The Compact Disk

Who has never used a CD to hold data, music or even computer software? You must be a crook if you have never; but better yet, further inane if you don’t know how they handle their crap! They are so cheap, that it is easy to produce them as long as you have a computer CD-R drive. A CD can store up to 74 minutes of music, yeah….my crunk mix CD plays for about 74minutes…let me hope yours does for approximately the same approximate length of time.

Let us do the math now

44,100 samples/channel/second×2bytes/sample ×2channels ×74minutes ×60 seconds/minute=783,216,000bytes

To carry 783MB on to a 12cm diameter CD….huh…bytes must then be very small.

Let’s get physical

Most part of a CD is an injection-molded piece of clear polycarbonate plastic. At manufacture, the plastic is impressed with microscopic bumps organized as a single, continuous and extremely long spiral track of data. A thin reflective aluminium layer is then sputtered on it to cover the bumps, then a thin acrylic layer sprayed over for protection.

What holds my data?

A CD has one spiral track of data circling from inner (center) to the outermost part, this can be less than 12cm in diameter if you want it to be! Infact there are plastic baseball cards and business cards put in a CD that hold about 2MB data before the size and shape of the card cuts off the spiral. Haa….this shit is funny, the data track is only 0.5microns (hope you know what a micron is-a millionth of a meter)  wide, does it even deserve the term wide….? 1.6 microns separate one track to another. What about the little bumps….they are even tinier, each 0.5microns wide, a minimum of 0.83 microns long and 125nm (a nm is abillionth of a meter) high. Take note that if someone tells you those are pits just turn the CD and show him the bumps, as in they appear like pits on one side and like bumps on the other side.

Then how is this small spiral track read?

The CD player reads data stored as bumps on the CD. It has three basic components

• Drive motor to spin the disc. Is precisely controlled to rotate between 200 and 500RPM depending on the track being read.

• A laser and a lens system tofocus in, on and read the bumps.

• A tracking assembly that moves the laser assembly so that the beam can follow the track and resolutions. The data is then processed into understandable data blocks and send to the DAC (in audio CD players) or to the computer (CD-ROM).

The main job of the CD player is to focus the laser on the track of bumps. The laser beam passes through the polycarbonate layer, reflects  off the aluminium layer and hits an opto-electronic device that detects the alterations in light phases upon reflection by the different surfaces . The bumps reflect light that differs with flats (aluminium layer) and the opto-electronic sensor reads these alterations in reflectivity from crest to trough and back as bits that finally constitute the bytes. The tracking beam that keeps the laser beam centered on the data track has a hell of a job to do…coz it has to move with the layer continuously outwards, as the bumps move fast, the laser even goes faster(v=wr). So, as the laser moves outward, the spindle motor must slow down the CD for bumps to travel past the laser at constant speed and the data comes off the disk at a constant rate.