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Optical Storage

Optical media like CDs and DVDs store and retrieve data using focused laser beams that reflect off specially structured disc surfaces, with sensors interpreting the reflected light as binary ones and zeros. The physical encoding of data relies on either removing or blocking portions of a reflective layer to create the patterns a laser can detect.

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About this video

Optical media is a category of storage that includes CDs and DVDs, and it operates on the principle that light can be used to read information encoded on a physical surface. Unlike ordinary light sources, optical drives use lasers, which produce a highly focused beam capable of precisely targeting microscopic areas of a disc. As the laser strikes the disc, the reflected light is captured by a sensor, which interprets the presence or absence of a reflection as a binary one or zero. This combination of laser, reflective surface, and sensor is the foundation of how all optical media functions. Data is encoded on a disc by disrupting its reflective layer in one of two ways. The subtractive method physically removes portions of the reflective surface so the laser finds nothing to bounce back from, producing a zero signal at those points. The additive method instead places a light-blocking material over the reflective surface, preventing the laser from reaching it and producing the same result. Both approaches create a pattern of reflective and non-reflective zones that the sensor reads as a continuous stream of ones and zeros as the disc spins. Writable optical media uses a variation of these principles to allow new data to be stored. A more powerful laser superheats a chemical layer embedded in the disc, making it permanently opaque at targeted spots. This effectively introduces the light-blocking effect of the additive method in precise locations, encoding user data onto a disc that was previously blank. While different optical formats and technologies introduce variations in implementation, this laser-and-reflection mechanism is the core of how optical storage works across CD and DVD media.

What you'll learn

What's covered

Optical Media (CDs & DVDs)

Aligned to

CompTIA Tech+
2.5 Compare and contrast storage types.
CompTIA A+ Core 1
3.3 Given a scenario, select and install storage devices.

Key terms

Optical Media
Storage media, such as CDs and DVDs, that uses laser light reflected off a disc surface to store and read binary data.
Laser
A highly focused beam of light used in optical media drives to read and write data by reflecting off or interacting with a disc surface.
Pit
A carved-out area on the reflective surface of optical media that disrupts laser reflection, representing a binary value during data reading.
Sensor
In an optical drive, the photodetector that receives laser light reflected off the disc surface; pits and lands reflect light differently, and the sensor converts those intensity variations into the binary ones and zeros the drive reads as data.
Subtractive Method
An encoding approach for optical media where reflective surface areas are physically removed to create non-reflecting zones that represent binary zeros.
Additive Method
An encoding approach for optical media where a material is added to block laser light from reaching the reflective surface, representing binary zeros.

Topics

Optical Storage Data Encoding Binary Representation Laser Technology Hardware Storage Media

Transcript

Optical Media

Our CDs and DVDs are what we call optical media. The word optical just means relating to sight, and what sight is is our eyes determining or interpreting light waves that are coming into them. Well, the same thing happens with these CDs and DVDs, this optical media, as we reflect light off of it and then determine what it's telling us, what is the data stored on it. And the type of light that we use are these lasers, so we shine lasers off of this in order to...

One thing to understand is that we do use lasers for this, not just regular light. A laser is a very focused beam of light, so we see right here it's a very focused beam, rather than just a regular flashlight that is all over the place.

Reflective Surfaces

Another important concept to how optical media works is that we use reflective surfaces. So what we see right here is the lasers coming out and hitting this DVD, reflecting up, and then I have this mister going so we can actually see the reflection and what's happening over here.

So what we have in this diagram is a reflective surface, we have a laser, we have that laser beam that's shooting off, reflecting off of that surface, and then we need some sort of sensor to be able to detect that light. So in this example right here, here's the laser, here's our reflective surface, and then this is going to represent our sensor that's sensing whether it's a one or zero.

Disrupting the Reflection

To store data and read data off of CDs, somehow we need to disrupt that reflective surface. So I'm just going to use a note card here as an example, with holes in it, and then move it along. What you can see is sometimes it gets reflected and sometimes it doesn't, creating ones and zeros.

So what we have here is we have some sort of backing to all of this, maybe it's some sort of plastic that holds this all together. The next is we have a reflective surface, and then on top of that we've got a protective layer right here. And all we have to do now, if we want to have a signal on here, is we can carve out certain areas that don't have reflective surfaces on them. So we can see that there are certain areas that are carved out.

So now what's going to happen is that laser is going to reflect and get a signal off of some of it, which would be maybe a one here, and then it's not going to reflect off of other parts. What's going to happen is it's not going to reflect here, the sensor is not going to receive anything, and so we categorize that as a zero. So let's see what happens when this laser goes across data.

Subtractive and Additive Methods

Now, there's really two methods that we could read from this disc. One of the methods is that we could have an absence of a reflective surface. So that's what we've been covering, is we have a reflective surface that doesn't exist in these spots right here, and so therefore those are zeros. But the other way that we could do this is we could actually block light on this other surface here. We could block light from hitting that reflective surface.

I'm just going to call one of these methods the subtractive method, which is we're eliminating the reflective surfaces to create that signal of zeros. I'm going to call this other method additive, that we're adding something that's going to block the reflective surface so that way it's going to represent zeros. And here's an example of where that laser now is hitting a blocked content there, so that way it's not getting to the reflective surface, so then the signal is not getting to the sensor, so therefore that would be a zero.

Writing to a CD

There are CD drives that we can write data to CDs. What we have is we have some sort of laser that superheats the chemical here, making it opaque, that is making it so that way the laser can't shine through it. And so that's how we write data to some of these CDs. Now, there are different technologies and they work a little differently.

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