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Digital Recording
WHAT IS DIGITAL RECORDING?
It is the video equivalent of going from vinyl records to CD audio. But why use
digital recording anyway? For three reasons: labour, labour and labour. It is all about
handling and storage and how much time is involved – if it takes 45 seconds to change
a tape, that could add up to a couple of hundred hours per year in some CCTV schemes.
What medium does digital recording use for storage? There are a
variety of media: hard disk, removable hard disk, RAID (redundant array of inexpensive
disks), DVHS, DV (digital video), DAT (digital audio tape), Exabyte, AIT (advanced
intelligent tape), CD-ROM or WORM (write once read many), DVD (digital versatile disk) or
PD (photo-optic disk).
How much can they store?
* HD up to 60GB
* Removable HD are usually a little smaller
* RAID up to 16TB
* DAT up to 12GB and stackable usually to 5 (No 6 is a cleaner)
* EXABYTE up to 14GB, stackable to 12TB.
(EXABYTE in capital letters is the name of a manufacturer of a
tape-based storage device. In lower case, exabyte means 2 to the power of 60.)
* AIT 2 up to 50GB and stackable to 32TB, includes embedded memory
(16k)
* DV up to 60GB
* CD ROM up to 700MB
* DVD up to 4.7GB.
They are not compatible, except you can play CDs on a DVD machine,
but not the other way round.
How long will a recording last?
It depends on the data compression used. A quick calculation for the
equivalent of an E180 tape: 50 fields per second for three hours equals 540,000 images.
Compress to, say, 20kB per image would mean the disk needed is 10.8GB.
Common compression types include JPEG, JPEG 2000 (a superseding
version, not compatible with JPEG), MPEG 1 and MPEG 2, wavelet and H261 and H263. Too much
compression can ruin the image, but some compression types give fixed quality. Smaller
pictures mean smaller files – but a smaller picture also means less detail. CIF
(Common Intermediate Format) is only 352 pixels by 288 high.
* What does the customer/installer need?
* A real time video output
* The ability to make prints
* The ability to export to something like CD.
How does the customer know which to buy?
There are a couple of questions to ask first:
* What do you need from the recordings?
* How long will you keep them? The recommendation is 31 days.
* What will you do with your old multiplexer
and VCR? – Don’t throw them out.
* How will you make copies?
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