Mad about Science: Disc storage

By Brenden Bobby
Reader Columnist

I had the privilege of being a part of a library ambassadorial voyage through the state of Idaho earlier this year. The goal of this mission was to make connections with other library professionals throughout the state, see what we were doing right, what we could be doing better, and trying to understand the needs of all Idahoans and how we can apply their needs to better serve the people of Bonner County.

Among the most interesting finds was that our library has perhaps the most robust DVD collection in the entire state. I’ve lived in Bonner County for my entire life, so I quickly understood why. Bonner County is a very rural area spread across a huge geographic area, with digital connectivity interrupted by mountains and very dated infrastructure outside of a few central areas. Complicating the connectivity issues is the astronomical cost of living in this area, which can force people to choose between internet access and a streaming service or food they need to survive to the next paycheck.

It became very clear that the library’s massive DVD collection isn’t just a convenience, but a lifeline for many residents.

But how does a DVD work? How does that little reflective disc store and replay your favorite movies or games?

The disc is only half the equation. The second half of the equation is the player in which you place the disc. The player will spin the disc rapidly, turn on a laser and voila: your music plays, your movie runs and Master Chief falls out of the sky to finish this fight. Meanwhile, there is a lot of microscopic magic going on in order to make this work as intended.

There are two structures on the surface of a disc to be aware of: pits and lands. Pits are divots in the structure and lands are the spaces between the divots. When the laser of the reader strikes the lands, it’s reflected back into a photodiode, or a mechanism that detects light. Whenever the laser strikes a bump created by the pit, it creates a disturbance that the photodiode picks up and then feeds into the computer as data. The data is then decoded and transformed into a medium that we as humans can use and understand.

This is an extreme simplification of the entire process.

Recording and replaying encoded data goes back to a time well before the development of the compact disc. One of the earlier media that laid the groundwork for compact discs was the phonograph, developed by Thomas Edison in 1877. The phonograph, like the vinyl record player that would follow years later, used a stylus at the end of an arm that would follow the grooves in a disc and vibrate as it moved. These vibrations were translated into soundwaves that could then be heard by a human. There are a few more steps involved with a modern record player that converts the sound waves into an electrical signal using something called piezoelectric crystal, which is a crystal that creates electrical energy when mechanical stress is applied to it.

Producing compact discs is a fascinating process that begins with a glass master disc. This master is put through a number of chemical processes to preserve it as well as adding a chemical layer that will keep the etched data configuration in the form of inverted pits and lands. This master is then fed into a stamping machine that is filled with molten polycarbonate plastic. The master stamps the data pattern into the polycarbonate that hardens, and is then removed from the stamper. A reflective metal layer is added to aid the laser and photodiode in reading the disc later — this is why CDs and DVDs are shiny. 

A layer of lacquer is added to help protect the disc and then a graphical topper is sometimes applied so that you know what disc you have in your machine. Sometimes, manufacturers will have two-sided discs to effectively double the amount of data they can store, but this isn’t common practice as it’s a little more expensive and complicated to produce and can be more prone to severe damage.

Polycarbonate is a powerful and pliable plastic with a huge variety of uses. Its tough and durable nature allows it to be easily curved into shapes with unique geometry. It is commonly used in personal protective equipment, particularly in the form of goggles and face shields. (My favorite use for tinted polycarbonate is to create tinted visors for cosplay helmets.) If you place two-way mirror film over the visor, you can create a cool effect right out of Halo: Combat Evolved. This is very similar to the process applied to a compact disc.

You may be wondering about the difference between CDs, DVDs, HD-DVDs and Blu-Rays. Essentially the process is identical across all of these medias, but the pits can be shrunken far more when using something like a Blu-Ray. This is because a Blu-Ray laser uses a much smaller wavelength of light in the ultraviolet spectrum than a CD, which uses a laser on the infrared side of the spectrum. Smaller pits means that there is more surface area on the disc for data, despite the disc being virtually the same size. The finer your laser, the smaller you can make your pits and the more data you can stuff onto your disc.

As a final bonus bit of information about discs, all of the information is stored in a spiral pattern on the inner two-thirds of the disc. There’s a modest gap around the exterior edge to account for smudges from your fingers or other damage. Light smudges and minor scratches generally won’t upset your player, but when dealing with points 75 times smaller than the width of a human hair, sometimes the disc just won’t work. Sorry, folks, it happens.

Stay curious, 7B.