5.5.06

For the Technically minded

The difference between CDs and DVDs Do the terms CD-R, CD-RW, DVD-R, DVD-RW, DVD+R and DVD+RW make you wonder which disks work in your computer and which will just be shiny drinks coasters? What are CDs and DVDs exactly? Collectively, CDs (Compact Disks) and DVDs (Digital Versatile Disks) are known as optical media because the data stored on them is ‘read’ by a laser. Don’t fall into the trap of thinking DVDs are just for video either – all recordable optical disks work in the same basic way: Binary data – a series of dots and dashes - is literally burnt into a heat-sensitive layer in the disk by laser. A reading laser can recognise these ‘burns’ and convert them back into data for your computer (or the chips driving your stereo and DVD players). The big difference between CDs and DVDs is really in the way stored data is formatted, which affects the amount of data a disk can hold. How much data are we talking about? A typical CD will hold 650MB to 750MB of data. How much is that? Well, if your average MP3 song is around 5MB, we’re talking about 130 odd songs per disk. (If you’re wondering, store-bought CDs don’t compress the music into MP3 code, which is why they hold fewer tracks.) A standard DVD disk, on the hand, can store 4.7GB (approximately 4,700MB) – enough for roughly two hours of video. Beyond that, it starts to get more confusing. The latest disk technology includes DVD-RAM disks (9.4GB), double-layer DVDs (8.4GB) and double-layer double-sided DVD (16GB+). As an indication of capacity, a double-layer DVD can store around four hours of digital quality video or up to 16 hours of VHS quality video. What about all the different types – CDs? The basic types of CD are CD-R and CD-RW. • CD-R stands for CD-Recordable. You can record data onto these once only, and then it’s archived safely for as long as you want it. CD-R disks are fast writing and can then be read by most disk drives or players. • CD-RW stands for CD-Rewriteable. The big difference is that these disks are re-writable: You can record, read and then rewrite fresh data onto the same disk, so you can update or change the data over and over again – up to 1,000 times. What about all the different types – DVD? DVDs have split into quite a few formats, and this is where compatibility issues can really become a problem. (Remember those shiny drinks coasters…?) • DVD-R and DVD+R are both write-once Recordable disks (like CD-R disks). You can record data onto them once, and it is then stored for posterity. • DVD-RW and DVD+RW are both re-writable (like CD-RW disks), so you can record, retrieve, and write over data on these disks. Again, they’ll stand 1,000 or more rewriting sessions, making them a flexible storage medium for large quantities of data. So what’s with ‘DVD-’ and ‘DVD+’ ? DVD- is a slightly older, slower DVD disk format. By comparison, DVD+ disks are certified for up to a 16x recording speed – the fastest recording speed of any DVD format. That’s equivalent to around 3.2MB/sec (and an entire MP3 track is around 5MB, remember). Are store-bought disks the same? No. While manufactured CDs and DVDs also store data in ways that can be read by a laser, the data isn’t burned into the disk. Usually, it’s mechanically stamped into a foil sheet. These disks don’t have the heat sensitive layer of recordable disks, so they’re less expensive and more robust. Finally, a storage tip. On that note, you should always be aware that your disks have that heat sensitive layer in them. So while they have the potential to safely hold your data for 100 years or more, if you expose them to heat all bets are off. If you really want your disks to last, keep them somewhere cool and dark. Not much will ruin them faster than exposure to heat such as direct sunshine – or the bottom of a coffee cup!But even then beware as a survey I read once said that whatever quality you purchase does not really guarantee they will be a lot of good and so I tried out some old ones of a decent and not so decent brand of these storage devices and found that the longer you keep them is not good as they seem to take forever to load and I recomend a flash or pen drive as they seem to be called now. This info was sent to me by a friend but I know it to be true.

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