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A few years back, when USB flash drives and cloud computing hadn’t become as popular as they are today, data was primarily stored and retrieved with the help of CDs. The shiny round discs, even to this day, are known for their ability to store data in the range of a few hundred megabytes to a couple of gigabytes.
What is a Compact Disc?
Compact disc (CD) is a digital optical disc data storage format. It was co-developed by Philips and Sony. CD format was originally developed to store and play only sound recordings but was later adapted for storage of data.
Data on a CD is encoded with the help of a laser beam that etches tiny indentations (or bumps, if you will) on its surface. A bump, in CD terminology, is known as a pit and represents the number 0. Similarly, the lack of a bump (known as a land) represents the number 1.
However, one of the biggest challenges with CDs is their delicate, smooth surface that easily gets scratched if not handled with the utmost care. Cracks and scratches on a disc’s surface severely hamper its ability to store data, and it also makes it difficult for a CD player trying to retrieve the data stored in it.
But why exactly does this happen? Why are scratched CDs harder to read/access?
Before we get to that, it’s imperative that we first understand how a CD works.
How does a CD Work?
A CD is usually around 12 centimeters (4.5 inches) in diameter and consists of a couple of thin circular layers that are attached one on top of another.
Most of a CD is composed of a plastic called polycarbonate. The bottom layer is a polycarbonate layer where data is encoded by using tiny bumps on the surface. Above this layer is a reflective layer, which is typically made of aluminum (gold is also used, although quite rarely).
Above the reflective layer is a protective layer of lacquer and plastic, which shields the layers below it. The artwork or label is printed on the lacquer layer (i.e., on top of the CD) via offset printing or screen printing.
CDs store information digitally, i.e., with the help of millions of 1s and 0s. Data on a CD is encoded with the help of a laser beam that etches tiny indentations (or bumps, if you will) on its surface. A bump, in CD terminology, is known as a pit, and represents the number 0. Similarly, the lack of a bump (known as a land) represents the number 1. Hence, a laser beam can encode the required data into a compact disc by using pits and lands (0 and 1, respectively).
Now that you know how a CD is encoded with data, let’s take a look at how a CD player actually reads that stored data.
How does a CD player work?
There are two main components inside a CD player that help read a CD: a tiny laser beam (known as a semiconductor diode laser) and an electronic light detector (basically, a tiny photoelectric cell). When you switch on the CD player, an electric motor inside the player makes the CD rotate at a very high speed (while reading the outer edge it rotates at 200 RPM, and while reading the inner edge it spins at 500 RPM).
The laser beam source inside the player switches on and scans along a track from the center of the disc to the outer rim. It focuses a 780 nm wavelength (near infrared) beam through the underside of the compact disc. When the beam falls on a land (1), it reflects straight back, but when the beam falls on a pit (0), it scatters.
When the photocell detects the reflected light, it understands that the laser must have encountered a land, and in turn, sends a signal to a circuit that produces the number 1. Similarly, when it detects no light, it (correctly) determines that there’s a pit at that location, so the circuit generates the number 0. This way, the photocell uses the intensity changes of the reflected beam to determine whether there’s a 1 or a 0 on the disc.
Why is it difficult for a CD player to read the contents of a scratched CD?
The data in a CD/DVD/Blu-Ray disc is not present on the shiny surface, but rather on the polycarbonate layer near the bottom of the disc. As mentioned earlier, a CD player has a laser beam that reflects/scatters off the underside, depending on whether it falls on a land/pit. The indentations on a disc’s surface are very, very small, which is why scratches and cracks mess up the way light bounces off the surface of the CD.
Basically, when the laser falls on a scratched area, it invariably scatters, even if there isn’t a bump at that specific location. Consequently, the photocell relays false information to the electric circuit, which makes it difficult for the CD player to read the data correctly.
However, scratches don’t necessarily render a CD useless. There is some error correction in the way data is encoded into a CD, which ensures that minor scratches on the disc’s surface won’t make the CD unreadable. It’s only when the scratches are severe, or if the CD is cracked, that CD/DVD players won’t be able to read it.