Compact Disc, DVD, and MP3 Technology
Compact Disc, DVD, and MP3 Technology
Compact discs (CDs), MP3s, and Digital Versatile Discs (DVDs) are currently some of the most popular ways to enjoy music and movies. Utilizing the latest in digital technology they have each been able to improve on the standards set by their predecessors.
A compact disc is a storage medium with digital data recorded on its surface. An optical beam reads the CD and reproduces the sound. Because the beam does not touch the CD, there is no wear and tear on the disc.
Compact discs store sound through a process known as Pulse Code Modulation (PCM). With this method, audio signals are sampled during short times intervals, and then converted into numerical data for storage in digital format.
To store audio in digital format requires large amounts of data. Optical discs were found to be ideally suited for this because they can store up to one million bits of data on an area as small as a pinhead.
How a CD Works. A CD can hold approximately 74 minutes of audio information. Not all of this information is music, though. Some of the space on a CD is used in error detection, synchronization, and display purposes.
Audio information on a CD is contained in the form of lands and pits (indentations). When it is read, a laser focuses on the lands and pits as the CD player's detector senses the differences in how they each reflect light. The detector then turns the reflected light into an electrical signal, which is then relayed to a circuit board and converted into sound.
Digital Versatile Discs
Digital versatile discs (DVDs) work in a manner very similar to CDs. Like a CD, a DVD is a disc layered with pits and lands. A DVD, though, can have more than one layer to it; a CD does not. Because of this, a DVD can hold about seven times more data than a CD.
How a DVD Works. A laser reads a DVD. The laser focuses on the bumps and flat areas of the disc, and a sensor in the DVD player senses differences in how these areas reflect light. These differences are then translated into video and audio signals, that the viewer watches as a movie.
Though DVDs have large storage capacities, uncompressed video data would never be able to fit on them. To put a movie on a DVD, it must first be encoded in the Moving Picture Experts Group (MPEG-2) format, which have standards for compressing moving pictures. Once a movie is in MPEG-2 format, it is stored on the disc. DVD players have a decoder in them that is able to uncompress the data quickly.
When an MPEG decoder uncompresses a movie, it must decide how it will encode the data. Each frame can be encoded in three ways: as an intraframe , predicted frame , or a bidirectional frame .
An intraframe contains the complete image data for that frame. This method provides the least compression . A predicted frame contains only data that relates to how the picture has changed from the previous one. In a bidirectional frame, data from the closest surrounding frames is looked at and averaged to calculate the position and color of each pixel.
The encoder decides to use different types of frames depending on what is being encoded. For example, if something is being encoded, and the frames stay similar from one to the next, predicted frames would be used.
A Moving Picture Experts Group audio Layer-3 (MP3) file compresses music files into a size that can easily be downloaded by a computer. A 3-minute song from a CD requires 30 megabytes of data. To try and download this on a computer would take an immense amount of time. By compressing the data into a much smaller file, it can be obtained much quicker.
Music can be compressed by a factor of 10 to 12 as an MP3, and yet maintain the original CD-quality sound. The sound is compressed using a compression algorithm . This algorithm comes from a technique called perceptual noise shaping . With this technique the qualities of the human ear are taken into account. There are certain sounds the human ear can hear, and sounds it can hear more than others. By recognizing sounds that will go unnoticed if removed, they can be omitted to make the file smaller.
CDs, MP3s, and DVDs have brought technology to the forefront of personal entertainment. By using the latest advances in technology they continue to improve what the public sees and hears.
see also Analog and Digital; Factors.
Brook E. Hall
Brain, Marshall. How DVDs and DVD Players Work. Marshall Brain's How Stuff Works. <http://www.howstuffworks.com/dvd.htm>.
Brain, Marshall. How MP3 Files Work. Marshall Brain's How Stuff Works. <http://www.howstuffworks.com/mp3.htm>.