Wednesday, February 16, 2011

Inside The Compact Disc

Everyone has seen a Compact Disc, heard music played from one, or used one to store files digitally. Not many people know how they work though. The CD is a modern marvel of science that is a true testament to the tenacity of the human spirit and ingenuity.
Creating a Compact Disc is also a more involved process than many people realize. The data is recorded onto the disc in microscopic increments which means that during production, even the smallest particles of dust will cause the recordings to be corrupted. The manufacturing process must be very accurate and precise because there is no room for error.
Turning Binary into Sound
The first challenge was developing the disc itself. Optical discs must be able to contain huge amounts of data because it takes around a million bits to playback one second of music. Fortunately, one million bits on a Compact Disc is about the size of the head of a pin. This made optical discs the ideal way to store the data for later retrieval. Another challenge was inventing a way to read back all of the compressed information on the CD fast enough to play back sound continuously, which was made possible by integrated circuit technology.
When a new audio CD is needed, a glass master must be created first. It is inspected carefully for quality assurance because a flawless master is what is really needed to manufacture good replicas. Once the glass master is finished, a stamper is created that is loaded into an injection molding machine, where the actual replicates are created. Through every step of the manufacturing process, quality and precision must be solidly maintained at the risk of creating waste.
A Compact Disc plays the sounds back in a system of groves laid out in a spiral over the surface of the disc. When the laser from the CD player moves along the spiral, it will encounter "pits" and "lands" that cause the laser to be reflected back at various intensities to the reader. The computer inside the CD player interprets this data as sound. Using this method eliminates the "fuzz" that you might hear on some older audio recordings, making CD's one of the most high quality audio formats around to this day.

Alternate Methods
The original formats laid out in the Red Book created by Sony and Philips were eventually discarded by many in order to produce audio CDs that were in excess of 79.8 minutes and in order to allow for digital rights management (DRM). Over the years compact discs have become available in 74, 80, 90, and 99 minute versions. The 80 minute CD (which is actually 79.8 minutes) is currently the standard size. However, the more music a compact disc can hold, the tighter its spiral must be (which allows for more playback). Not all CD players can handle the tighter spirals, especially the older CD players that existed before the higher capacity discs hit the market. Eventually though, manufacturers updated their products and now you are much less likely to have problems playing 80 minute discs. The 99 minute discs are another story though. Your best bet is to avoid them.

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