Optical Media Explained
#15 | 15:25 |
Monday February 20, 2006
Monday February 20, 2006
Lab Rats grab a bag of marshmallows and fire up their blow torch and show you how optical discs work. Includes coverage of CDs, DVDs and blu-ray and HD DVD.
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- Call for Help Episode #85 (3 minutes ain't enough)
- Typically, the laser will actually hit the disc from the bottom, as the label side is usually on the top.
- Techically, 0's and 1's are created by the Transitions between puts and lands, but you get the idea.
- A pit reflects light more dimly than a land, and a diode is used to measure the difference in reflectivity.
- Though the shape of the disc is the same, data CS's , Audio CD's, DVD's all have a different sata structure.
- Master "stampers" are made from a glass master with nickel coating, with the pits burned into an emulsion layer.
- Unlike old-school vinyl records, data on CD's and DVD's starts on the inside, and moves outwards.
CD-R = Compact Disc Recordable
- Burnable Media uses a special layer of dye that changes properties when hit by the "write" laser.
CD-RW = Compact Disc Re-Writeable
- about 100 re-writes per disic
- The alloy used for CD-RW is typically comprised of silver, indium, antimony and tellurium, or AglnSbTe.
- The burning process melts the alloy into an amporphous state. Erasing data anneals it back to cyrstalline.
The two most common sizes of CD
- 650 MB (about 74 minutes)
- 700 MB (about 80 minutes)
- CD's use a 780 nanometer wavelengh.
- Beyond 700 MB, media may be unusable on older machines,as the space between data tracks becomes too tight.
Single-Layer DVD = Capacity of 4.7 GB
- more than seven times the size of a CD.
- DVD uses a laser with a 650 nanometer wavelengh.
Dual-Layer DVD = Capacity of X.X GB
- polycarbonate, dye and reflective layers brng the count up to 7 or 8 layers.
- With mulit-layer discs, one of the layers must be semi-transparnet so the laser can re-focus right through it.
- It's like the way a wire screen vanishes when your eyes refocus on an object on the other side of a screen door.
FUN FACT: on a second layer of a dual-layer DVD, the data goes from the outside to the inside, instead.
Next Generation DVD Media:
- HD-DVD is more than three times the size of DVD for a single layer.
- Blu-Ray is more than five times the size.
- HD-DVD is 15 GB per layer, while Blu-Ray discs can cram up to 25 GB per layer.
- A dual-layer Blu-Ray DVD can fit 50 GB of data.
- Triple-layer HD-DVD can handle 45 GB.
- Triple-layer HD-DVD and dual-layer Blu-Ray are still in the development stage.
- So all these numbers could change.
- This minimizes the "stutter" if the DVD player changes layers in the middle of a long motion picture.
- Both HD-DVD and Blu-Ray use a 405 nanometer blue laser to read data from the disc.
- Remember, we are trained professionals. Do not try this at home, unless you're bored.