)

Where H= Hours of recording time needed, R= Rate of image capture, (images per second), C= Compression

ratio (ex: 15 for 15:1), S= Size of Hard Drive, in Gigabytes.

This was reduced from:

H x 3600 [# of seconds in an hour] x R x 349,920 [total bytes of an uncompressed NTSC field] / C = S

(or H x 3600 x R x 414,720 [total bytes of an uncompressed PAL field] / C = S)

This is the same as:

H x 1,259,712,000 x R / C = S (NTSC)

H x 1,492,992,000 x R / C = S (PAL)

By dividing both sides by 1 billion (Manufacturer’s measurement of a GB), and rounding off to 3 decimal places,

you get:

H x R x 1.26 / C = S (NTSC)

H x R x 1.49 / C = S (PAL)

Therefore, S is equal to the size of Hard drive required, in GB, using the measurement standard of the Hard Drive

manufacturer.