On a cd rom the pits and lands are etched into the

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surface of the CD doesn't reflect, while a land or flat spot does reflect. On a CD-ROM the pits and lands are etched into the bottom of the CD at the factory. You can't add any data to it and you can only read data from it. Here's how it works. We have our CD disc right here. We're looking at it from its edge so it looks flat, and we have a laser right here. The laser continuously shines its beam onto the bottom surface of the CD-ROM. If it hits a pit, then nothing is reflected back. If it hits a land, a reflection's reflected back. By spinning the disc and moving the laser head underneath the disc, the drive can read the binary information. The reflection is captured by a photosensitive device down here, which then converts the pulse of the laser light into zeros and ones. This is binary data that we'll use to process the information off the disc. In this way a CD can store binary digital data. In fact, a CD can store approximately 650 megabytes of data, but there are varieties that you can purchase at the store that are 700 megabytes. Let's take a look at how a CD-RW drive works. A CD-RW drive stores binary PC data just like a CD-ROM. However, the way it does it is a little bit different. The bottom surface of the CD-RW drive is coated with a photoreactive crystalline coating. A CD-RW drive has a second high-powered write laser.When this laser hits the bottom of this photoreactive material on the bottom of the CD-RW disc, it causes crystals to form. This is called phase shifting or phase shifting media. Wherever a crystal is formed on the bottom of a CD-RW disc is like a land on a CD-ROM disc because it reflects light. Summary 5:08-5:19 So to review, we learned about CDs, how data is stored to a CD, and the different formats of a CD, like CD-ROM and a CD rewritable. We also talked about the different capacities and speeds of each type of compact disc. DVD 0:00-2:03
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In this lesson, let's talk about DVDs. DVD stands for digital versatile disk. In general, they've replaced compact discs for storage. A DVD can hold many different types of data. DVDs can hold theater-quality video, audio, and computer data. If you look at a typical DVD disc, you'll see that the physical dimensions of that disc are the same as those of a CD. However, a DVD can hold a lot more data than a CD can. A CD can hold around 737 megabytes of data, but a DVD can hold 4.7 gigabytes of data. So how do you get so much more data on a DVD than a CD if they both have roughly the same physical dimensions? The difference is in the width of the data track on the underside of the DVD. The track that's used to store data on the bottom of the CD is quite wide. On a DVD the data track is narrower, something like this. So in other words, by shrinking the width of the track on a DVD, we're able to include more tracks on the DVD and store more data on a DVD than an older CD.
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