How do CDs work

What is it?

Compact Disc (CD) is an optical disc that is used to store data.
CDs can store any type of data like audio, video, text, pictures, etc in digital format.

Lable Side (Top)

12 cm

Shiny Side

(Read/Write by LASER)

Types of CDs

There are 3 - types of CDs.

CD

CD-ROM

CD Read Only Disc
You can not add or delete information on it.
Used to commercial programs and data.

CD-R

CD-Recordable.
We can burn files to the disc more than one time.
Can not delete files from it.

CD-RW

CD ReWritable.
We can burn files more than once.
And delete files.

You can find which type of CD/DVD you have on the label side of CD.

How does it work?

In a CD, data is stored digitally (as a long string of numbers). After the music has been recorded, it is converted into numbers by a process called sampling.
A piece of electronic equipment measures the sound, turns the measurement into a number, and stores it in binary format (0s and 1s).
The master is "burned" with a laser beam that etches bumps (called pits) into its surface. A bump represents the number zero.
The lack of a bump (which is a flat, unburned area on the disc, called a land) represents the number one.

A CD has a continuous spiral of about 3-5 billion pits and lands.
The spiral would stretch for about 6 km.

Recordable CDs

CD writers are widely referred to as CD burners.
They simply use a laser to change the light-sensitive dye.
The CD-R writer has a higher-powered laser than normal, which generates heat when it strikes the disc, "burning" the dye and making a tiny black spot.
Later, when a CD reader aims its laser at that spot, the light is completely absorbed and doesn't reflect back.

Rewritable CDs

Instead of having a layer of dye, a CD-RW has a layer of a metallic alloy that can exist in two different solid forms and change back and forth between them. It's called a phase-change or phase-shift material.
Sometimes it's crystalline, with its atoms/molecules arranged in orderly ways, so it's translucent and light can pass straight through it; other times, its atoms/molecules are jumbled up in a much more random and disorderly form called an amorphous solid, which is opaque and blocks light.

CD Driver

A CD driver has 3 main parts.

Driver Motor: Spins the optical disc (CD).
Shoot the laser onto the disc & figure out the Laser & Lens: reflection.
Tracking Machine: Moves the laser from the inside of the disc to outside of the disc.

The data on a CD is stored as small notches on the disc.

In a CD, data is stored digitally (as a long string of 0s and 1s)

And is read by a laser from an optical drive. The drives translate the notches (which represent 1's and 0's) into usable data.

Driver Motor and Laser

Inside your CD player, there is a miniature laser beam (called a semiconductor diode laser) and a small photoelectric cell (an electronic light detector).
The disc rotates at high speed (up to 500rpm).
The laser beam switches on and scans along a track, with the photocell, from the center of the CD to the outside.
The motor slows the disc down gradually as the laser/photocell scans from the center to the outside of the disc.
The laser (red) flashes up onto the shiny (under) side of the CD, bouncing off the pattern of pits (bumps) and lands (flat areas) on the disc. The lands reflect the laser light straight back, while the pits scatter the light.
Thus the scanning laser and electronic circuit gradually recreate the pattern of zeros and ones (binary digits) that were originally stored on the disc in the factory.
A DAC decodes these binary numbers and converts them back into a changing pattern of electric currents.
A loudspeaker transforms the electric currents into audible sounds.

Here the optical sensor does not get reflected light back on the burned area (bump/ pits) takes as 0.

Here the optical sensor gets reflected light back in the unburned area (flat/ land) takes as 1.

How do CDs work

What is it?

Compact Disc (CD) is an optical disc that is used to store data.

CDs can store any type of data like audio, video, text, pictures, etc in digital format.

Lable Side (Top)

12 cm

Shiny Side

(Read/Write by LASER)

Types of CDs

There are 3 - types of CDs.

CD

CD-ROM

CD Read Only Disc

You can not add or delete information on it.

Used to commercial programs and data.

CD-R

CD-Recordable.

We can burn files to the disc more than one time.

Can not delete files from it.

CD-RW

CD ReWritable.

We can burn files more than once.

And delete files.

You can find which type of CD/DVD you have on the label side of CD.

How does it work?

In a CD, data is stored digitally (as a long string of numbers). After the music has been recorded, it is converted into numbers by a process called sampling.

A piece of electronic equipment measures the sound, turns the measurement into a number, and stores it in binary format (0s and 1s).

The master is "burned" with a laser beam that etches bumps (called pits) into its surface. A bump represents the number zero.

The lack of a bump (which is a flat, unburned area on the disc, called a land) represents the number one.

A CD has a continuous spiral of about 3-5 billion pits and lands.

The spiral would stretch for about 6 km.

Recordable CDs

CD writers are widely referred to as CD burners.

They simply use a laser to change the light-sensitive dye.

The CD-R writer has a higher-powered laser than normal, which generates heat when it strikes the disc, "burning" the dye and making a tiny black spot.

Later, when a CD reader aims its laser at that spot, the light is completely absorbed and doesn't reflect back. ​

Rewritable CDs

Instead of having a layer of dye, a CD-RW has a layer of a metallic alloy that can exist in two different solid forms and change back and forth between them. It's called a phase-change or phase-shift material.

Sometimes it's crystalline, with its atoms/molecules arranged in orderly ways, so it's translucent and light can pass straight through it; other times, its atoms/molecules are jumbled up in a much more random and disorderly form called an amorphous solid, which is opaque and blocks light.

CD Driver

A CD driver has 3 main parts.

Driver Motor: Spins the optical disc (CD).

Shoot the laser onto the disc & figure out the Laser & Lens: reflection.

Tracking Machine: Moves the laser from the inside of the disc to outside of the disc.

The data on a CD is stored as small notches on the disc.

In a CD, data is stored digitally (as a long string of 0s and 1s)

And is read by a laser from an optical drive. The drives translate the notches (which represent 1's and 0's) into usable data.

Driver Motor and Laser

Inside your CD player, there is a miniature laser beam (called a semiconductor diode laser) and a small photoelectric cell (an electronic light detector).

The disc rotates at high speed (up to 500rpm).

The laser beam switches on and scans along a track, with the photocell, from the center of the CD to the outside.

The motor slows the disc down gradually as the laser/photocell scans from the center to the outside of the disc.

The laser (red) flashes up onto the shiny (under) side of the CD, bouncing off the pattern of pits (bumps) and lands (flat areas) on the disc. The lands reflect the laser light straight back, while the pits scatter the light.

Thus the scanning laser and electronic circuit gradually recreate the pattern of zeros and ones (binary digits) that were originally stored on the disc in the factory.

A DAC decodes these binary numbers and converts them back into a changing pattern of electric currents.

A loudspeaker transforms the electric currents into audible sounds.

Here the optical sensor does not get reflected light back on the burned area (bump/ pits) takes as 0.

Here the optical sensor gets reflected light back in the unburned area (flat/ land) takes as 1.

Later, when a CD reader aims its laser at that spot, the light is completely absorbed and doesn't reflect back.